Bad Idle Air Control Valve?


tidalcock
09-15-2004, 04:11 PM
Can the IAC valve go bad or not perform properly without throwing a code? My '01 ext cab with a 4.8 has developed a lower and sometimes erratic idle. Dealer has cleaned the throttle body, checked for codes, but says there is no problem. I have replaced the PCV valve and the plugs, but to no avail. I have owned and driven this truck for a year and I know when it is not running right. Man this is frustrating!

silveradoman
09-16-2004, 12:49 AM
Check it like this, turn on the engine and let it run until warm (the thermostat has to open) and have somebody put the truck in gear and turn the AC on. If the engine stalls or gets erratic on you, you definetly have a bad iac valve. If it reacts "normal" turn the engine off and disconnect the connector from the iac valve and measure the resistance with an ohmmeter between the first two terminals, and the last two terminals, both readings should be the same around 60 ohms plus minus a few. If you get an open circuit, you have to replace the valve, other than that I donīt know what else to do.

chuck16
09-16-2004, 01:27 AM
I agree with Silveradoman. One other thing you may want to check is your gaskets. My old truck use to that and I thought it was the IAC. I changed the valve to no avail. Feeling frustrated and let down I took it into somebody.
Turns out it was the throttlebody gasket between the intake manifold and the throttlebody. If there is even the tiniest of leaks in there it will cause the engine to idle lower, erratic or stall.
Happy Hunting. :D

GMMerlin
09-16-2004, 05:22 AM
Can the IAC valve go bad or not perform properly without throwing a code? My '01 ext cab with a 4.8 has developed a lower and sometimes erratic idle. Dealer has cleaned the throttle body, checked for codes, but says there is no problem. I have replaced the PCV valve and the plugs, but to no avail. I have owned and driven this truck for a year and I know when it is not running right. Man this is frustrating!

First make sure the throttle body is clean. next there are some GM TSBs for low erratic idle that include a calibration update.
On some vehicles a small vacum leak at the intake gasket or a cracked intake manifold will cause some idle concerns and not set a DTC
Usually these two will resolve your concern.
Personally I have not seen a bad IAC valve on these vehicles.

tidalcock
09-16-2004, 08:01 PM
Thanks everyone. I will look into these further. The throttle body gasket is sound and tight. How would I check for a crack or leak on the intake manifold? GM Merlin, I printed one of the TSB's that you had posted earlier and took it to the dealer. The mechanic said that he had personally performed the plug installation in the butterfly valve several times on other vehicles with no improvement. Should I believe him, and are there any other TSB's relating to this issue that I could research? If so, where can I find them?

gschretter
09-16-2004, 08:13 PM
Can you use normal carb cleaner to clean the TB while the engine is running?

Thanks

gtmud
09-17-2004, 06:07 AM
How amny miles do you have on your truck? Not sure if this will help, but I have the 4.3ltr 91' and had a bad idle problem, turned out the TBI unit's PCV vacuum connection was PACKED with carbon, and the fuel pressure regulator had a broken spring. Becareful removing the regulator, spring is under allot of tension. Hope this helps.

gtmud
09-17-2004, 06:11 AM
Also, spraying carb cleaner down the throat of the TBI does nothing but clean the immediate area, not the injectors, or passages that your fuel goes through internally. You can dis-assemble the TBI yourself and get a "rebuild kit" for it. It will come with all the gaskets, and screens that go on the injectors. Very easy to do. Be careful, when you remove the injectors, there is a tiny screen filter, just under the seat area of the injector along with one at the base.

tidalcock
09-17-2004, 01:25 PM
answers to above posts. the truck has about 55,000 miles. After the dealer charged me $200 to clean the throttle body and the fuel system, the truck showed no improvement. so, I took it upon myself to remove the throttle body and clean it myself. You wouldn't believe the crap left behind after the dealer's cleaning. the only thing that got cleaned was my wallet!

Anyway, the additional cleaning did not make any difference either. I have also checked the resistance/voltage on the TPS as the butterfly valve was modulated, and found the readings to be within normal limits. Additionally, the readings on the IAC appear to be normal as well. am getting tired of this guessing game and disappointed that the dealer can't even diagnose the problem.

As always, thanks for the suggestions and keep them coming. If I ever find out what is causing this problem, everyone on this board will know about it so that the information can be passed along!

tidalcock
09-18-2004, 11:33 PM
Still looking for answers. anyone have similar problems and/or fixes?

tidalcock
09-20-2004, 11:31 AM
Any ideas? Anyone?

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