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Old 04-20-2012, 05:30 PM   #1
austinseanchris
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Default IAC Gone Bad? Symptoms? Rough Idle and hesitation...PLEASE HELP!

Hello all...I posted a new thread last week with not much help in replies and hoping to get a little more this time. Basically when starting the car up, you can hear the car wanting to hestitate a little at idle. When taking it down the road, it stutters and hesitates. No check engine codes, etc. Now, thus far, this is what I have checked. I checked the PCV in manifold. It still rattles and restricts air flow one way, so that seems to check out ok. Secondly, the EVAP box on passengers side was checked as well. I removed and blew through both vacuum fittings and no air passed thru. This IAC thing has been on my mind after all I've read. So I decided to take the whole throttle body off. And btw, this is a brand new(well, been on the car about a year) BBK 52mm throttle body and has worked fine since installing. Now I did use original IAC that was on factory tb. The intake was not gunked up and was very clean. I took off IAC and it was covered in carbon. I cleaned all that off and it looks new. Put everything back together. I decided to check all my vacuum lines and BAM, I thought I had found the problem. I found a hose coming out of the emission air pump(if front left nose area) that was dry rotted and broken and not even connected to the SLP cold air intake I have on there. I fixed that and thought to myself...all this BS over a da#* vacuum hose. Time for a test run. Cranked it up...still seemed a little choppy. I took it out anyways and sure enough...very stuttery and choppy.

I called a friend of my who is a ford mechanic that I know and he told me while the car was running to "unplug the IAC". He stated "if the car shuts off and dies, than the IAC is doing its job(at least on a ford)". He said "if the car continues to run, then thats a definate problem." So I did try that. While running, I unplugged it and it made no difference. The car continued to run just as before, no better, no worse. So now I'm lost...

I have no cats, no o2 sensors and no egr, so all that can be eliminated. The car has been dyno tuned around all those issues. New plugs and new wires which were all re-checked to make sure they were seated well. Also has a new coil on it as well. Also a side note...has an aftermarker JET MAF sensor that was installed with BBK tb and has worked fine as well. Any thoughts on IAC issue? Any other ideas? Not sure what symptoms of faulty injectors or them starting to go out are? Ughhh...these computer cars are a nightmare!!! Too much electrical BS!
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Old 04-20-2012, 06:37 PM   #2
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IAC is only for idle control so if the problem occurs when you've got your foot in it, it has nothing to do with the IAC.

GM IAC test is to note rpm. Turn it off and then turn it back on. Rpms should now be higher than what you noted but return to that figure in a matter of seconds. If yours does that, the IAC is working. This is also the test for Ford, Chrysler, Toyota and pretty much any car that has an IAC. It works that way because the ECM is programmed to retract the IAC pintle when you turn it off. That allows ample (more) air flow for a quick start which is met by an extra shot of fuel from the injectors. Kinda like goosing a carb only the ECM is now doing it for you.

If you had the IAC off or disconnected while it was running, first start after reinstalling/reconnecting should be with the accelerator slightly depressed. Let it run for 10 seconds. Off for 5 seconds. Then a normal - foot off the accelerator - restart. Using a scanner - Actual rpm should be 100 +/- rpms from Targeted in Park/Neutral; 50 +/- rpms from Targeted in Drive.

Wear and tear is built into idle control - it takes a hell of lot to gunk one up to the point that it no longer works and even though I managed to jam a butterfly and pop it's rivets on a truck I once owned by never cleaning the throttle body - the IAC was still functioning as it should when I replaced the flapper. The wear and tear factor has to be there -well at least out here - because the basic warranty, by Statute, is 50/5. Regardless of where you are, your Vette came with the same part and tune.
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Old 04-20-2012, 09:31 PM   #3
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Originally Posted by SunCr View Post
IAC is only for idle control so if the problem occurs when you've got your foot in it, it has nothing to do with the IAC.

GM IAC test is to note rpm. Turn it off and then turn it back on. Rpms should now be higher than what you noted but return to that figure in a matter of seconds. If yours does that, the IAC is working. This is also the test for Ford, Chrysler, Toyota and pretty much any car that has an IAC. It works that way because the ECM is programmed to retract the IAC pintle when you turn it off. That allows ample (more) air flow for a quick start which is met by an extra shot of fuel from the injectors. Kinda like goosing a carb only the ECM is now doing it for you.

If you had the IAC off or disconnected while it was running, first start after reinstalling/reconnecting should be with the accelerator slightly depressed. Let it run for 10 seconds. Off for 5 seconds. Then a normal - foot off the accelerator - restart. Using a scanner - Actual rpm should be 100 +/- rpms from Targeted in Park/Neutral; 50 +/- rpms from Targeted in Drive.



Wear and tear is built into idle control - it takes a hell of lot to gunk one up to the point that it no longer works and even though I managed to jam a butterfly and pop it's rivets on a truck I once owned by never cleaning the throttle body - the IAC was still functioning as it should when I replaced the flapper. The wear and tear factor has to be there -well at least out here - because the basic warranty, by Statute, is 50/5. Regardless of where you are, your Vette came with the same part and tune.
Based on the sounds of that...I dont know what to think now. Should car shut off or die when unplugging iac? Do you have any opinions on what this may be?
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Old 04-21-2012, 10:21 AM   #4
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Studdering & hesitation... really sounds like a misfire. Possibly the Optispark starting to go bad, one of the new plug wires melted/touching/grounding to a manifold, or a cracked/broken plug.

Doesn't sound like the IAC... if it were acting up, the MAF still meters all of the air going through the intake, whether it be through the IAC passage or TB butterflies, so it would still command the correct amount of fuel.
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Old 04-21-2012, 12:09 PM   #5
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Stalling when you unplug the IAC isn't unusual. Running electronic fuel injection without oxygen feedback - you say you've removed the O2's? - makes me wonder what you're trying to accomplish.
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Old 04-21-2012, 04:28 PM   #6
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Stalling when you unplug the IAC isn't unusual. Running electronic fuel injection without oxygen feedback - you say you've removed the O2's? - makes me wonder what you're trying to accomplish.
Hard to run 02 sensors and cats w/ full length headers...But its not stalling when you unplug IAC...thats the thing. It isnt doing anything at all by unplugging it...
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Old 04-21-2012, 05:45 PM   #7
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Try clipping a timing light successively onto each plug wire to see if you can find a missed flash corresponding to the engine miss.
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Old 04-21-2012, 05:47 PM   #8
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Originally Posted by austinseanchris View Post
Hard to run 02 sensors and cats w/ full length headers...But its not stalling when you unplug IAC...thats the thing. It isnt doing anything at all by unplugging it...
You got a PCM engine computer that is trying to set A/F mixture ratio, timing and other things based on all its inputs from the sensors on the engine. It is expecting to use the O2 sensors when it goes into closed loop operation. But you have removed them hence changing the whole formula for proper engine operation. The software in the PCM defaults to a richer condition to protect the engine and basically is probably confused.

If you want to check you Fuel trim and BLM numbers, then you will know a bit more about the A/F ratio. You basically sabotaged the GM setup and plan to make the engine run correctly with the correct inputs to the PCM but which now is lacking them. If you want to make that kind of radical change, you then need to change the PCM software and do whatever is necessary to make it run correctly across the RPM range.

I think with these parts removed from your car as it is right now, all bets are off the table with trying to get it fixed or running right. You have change the rules of the game, you may get different results from expected troubleshooting procedures. I think you would be in better shape getting the O2s installed and connected properly and getting a expert tuner to make changes in the PCM to re-tune it.

Sorry to be so blunt, just my opinion.

Last edited by pcolt94; 04-21-2012 at 08:47 PM.
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Old 04-24-2012, 10:27 AM   #9
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Quote:
Originally Posted by pcolt94 View Post
You got a PCM engine computer that is trying to set A/F mixture ratio, timing and other things based on all its inputs from the sensors on the engine. It is expecting to use the O2 sensors when it goes into closed loop operation. But you have removed them hence changing the whole formula for proper engine operation. The software in the PCM defaults to a richer condition to protect the engine and basically is probably confused.

If you want to check you Fuel trim and BLM numbers, then you will know a bit more about the A/F ratio. You basically sabotaged the GM setup and plan to make the engine run correctly with the correct inputs to the PCM but which now is lacking them. If you want to make that kind of radical change, you then need to change the PCM software and do whatever is necessary to make it run correctly across the RPM range.

I think with these parts removed from your car as it is right now, all bets are off the table with trying to get it fixed or running right. You have change the rules of the game, you may get different results from expected troubleshooting procedures. I think you would be in better shape getting the O2s installed and connected properly and getting a expert tuner to make changes in the PCM to re-tune it.

Sorry to be so blunt, just my opinion.
Well this all makes sense and sounds correct, but the only problem being when everything was removed, the PCM has been changed too. Everything was custom dyno-tuned to where they went in and changed the ratios and setting on the factory PCM. Somewhat like what PCMforless does. The only difference is there your buying software that is already set that has setting that work with most application. With mine, it was custom set to my application for fine tweaking. And the other side of that is that it has ran fine for about 4 months now with no problems. If it was going to act up or give me problems due to removal of these parts, I think it would have done so as soon as they were removed instead of operating fine for 4 months and then all of the sudden out of the blue one day deciding to cause problems...? I dont think that can be it...that wouldnt make any sense...
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Old 04-24-2012, 12:28 PM   #10
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As was stated above, the IAC is only used at idle for when the throttle blades are not being opened by your foot.

Does this engine still have a MAF sensor? If so, try removing the electrical connector on it (with engine not on) and note any differences.

Make sure all plugs are tight in the heads and wires are on tight. Also run engine at night with no lights around and check for arcing of plug wires.
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Old 04-24-2012, 02:29 PM   #11
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As was stated above, the IAC is only used at idle for when the throttle blades are not being opened by your foot.

Does this engine still have a MAF sensor? If so, try removing the electrical connector on it (with engine not on) and note any differences.

Make sure all plugs are tight in the heads and wires are on tight. Also run engine at night with no lights around and check for arcing of plug wires.
Yea, checked all plugs yesterday(removed and looked to see if any were fouled) and they all looked fine. Also checked wires and all are seated and are tight in good shape. I havent tried the arcing idea. Will def check that tonight. And I have checked to make sure MAF was connected and it is. So unhook it...what am I looking for with that unplugged?
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Old 04-24-2012, 03:12 PM   #12
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I suspect an opti problem. How many miles are on it?
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Old 04-24-2012, 03:57 PM   #13
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I suspect an opti problem. How many miles are on it?
85,500k
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Old 04-24-2012, 04:14 PM   #14
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If you have 85,000 on a opti and are having the problems you are describing, I would bet the opti is the problem.
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Old 04-24-2012, 04:25 PM   #15
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If you have 85,000 on a opti and are having the problems you are describing, I would bet the opti is the problem.
Looks like I'll be doing those heads sooner than later huh? That was planning on being a late summer project...
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Old 04-24-2012, 04:57 PM   #16
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That would probably be the easiest time to get to the opti when you unbury the engine to do the heads. You should be seeing the new AFR's soon!
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Old 04-24-2012, 09:30 PM   #17
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Quote:
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Everything was custom dyno-tuned to where they went in and changed the ratios and setting on the factory PCM.
Well said so we'll move on.

Before replacing the opti which at this point is a possibility, check out the TPS. It's been known to cause some funny things. From memory, I think the output of the wiper should go from about 0.64 volts to 4.0 volts. It should measure real smooth when the wiper is moving which is most important at the low end. You can measure this with just any basic DVM tapping into the DK Blue wire pin C of the sensor. Use a pin or such to either puncture wire or what I do, just slide pin up side of wire into connector an bit till the measurement is seen. Move throttle slowly and watch the meter.
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Old 04-25-2012, 10:33 PM   #18
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Replacing the OptiSpark unit is not that difficult on an LT1 engine. I have done it twice on my '95. I do not see how removng the intake and heads would make much difference in replacng the OptiSpark unit...I just do not.

Anyways, to the orignal poster....do not get a cheap knock-off OptiSpark unit or you will be doing it again.

Do not loose the water pump drive shaft either.
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Old 04-25-2012, 10:33 PM
 
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