Anyone know what inputs control the IAC?

craigh

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I have a Gen 1 V6 5 speed. Car does not idle down when shifting into neutral and either coasting or coasting and braking to a stop. Idle hangs and bounces around up to 2500 RPM. It will usually do this until the car comes to a stop. Sometimes the idle will go down if I step on the clutch. The idle not coming down makes it difficult to use engine braking. This is strange situation and I don't know what could possibly be the connection between the clutch being depressed and the finally idle going down. This is also intermittent so it makes not sense to me. Wondering if anyone knows what engine control inputs would affect the IAC. BTW, I have replaced the IAC and the TPS, checked for vacuum leaks, replaced the PVC elbow and none of this made no change in the idle not coming down. Any thoughts?
 
The idle sits at around 750 on my V6 with automatic. All my idle wondering issues came from the IAC valve. But nothing would make it spike to 2500. Strange, just a wild thought here, check your accelerator linkage. Vacuum leaks make the engine run rough, no spikes in RPM.
 
Hi CraigH, my 2000 does the exact same thing. Pain in the ass, especially when the idle goes to normal as soon as the car is stopped in traffic for any reason. I had to just downshift low enough so that engine braking could work instead of the opposite, which is this condition causing the car to speed up on no throttle. I didn't get to use mine very much this year since my daughter's car was offline from March until November, so she was using it almost exclusively. Needless to say, I didn't miss this feature. My car has been doing this for years. I never thought it was the IAC, since it does have control when not moving. Something is telling it to open up more air, but what that is remains a mystery. I am glad that I have a stock sounding walker exhaust, otherwise, it would be more embarrassing when pulling up to a light. I have to wonder if a re-flash would help.
 
AFAIK, engine RPM, AC on/off, Power steering load (pressure) switch, engine temperature, start time, and in the case of the manual, I think it tries to rev match based on car speed.
 
The strange part is that these symptoms were never there in the earlier years. I noticed my car doing this perhaps 5-7 years ago. I do notice the engine slightly speeding up by incidental contact of the steering wheel, when the car is at idle and not moving. It would seem as though datalogging could help isolate the cause of this high idle reaction. It happens whether a/c is in use or is not in use. On back roads, where the speed limit is in the 30-40 mph range, the car will actually go faster than it should be going due to this increase in engine speed, which is what Craig and I both noticed; the only way to use engine braking is by downshifting to a gear that would result in higher engine rpm than what the PCM is trying to hold it at, which is not all that desirable.
 
AFAIK, engine RPM, AC on/off, Power steering load (pressure) switch, engine temperature, start time, and in the case of the manual, I think it tries to rev match based on car speed.
Joe, what makes you think that it matches the RPM based on car speed? I don't know how this would work as the computer does not know what gear you are in. Just kind of thinking out loud on this. I think I will do some research tonight.

Thoughts?
 
tireman, mike and joe. I appreciate your insight and trying to help with this issue. I can usually think my way out of an issue, but this one has me stumped. BTW, I did make an IAC choke plate and tried it. It did idle down lower but the cold running performance was miserable and I took it off. I think I used a choke plate with about a 3/16" hole in it.
 
Craig, perhaps you could try wiring a cutout switch into the actuation coil of the IAC. Leave closed during warmup and open the switch while cruising. If the engine starts stumbling in stopped traffic, turn the switch back on, etc. I thought about this yesterday and wondered if the system would indicate a malfunction from this. I know that on my Subaru, if I turn on the key with any components disconnected, like MAF sensor or cam position sensor for instance, it will flag a high voltage condition due to lack of signal continuity. To me, if my LS is still on the road after the current Subie Season is over, I may actually try that.
 
Craig, perhaps you could try wiring a cutout switch into the actuation coil of the IAC. Leave closed during warmup and open the switch while cruising. If the engine starts stumbling in stopped traffic, turn the switch back on, etc. I thought about this yesterday and wondered if the system would indicate a malfunction from this. I know that on my Subaru, if I turn on the key with any components disconnected, like MAF sensor or cam position sensor for instance, it will flag a high voltage condition due to lack of signal continuity. To me, if my LS is still on the road after the current Subie Season is over, I may actually try that.


Mike, Interesting concept. Are you thinking this would be a permanent solution or would this help troubleshoot it somehow?
 
Note that it does seem that the PCM checks for the presence of the IAC. You could get around that by using a double-throw switch. In one position it would connect to the IAC, and in the other position to a dummy load. You could figure out the resistance value and power rating needed for a resistor, or you could just get another IAC from a junkyard and wire to that (with it not installed on the engine).
 
Craig, I would just make it permanent and see how it goes, of course, utilizing Joe's suggested method of creating a dummy load using either the real component or a resistive element to simulate the IAC. And yes Joe, that is a great idea for the DPDT switch! You are always thinking!
 

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