Mercruiser Iac Valve Test

• Marine
• Truck
• Hot Rod
• RV
• Parts
• Info

• Mercruiser 350 Mag Iac Valve
• Mercruiser Iac Valve Replacement
• Mercruiser Iac Valve Location

MERCRUISER ALARMS AND FAULT CODES Applies to Mercruiser engines with PCM 555, including 496 MAG and 8.1S. Similar to ECM 555 engines (4.3, 5.0, 350, 6.2). 'Pwr Limit' = percentage of power available (may be limited by Engine Guardian). Engine Coolant Temp (ECT) sensor (near thermostat) measures temperature of closed-circuit coolant. The idle control valve in your vehicle is mounted on the throttle body and controls the air flow into the engine from around the throttle body. The valve, as the name suggests, controls the idle. When this valve is dirty, your vehicle will have a hard time idling. Check to see if the valve is functioning properly. New IAC (Idle Air Control) for Mercruiser and Volvo Penta engines Replaces Mercury Mercruiser 805224A1 MCM/MIE GM V-6 and V-8 (305, 350, 454 and 502 cid) EFI engines Symptoms of a failed IAC can be rough idle, fluctuating idle, engine stalls when coming down from full throttle, hard to start or high rpm on start › See more product details.

We at Full Throttle Marine are one of the few facilities that have a prop shaft dyno capable of up to 800 HP and an engine dyno room plumbed to run marine engines in a complete wet environment. Thanks to one of our customers, Mark LeBlanc, an opportunity arose to do some comprehensive testing on a Mercruiser, 496 Mag. HO. The engine is installed in a 2006 Nordic Heat. He brought us the boat to upgrade the engine with a Raylar 525HO kit.

Our first order of business was to get some baseline numbers on the boat. We headed to our local lake, GPS in hand. Weather conditions were 65 degrees, flat water and approximately 900 ft. elevation. We made 5 different runs, with 2 different drivers. The slowest speed was 62.6, fastest 63.5. Average RPM was 4920 on the smartcraft digital tach. Let me interject at this point the Heat really does not like glass smooth water. I've personally driven this boat a couple of MPH faster in more chop. Nevertheless, those were the conditions presented to us, now we have our baseline. Back to the shop for a dyno pull. We hook up our Land & Sea 13' Siamese-rotor prop shaft dyno with automatic load valve and DynoMax 2000 data acquisition software on the lap top. We made a total of 4 pulls all within a couple of HP of each other. Final prop shaft HP was 387.8 at 5000 RPM, using SAE Standard correction method.

The Nordic is equipped with dual gas tanks. This made it handy to test one myth that is commonly questioned. Will I gain any power by running premium fuel? We filled one tank with 87 octane the other with 93. Peak HP came in at 387.3. That is ½ of a HP less than with 93 octane. Studying the dyno sheets shows the 87 test to be within 1-2 HP of the 93 octane across the board, which falls within the acceptable tolerance of repeatability. I think we can safely say there was no difference between the 2 type fuels on this engine.

OK, let's make some horsepower! Off come the cast iron Mercruiser exhaust manifolds(earlier years have aluminum manifolds). We have a brand new set of polished Dana Marine exhaust manifolds with the Vortex IV risers. Talk about a nice kit! Everything was neatly labeled and went on without a hitch. Total install time was less than 2 hours. We had ordered these with the optional 02 sensor blocks so we could run our dyno's oxygen sensor. From testing done by Dana Marine and Raylar, I was expecting the engine to run a bit lean requiring a fuel pressure adjustment, but decided not to touch it to begin with. Again we made several pulls. The best of which netted a peak HP reading of 408.7 at 5000 RPM. The A/F ratio was right around 13.6:1 which is where Larry said the 496 made best power, so we didn't touch it. We'll fine tune it after the Raylar kit. That is a 21 HP increase at peak RPM. I was expecting more based on advertised claims*.

Well, let's go to the lake and see what we gained. My first observation was the engine sounded better and had more throttle response. We idled out of the no wake zone and nail it. Seat of the pants told me it was much stronger out of the hole. I didn't measure time to plane, but it was noticeably better. Cool, so far so good. Weather conditions were very similar to the baseline testing, except there was more wind. I could not find a glassy smooth stretch of water like we had the first day. With the added chop the boat aired out better. Again we made numerous passes, same lake, same GPS, same driver. Our slowest speed was 64.6 and fastest was 65.5. RPM increased by 80.

I've set up and run many Nordic Heats. I know that it takes approximately 17 HP increase to gain 1 mph in this boat. Interestingly we gained 2 with 21 HP. A quick calculation tells us that the prop slip is less on this test (same prop as before). Why would that be? A couple of thoughts come to mind. First, we saved about 50 lbs with the aluminum exhaust over the cast iron. Secondly, there was more chop on the lake. Remember my comment about this boat liking chop? There you have it. We hoped for a little more increase based on Dana's 48 hp increase claim*, but based on all of my experiences with exhaust this is about what I expected. Aptoide foxfi. Did I mention they look really cool?

*Dyno tests performed by Dana Marine were with wet exhaust manifolds, but the water was collected and not mixed into the exhaust gasses as they are in a marine application. This may well account for the higher hp increases they were able to achieve during dyno testing as opposed to 'in the boat' data.

496 Dyno Testing At The Crank

Now it's off to the shop to pull the engine and bust some more myths.

Well, the engine's out and on the dyno stand with the Dana exhaust in place. One question that comes to mind. On the Nordic installation the exhaust uses an S pipe, so the exhaust makes 2 almost 90 degree turns before exiting the boat. Could this be costing us some horsepower? Easy enough to prove. We pulled the complete exhaust system out of the boat and set it up on the dyno. We made a couple of pulls with a resultant 454 HP.

Next we removed the boat exhaust and routed it straight. No change. Shoots that theory down. Back on with the stock exhaust manifolds. Now we get 431 HP. That is a 23 HP improvement at the crankshaft with the Dana exhaust, 21 HP at the prop. Well what do you know, we have an opportunity to verify another myth. Let's remove the turbulators and see what we get. How about 438.8 HP. That's 8 HP, or about ½ mph on this boat.

Every little bit counts though. We have one last opportunity. I have a set of custom Lightning headers hanging on the wall that came off my 28' Heat with 900+ HP. We bolted them up and fire it up. Sure sounds sweet. Even more responsive than the Dana's. One quick dyno pull and there's no more speculation. 451 horsepower. That's 3 less than the Dana's. Interesting! They did make 10-15 ft.lbs. more torque at all lower RPM, with the Dana's finally passing them at 5000 RPM.

One last Myth to bust. How much horsepower does a Bravo drive absorb? For those of you that are observant, you're probably ahead of me. It's certainly not 25 or 30 HP like many have come to believe. It is a percentage. In this case with a Bravo X drive, right at 10%. I've done testing on 600 HP engines and lost 60+ HP to the drive.

Adding The Raylar 525HO / Stage 1 Marine Kit

Several months have passed and we've completed the Raylar install and testing. Sorry it's taken so long. Virtual dj 7 home crack. Just too much work and not enough time. As this article is directed at results, I'll save the details of the install for another time. The instructions were thorough and it went together without a hitch. Now it's back to the dyno to see what we accomplished. The one caveat to the project is that we sent the PCM to Whipple for their stage 2 programming. They tweak the fuel mixture, raise the rev limiter and adjust the spark timing to be much more aggressive. This requires a 120 degree thermostat and 91 octane fuel.

We installed the stock exhaust manifolds, with turbulators removed for our initial tests. We made several pulls, tweaking the fuel pressure up and down to get a feel for what air fuel ratio worked best. It made best power at 13:1 air fuel ratio. More than a few points either direction and the power started falling off. Off course this is what L arry had already told us, but being hard headed we had to see for ourselves. The numbers? 530 HP at 5400 RPM. We've already established that the turbulators cost us 8 HP, so I didn't see a need to revisit that. Keep in mind that Raylar's testing was done without water mixing with the exhaust, whereas these tests had complete wet exhaust. This would certainly account for some HP loss as compared to his testing.

Next, we bolt on the Dana exhaust manifolds. Again we make several pulls to adjust the A/F ratio and arrive back at the 13:1 number. This time we get 553 HP at 5400 RPM. Very nice numbers and right in line with the manufactures claims. Let's see what it does in the boat. This time the test conditions were the same altitude as before, about 10 degrees warmer air temperature and smooth water. We were able to consistently get 72 mph with a best of 72.8. Cruise A/F ratio is a bit lean so we were forced to raise the fuel pressure a couple of pounds to bring that in line for now. We'll send the PCM back to Whipple for some fine tuning in the fall.

In summary, with the Raylar 525HO kit, Whipple stage 2 programming and Dana exhaust we picked up 122hp & 9 mph. As a comparison, these numbers are slightly better than what that boat will run with a Merc 525 EFI under the same conditions.

496HO Mercury Bravo Drive Figures

• > Stock Manifolds / 87 Octane - 387.3hp
• > Stock Manifolds / 93 Octane - 387.8hp
• > Dana Manifolds - 408.7hp

496HO Mercury Crankshaft Figures

• > Stock Manifolds - 431hp
• > Stock Manifolds / Turbulators Removed - 438.8hp
• > Lightning Longtube Headers - 451hp
• > Dana Manifolds / S Pipe Removed - 454hp
• > Raylar 525HO Kit / Stock Manifolds - 530hp
• > Raylar 525HO Kit / Dana Manifolds - 553hp

Want Even More Power?

• > 511cid Stroker Kit / 540cid Short Block
• > CNC Extreme Aluminum Cylinder Heads
• > Cool Gap Intake Manifold
• > Aluminum Intake Manifold
• > Long Tube Headers
• > Shorty Headers
• > Performance Spark Plug Wires

Throttle Position Sensor (TPS)

The job of the TPS is to tell the computer what the position of the throttle is.This sensor is vital in helping the computer determine if the throttle is closed or open; or how fast the throttle is opened or closed.The throttle position sensor is a simple potentiometer that uses ground and 5-volt reference inputs to produce a varying output signal depending on the position of its detection arm or shaft.At rest, this sensor outputs a relatively low voltage signal; as the arm/shaft is turned (as it would when the throttle opens), the output voltage increases. If this sensor is out of adjustment or is failing, the result could be stalling, idle surge, flat throttle response, hesitation, or erratic engine operation.Most of the TP sensors you find on stock Fiero engines are the adjustable type while those found on newer GM engines are usually non-adjustable.If the TPS on your engine is adjustable, it must be set correctly in order for the computer to function normally.

Some throttle position sensors are adjustable. In order to adjust the TPS, you will need a scan tool (or laptop/PC running scan tool software) or a digital volt meter.You will also need the proper tool to loosen the retaining screws.If you have a scan tool, all you need to do is pull up the TPS voltage data.If you are using a digital volt meter, then voltage will need to be measured across the blue and black wires going to the TPS.I suggest this measurement be made by going directly to the connectors at the ECM (located between the seats)so the weather seal is not broken in the wiring/connection out in the engine compartment.If this seal is broken, moisture can corrode the connections to the TPS and create all sorts of problems.Once you get your TPS voltage display up, turn the ignition on but do not start the engine.The voltage with closed throttle should be between 0.400 and 0.625 volts for most applications (check appropriate reference guides for your application's specified voltage range).If the voltage you get is not within spec, loosen the TPS retaining screws and adjust as necessary.Normal voltage reading for wide open throttle (WOT) is above 4.00 volts.

There are trouble codes that are associated with the TPS. One code will set if the TPS voltage is too high when the computer expects to see it lower.Another code will set if the TPS voltage is lower than the computer expects to see. The TPS code for low voltage is the most common and will usually set if the TPS is out of adjustment or the sensor has failed.The first thing you should do when you get a TPS code is to check adjustment and signal output of the TP sensorbefore replacing it.Be sure to wiggle all connections while watching scan data/voltage readout to make sure the problem is not a loose or bad connection.

There are circumstances that could occur with a failing throttle position sensor that may not set a trouble code.One of the most common symptoms of a failing TPS would be a tip-in hesitation or stumble when you apply throttle to take off from a stop.This can be caused by a dead spot in the TP sensor's internal circuitry, which usually causes the output voltage signal to not change (or it drops out) when the throttle opens.Unfortunately this type of failure is not easy to diagnose without the proper tool – a digital waveform scope.Most digital volt meters and scan tool displays will not respond fast enough to show this type of a glitch; but some may.If you do find this fault, then the obvious fix is to replace the TP sensor.

Idle Air Control Valve (IAC)

The IAC valve GM uses for most of their engines is a stepper motor actuated valve.A stepper motor is a device that moves a predetermined amount per electrical signal it receives by whatever device is controlling it.The GM IAC has a pintle that extends or retracts into or out of an idle (bypass) air passage in a throttle body or intake manifold attached to the engine.Typically as the IAC's pintle extends, air flow to the engine is restricted as the air passage is shut off.As it retracts, air flow to the engine is increased as the air passage is opened up.This air passage is a simple bypass for incoming air to take around the throttle blade.

Diagnosing problems associated with the IAC aren't simple.There are many other causes that can make a IAC valve appear faulty. In many cases, there may only one or two trouble codes in the ECM/PCM assigned to the IAC, but these trouble codes can set for a variety of reasons.Basically, this trouble code sets if the ECM cannot make the engine idle at a set (desired) speed by control of the IAC valve's position.There are limits set up within the ECM that only allow it to move the IAC in and out of the idle air passage so far.The IAC valve position is referred to as IAC counts.Scan tool data indicating '0' IAC counts means the IAC valve is fully extended (shutting off idle airflow to the engine); and '255' IAC counts means the IAC valve is fully retracted, allowing as much air to enter the engine via the throttle bypass passage as possible.Some ECMs may never allow the IAC to reach 255 counts.Generally, anything you see over 160 counts should be considered to be a near- or fully-open idle air passage.

On a normal operating engine, it is typical to see high IAC counts (100 or more) when the engine is idling cold and during the warm-up cycle.As the engine warms, the IAC counts should decrease.By the time the engine reaches operating temp (fully warmed up), the IAC position should drop into the range of about 20-50 counts in park or neutral.If the scan data you get reports counts lower than this, then that can indicate one or more of the following problems exists:

·There is a vacuum leak allowing unmetered air to enter the engine

·The throttle stop screw is adjusted incorrectly (throttle being held open too far; more on this later)

·There is a problem with the throttle cable or cruise control system that isn't allowing the throttle to close all of the way

·The IAC valve itself is faulty

Now if you see the scan data reporting IAC position higher than 20-50 counts on a fully warmed up engine, this could indicate one or more of the following problems exist:

·There is carbon buildup on the IAC pintle, or in the idle air passage restricting air flow

·There is carbon buildup on the throttle blade or throttle body bore

·The throttle stop screw is adjusted incorrectly (throttle resting closed too much)

·There is a mechanical problem with the engine resulting in lower than expected vacuum levels at idle (this will require the IAC to open further so the engine gets the required amount of air to maintain the preset idle speed)

·There is increased load on the engine (such as what would occur if the automatic transmission was shifted into gear)

·The IAC valve itself is faulty

Any one of the above issues can cause a an IAC fault code to set in the ECM.As you can see, there are many issues other than a faulty IAC valve that can cause a code to set.So before replacing the IAC valve, you should check all of these possible issues first.

The IAC valve cannot be tested using conventional electrical testing means.There are special tools available that are designed to test GM IAC valves, but I have discovered most shops don't have these tools anymore.To be quite honest, you don't really see many IAC valves fail.When they do, they usually freeze up or get stuck in a fixed position.

The ECM 'resets' the IAC valve when the car is operated at normal road speeds (35mph or more).During this time, the IAC valve is typically extended out all the way (IAC counts = 0), thus closing off the idle air passage.This helps the ECM 'learn' the position of the IAC valve.Any time the IAC valve is replaced, this 'learn' procedure should be performed.

Throttle Stop Screw (minimum air setting)

The throttle stop screw's primary function is to prevent the throttle blade from closing too far and getting wedged/stuck in the throttle bore.However, it serves as a secondary function to adjust the minimum air setting.The 'minimum air setting' is what is used to describe the amount of air that is allowed to enter the engine thru a 'closed' throttle.Because the throttle valve cannot be allowed to completely close (because this would result in it getting wedged/stuck closed in the throttle bore), some air will always be allowed to enter the engine around the throttle valve.

On a 100% factory stock engine, you should never need to adjust the throttle stop screw.This is the reason why GM installs a tamper-proof plug over the throttle stop screw on the throttle body.But there are times when the adjustment of this screw is necessary.One example of this is when the engine is modified, or a different throttle body is being used than what originally came with the engine.

Larger displacement engines require more air to maintain a set idle speed.Aftermarket camshafts with lots of duration or lots of overlap tend to lower the amount of vacuum an engine can generate at idle.Lower vacuum levels translate to less pressure differential between the intake manifold and outside (ambient) air.This means there isn't as much pressure difference to force air into the engine around the throttle blade or thru the IAC passage at idle.And either the IAC needs to open up or the throttle blade must be opened more to allow more air to enter the engine.Engines that have higher compression or are new/rebuilt can have higher internal loads/friction which can also result in a drop of idle vacuum levels.Basically any condition that increases load on the engine will result in the vacuum level to drop at idle, which will require the IAC or throttle blade to be opened up to compensate.

I've set up and run many Nordic Heats. I know that it takes approximately 17 HP increase to gain 1 mph in this boat. Interestingly we gained 2 with 21 HP. A quick calculation tells us that the prop slip is less on this test (same prop as before). Why would that be? A couple of thoughts come to mind. First, we saved about 50 lbs with the aluminum exhaust over the cast iron. Secondly, there was more chop on the lake. Remember my comment about this boat liking chop? There you have it. We hoped for a little more increase based on Dana's 48 hp increase claim*, but based on all of my experiences with exhaust this is about what I expected. Aptoide foxfi. Did I mention they look really cool?

*Dyno tests performed by Dana Marine were with wet exhaust manifolds, but the water was collected and not mixed into the exhaust gasses as they are in a marine application. This may well account for the higher hp increases they were able to achieve during dyno testing as opposed to 'in the boat' data.

496 Dyno Testing At The Crank

Now it's off to the shop to pull the engine and bust some more myths.

Well, the engine's out and on the dyno stand with the Dana exhaust in place. One question that comes to mind. On the Nordic installation the exhaust uses an S pipe, so the exhaust makes 2 almost 90 degree turns before exiting the boat. Could this be costing us some horsepower? Easy enough to prove. We pulled the complete exhaust system out of the boat and set it up on the dyno. We made a couple of pulls with a resultant 454 HP.

Next we removed the boat exhaust and routed it straight. No change. Shoots that theory down. Back on with the stock exhaust manifolds. Now we get 431 HP. That is a 23 HP improvement at the crankshaft with the Dana exhaust, 21 HP at the prop. Well what do you know, we have an opportunity to verify another myth. Let's remove the turbulators and see what we get. How about 438.8 HP. That's 8 HP, or about ½ mph on this boat.

Every little bit counts though. We have one last opportunity. I have a set of custom Lightning headers hanging on the wall that came off my 28' Heat with 900+ HP. We bolted them up and fire it up. Sure sounds sweet. Even more responsive than the Dana's. One quick dyno pull and there's no more speculation. 451 horsepower. That's 3 less than the Dana's. Interesting! They did make 10-15 ft.lbs. more torque at all lower RPM, with the Dana's finally passing them at 5000 RPM.

One last Myth to bust. How much horsepower does a Bravo drive absorb? For those of you that are observant, you're probably ahead of me. It's certainly not 25 or 30 HP like many have come to believe. It is a percentage. In this case with a Bravo X drive, right at 10%. I've done testing on 600 HP engines and lost 60+ HP to the drive.

Adding The Raylar 525HO / Stage 1 Marine Kit

Several months have passed and we've completed the Raylar install and testing. Sorry it's taken so long. Virtual dj 7 home crack. Just too much work and not enough time. As this article is directed at results, I'll save the details of the install for another time. The instructions were thorough and it went together without a hitch. Now it's back to the dyno to see what we accomplished. The one caveat to the project is that we sent the PCM to Whipple for their stage 2 programming. They tweak the fuel mixture, raise the rev limiter and adjust the spark timing to be much more aggressive. This requires a 120 degree thermostat and 91 octane fuel.

We installed the stock exhaust manifolds, with turbulators removed for our initial tests. We made several pulls, tweaking the fuel pressure up and down to get a feel for what air fuel ratio worked best. It made best power at 13:1 air fuel ratio. More than a few points either direction and the power started falling off. Off course this is what L arry had already told us, but being hard headed we had to see for ourselves. The numbers? 530 HP at 5400 RPM. We've already established that the turbulators cost us 8 HP, so I didn't see a need to revisit that. Keep in mind that Raylar's testing was done without water mixing with the exhaust, whereas these tests had complete wet exhaust. This would certainly account for some HP loss as compared to his testing.

Next, we bolt on the Dana exhaust manifolds. Again we make several pulls to adjust the A/F ratio and arrive back at the 13:1 number. This time we get 553 HP at 5400 RPM. Very nice numbers and right in line with the manufactures claims. Let's see what it does in the boat. This time the test conditions were the same altitude as before, about 10 degrees warmer air temperature and smooth water. We were able to consistently get 72 mph with a best of 72.8. Cruise A/F ratio is a bit lean so we were forced to raise the fuel pressure a couple of pounds to bring that in line for now. We'll send the PCM back to Whipple for some fine tuning in the fall.

In summary, with the Raylar 525HO kit, Whipple stage 2 programming and Dana exhaust we picked up 122hp & 9 mph. As a comparison, these numbers are slightly better than what that boat will run with a Merc 525 EFI under the same conditions.

496HO Mercury Bravo Drive Figures

• > Stock Manifolds / 87 Octane - 387.3hp
• > Stock Manifolds / 93 Octane - 387.8hp
• > Dana Manifolds - 408.7hp

496HO Mercury Crankshaft Figures

• > Stock Manifolds - 431hp
• > Stock Manifolds / Turbulators Removed - 438.8hp
• > Lightning Longtube Headers - 451hp
• > Dana Manifolds / S Pipe Removed - 454hp
• > Raylar 525HO Kit / Stock Manifolds - 530hp
• > Raylar 525HO Kit / Dana Manifolds - 553hp

Want Even More Power?

• > 511cid Stroker Kit / 540cid Short Block
• > CNC Extreme Aluminum Cylinder Heads
• > Cool Gap Intake Manifold
• > Aluminum Intake Manifold
• > Long Tube Headers
• > Shorty Headers
• > Performance Spark Plug Wires

Throttle Position Sensor (TPS)

The job of the TPS is to tell the computer what the position of the throttle is.This sensor is vital in helping the computer determine if the throttle is closed or open; or how fast the throttle is opened or closed.The throttle position sensor is a simple potentiometer that uses ground and 5-volt reference inputs to produce a varying output signal depending on the position of its detection arm or shaft.At rest, this sensor outputs a relatively low voltage signal; as the arm/shaft is turned (as it would when the throttle opens), the output voltage increases. If this sensor is out of adjustment or is failing, the result could be stalling, idle surge, flat throttle response, hesitation, or erratic engine operation.Most of the TP sensors you find on stock Fiero engines are the adjustable type while those found on newer GM engines are usually non-adjustable.If the TPS on your engine is adjustable, it must be set correctly in order for the computer to function normally.

Some throttle position sensors are adjustable. In order to adjust the TPS, you will need a scan tool (or laptop/PC running scan tool software) or a digital volt meter.You will also need the proper tool to loosen the retaining screws.If you have a scan tool, all you need to do is pull up the TPS voltage data.If you are using a digital volt meter, then voltage will need to be measured across the blue and black wires going to the TPS.I suggest this measurement be made by going directly to the connectors at the ECM (located between the seats)so the weather seal is not broken in the wiring/connection out in the engine compartment.If this seal is broken, moisture can corrode the connections to the TPS and create all sorts of problems.Once you get your TPS voltage display up, turn the ignition on but do not start the engine.The voltage with closed throttle should be between 0.400 and 0.625 volts for most applications (check appropriate reference guides for your application's specified voltage range).If the voltage you get is not within spec, loosen the TPS retaining screws and adjust as necessary.Normal voltage reading for wide open throttle (WOT) is above 4.00 volts.

There are trouble codes that are associated with the TPS. One code will set if the TPS voltage is too high when the computer expects to see it lower.Another code will set if the TPS voltage is lower than the computer expects to see. The TPS code for low voltage is the most common and will usually set if the TPS is out of adjustment or the sensor has failed.The first thing you should do when you get a TPS code is to check adjustment and signal output of the TP sensorbefore replacing it.Be sure to wiggle all connections while watching scan data/voltage readout to make sure the problem is not a loose or bad connection.

There are circumstances that could occur with a failing throttle position sensor that may not set a trouble code.One of the most common symptoms of a failing TPS would be a tip-in hesitation or stumble when you apply throttle to take off from a stop.This can be caused by a dead spot in the TP sensor's internal circuitry, which usually causes the output voltage signal to not change (or it drops out) when the throttle opens.Unfortunately this type of failure is not easy to diagnose without the proper tool – a digital waveform scope.Most digital volt meters and scan tool displays will not respond fast enough to show this type of a glitch; but some may.If you do find this fault, then the obvious fix is to replace the TP sensor.

Idle Air Control Valve (IAC)

The IAC valve GM uses for most of their engines is a stepper motor actuated valve.A stepper motor is a device that moves a predetermined amount per electrical signal it receives by whatever device is controlling it.The GM IAC has a pintle that extends or retracts into or out of an idle (bypass) air passage in a throttle body or intake manifold attached to the engine.Typically as the IAC's pintle extends, air flow to the engine is restricted as the air passage is shut off.As it retracts, air flow to the engine is increased as the air passage is opened up.This air passage is a simple bypass for incoming air to take around the throttle blade.

Diagnosing problems associated with the IAC aren't simple.There are many other causes that can make a IAC valve appear faulty. In many cases, there may only one or two trouble codes in the ECM/PCM assigned to the IAC, but these trouble codes can set for a variety of reasons.Basically, this trouble code sets if the ECM cannot make the engine idle at a set (desired) speed by control of the IAC valve's position.There are limits set up within the ECM that only allow it to move the IAC in and out of the idle air passage so far.The IAC valve position is referred to as IAC counts.Scan tool data indicating '0' IAC counts means the IAC valve is fully extended (shutting off idle airflow to the engine); and '255' IAC counts means the IAC valve is fully retracted, allowing as much air to enter the engine via the throttle bypass passage as possible.Some ECMs may never allow the IAC to reach 255 counts.Generally, anything you see over 160 counts should be considered to be a near- or fully-open idle air passage.

On a normal operating engine, it is typical to see high IAC counts (100 or more) when the engine is idling cold and during the warm-up cycle.As the engine warms, the IAC counts should decrease.By the time the engine reaches operating temp (fully warmed up), the IAC position should drop into the range of about 20-50 counts in park or neutral.If the scan data you get reports counts lower than this, then that can indicate one or more of the following problems exists:

·There is a vacuum leak allowing unmetered air to enter the engine

·The throttle stop screw is adjusted incorrectly (throttle being held open too far; more on this later)

·There is a problem with the throttle cable or cruise control system that isn't allowing the throttle to close all of the way

·The IAC valve itself is faulty

Now if you see the scan data reporting IAC position higher than 20-50 counts on a fully warmed up engine, this could indicate one or more of the following problems exist:

·There is carbon buildup on the IAC pintle, or in the idle air passage restricting air flow

·There is carbon buildup on the throttle blade or throttle body bore

·The throttle stop screw is adjusted incorrectly (throttle resting closed too much)

·There is a mechanical problem with the engine resulting in lower than expected vacuum levels at idle (this will require the IAC to open further so the engine gets the required amount of air to maintain the preset idle speed)

·There is increased load on the engine (such as what would occur if the automatic transmission was shifted into gear)

·The IAC valve itself is faulty

Any one of the above issues can cause a an IAC fault code to set in the ECM.As you can see, there are many issues other than a faulty IAC valve that can cause a code to set.So before replacing the IAC valve, you should check all of these possible issues first.

The IAC valve cannot be tested using conventional electrical testing means.There are special tools available that are designed to test GM IAC valves, but I have discovered most shops don't have these tools anymore.To be quite honest, you don't really see many IAC valves fail.When they do, they usually freeze up or get stuck in a fixed position.

The ECM 'resets' the IAC valve when the car is operated at normal road speeds (35mph or more).During this time, the IAC valve is typically extended out all the way (IAC counts = 0), thus closing off the idle air passage.This helps the ECM 'learn' the position of the IAC valve.Any time the IAC valve is replaced, this 'learn' procedure should be performed.

Throttle Stop Screw (minimum air setting)

The throttle stop screw's primary function is to prevent the throttle blade from closing too far and getting wedged/stuck in the throttle bore.However, it serves as a secondary function to adjust the minimum air setting.The 'minimum air setting' is what is used to describe the amount of air that is allowed to enter the engine thru a 'closed' throttle.Because the throttle valve cannot be allowed to completely close (because this would result in it getting wedged/stuck closed in the throttle bore), some air will always be allowed to enter the engine around the throttle valve.

On a 100% factory stock engine, you should never need to adjust the throttle stop screw.This is the reason why GM installs a tamper-proof plug over the throttle stop screw on the throttle body.But there are times when the adjustment of this screw is necessary.One example of this is when the engine is modified, or a different throttle body is being used than what originally came with the engine.

Larger displacement engines require more air to maintain a set idle speed.Aftermarket camshafts with lots of duration or lots of overlap tend to lower the amount of vacuum an engine can generate at idle.Lower vacuum levels translate to less pressure differential between the intake manifold and outside (ambient) air.This means there isn't as much pressure difference to force air into the engine around the throttle blade or thru the IAC passage at idle.And either the IAC needs to open up or the throttle blade must be opened more to allow more air to enter the engine.Engines that have higher compression or are new/rebuilt can have higher internal loads/friction which can also result in a drop of idle vacuum levels.Basically any condition that increases load on the engine will result in the vacuum level to drop at idle, which will require the IAC or throttle blade to be opened up to compensate.

Mercruiser 350 Mag Iac Valve

Setting the throttle stop screw can beaccomplished a couple of different ways.If you have a scan tool, I recommend allowing the engine to warm up to operating temperature and then adjust the throttle stop screw in or out until the observed IAC position counts come to rest within the spec range Iprovided earlier.After adjusting the throttle stop screw, it may be necessary to adjust the Throttle Position Sensor, which we discussed in my Jan/Feb 2008 segment.On cars that don't have an adjustable throttle position sensor, the ECM automatically learns the 'closed throttle' voltage when the ignition is keyed on (after the key has been off for at least 10 seconds).If you don't have a scan tool and you are working with a pre 1994 model year ECM, you should be able to adjust the minimum air setting by doing the following steps…

1)With the IAC valve connected, ground the diagnostic (ALDL) terminal (same as you would do to flash trouble codes thru the check engine light).

Mercruiser Iac Valve Replacement

2)Turn ON the ignition, but do NOT start the engine.Wait at least 30 seconds.

3)With the ignition still on, disconnect the IAC electrical connector.

Mercruiser Iac Valve Location

4)Remove grounding of the diagnostic (ALDL) connector and start the engine.Allow the engine to fully warm up and go into closed loop.

5)Adjust the idle stop screw so the engine idle speed obtains 550rpm in drive (auto trans) or 650rpm in neutral (manual trans).

6)Turn the ignition off.Disconnect power from the ECM for at least 10 seconds to clear codes (in case any are present) and reconnect the IAC electrical connector.