1200 rpm at idle, no vac leaks - Maintenance and Repair Forum

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1200 rpm at idle, no vac leaks
Friday, July 29, 2005 5:17 AM
My rpm at idle is 1200, i cant get it to lower unless i turn the ac on... there are no vac leaks, i replaced the gasket and vac hose that was cracked, throttle body is clean as a whistle... iac counts are around 50-60 at idle last time i had a scan tool(monday)... im stumped... any help would be appreciated.

Re: 1200 rpm at idle, no vac leaks
Friday, July 29, 2005 6:32 AM
Sounds like you may have the same fried PCM problem that I do. But I would check everything else before making that decision. I checked every single thing (sensor, valve, hose...).

The PCM controls engine idle speed with the IAC valve. To increase idle speed, the PCM retracts the IAC valve pintle away from its seat, allowing more air to bypass the throttle bore. To decrease idle speed, it extends the IAC valve pintle towards its seat, reducing bypass air flow. A "Scan" tool will read the PCM commands to the IAC valve in counts. Higher the counts indicate more air bypass (higher idle). The lower the counts indicate less air is allowed to bypass (lower idle). A garage down the street measured the counts on my car and it was 60. Also, if you open the TB (going from idle to WOT) the counts "should" change, but in my car they are frozen at 60. This means the IAC pintle is being instructed to stay in one spot all the time. This is not normal.
Re: 1200 rpm at idle, no vac leaks
Friday, July 29, 2005 7:42 AM
I did i IAC Control test on a snap on scanner, and the pcm thinks that its runnign at base idle.. 600 rpn, because when i increase the iacs above 600 the rpm will raise, but when try to lower the iac count, the iac will never close and never go pass 600 rpm (50 iac counts) If u take the rpm to 400 on the scanner the iac should close almost and car should start running really rough if not stalling, i did this on my fathers 00 lumina. My car at 300-400 rpm the iacs never decrease below 50 iacs
Re: 1200 rpm at idle, no vac leaks
Friday, July 29, 2005 7:59 AM
Sorry, I forgot to add, that if i create a vac leak ( like the brake booster) the pcm doesn't try to lower the iac counts. It will lower them maybe 1 or 2 iac counts but never does it even go close to 0
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