High Idle / DTM 35 / Idle Air Control Error

sullivanjay · December 17, 2005

Hi, I have a 1995 ½ Ton 4X4 with 5.7 liter automatic. I have a DTC code 35. (Idle air control error) Symptom is a high idle usually around 1100 and as high as 1500. The MIL lamp will only light if I take it out of gear and put it in neutral or park and wait at least 20 seconds. If I put it back in gear it will go out. I have checked for vacuum leaks and checked connections to IAC. I sprayed WD40 around base of TB with no RPM change. Anyone know how to exercise the IAC without the GM tool? Any ideas what could be the cause of this? / Thanks, Jay

GM-Tech · December 18, 2005

Get a good light and peek down the port that leads to the IAC when this condition exists. If the IAC's pintle is fully seated, you should be able to see that. And if it is fully seated, air is getting in somewhere other than the IAC.

The TB gaskets on these were good for having pieces missing. I guess sucked into the engine. I've never tried WD-40 to test for a leak so I'm not sure how good a test that is. Try something a little more volatile, like carb cleaner.

sullivanjay · December 18, 2005

Thanks, I was hoping for a way to test / move the pintle. Parts about 50.00 dollars and hope to be sure it broken if I do decide to buy it. Had some of the "old" WD40 but will try some carb cleaner also as soon as I can. Did not know you could see the pintle seated from the top. Will definitly give it a shot and probaly just take it off and clean it. Thanks again / J

GM-Tech · December 18, 2005

I don't know any way to run the IAC without the special tool we use. The signal is pulsed, but I've never tried to simulate that. I just plug in our IAC tester.

sullivanjay · December 18, 2005

Thanks for the help. Looking down the throat of the TB I could not see the tip of pintle at all. I did have carb cleaner this time and used it around the base gasket of the TB and was able to change the RPM in one spot. Stuck my finger down at the suspect area and was able to hear the vacuum leak big time. So tomorrow I will get a new base gasket installed and hopefully it will end there. Thanks / J

GM-Tech · December 18, 2005

Odd. If there's a vacuum leak that causes the idle to be higher than the computer wants it to be, you should see the pintle seated, trying to slow the engine down. Are you sure you were looking down the right spot? You should have been able to see the hole that the pintle goes into.

sullivanjay · December 18, 2005

I was looking at it with the engine off. Was not sure what I was looking for but could imagine from the picture in the manual. (Factory - Helms) Was surprised that the hole I think it was was as large as it is. Definitely no cone shaped object protruding through that hole. Should be easy to see if I was looking in the right place. I'm sure of the vacuum leak so will start there. Just for a learning experience I will look at it again while running the engine. Your statement makes way to much sense for me not to be able to see it. Will get this done Monday or Tuesday and let you know how I make out. / J

Interesting observation: I remote start the car. (been cold in Boston lately) It will run for 15 minutes before it shuts off. Usually I'm in it in about ten minutes. When I get in there will bethe high idle condition but no check engine light on. I can drive it all day and the check engine light will not come on. If I put the car in park or neutral after the initial start and wait at least 20 seconds I will get the check engine light. As soon as I put it in gear and drive away it will go out which I can understand. What I don't understand is why no light from the initial start?

GM-Tech · December 19, 2005

I was looking at it with the engine off.

What would be the point of that? Note that I said, "peek down the port that leads to the IAC when this condition exists".

When you shut the engine off, the computer retracts the IAC in preparation for the next start so I would expect to be fully withdrawn from the hole and out of sight.

sullivanjay · December 20, 2005

Hey it's hard to look for something that you don't know even know where or what it looks like. Anyway checked it while running before I replaced the gasket and could see the pintle shut while running easily. New gasket has fixed it. Thanks for the help! / J