View Full Version : help! erratic idle, no codes :-x

12-29-09, 12:41 PM
So a few days ago I turned on the car and the car didn't start. A little throttle got that going, and then it was back to normal... or was it.?

I noticed braking sometimes requires more effort, and then I realized my idle is sometimes high. Braking these times is as if I am also applying the gas.

NO CODES (this stumps me)

Braking to a stop would reduce RPMs but not as low as they should, and if I'm in traffic, the temperature needle moves a pinch, which is unusual without the AC on. Putting it in neutral, rises the RPMs to 1500-2000, but pressing on the gas may allow that to go back down.

Could it be the IAC? I've also been searching here and found these could be associated with vacuum leaks.

Then there's something about the overpressurization valve at the other side of the manifold.

THe throttle plate didn't seem to be involved as I manually tried closing it when idling, and it was already closed. So the more I think of it, it may be the IAC, however I have no clue if it could be elsewhere. Not sure about the overpressure valve or where to begin looking for a random vacuum leak.

I authored this a few months ago, it can't hurt to try it again... http://www.cadillacforums.com/forums/cadillac-tech-tips/167181-tb-iac-idle-air-control-valve.html back then it was the throttle plate that was sticky. Back then, I could even feel the resistance in the gas pedal!

Dave S Texas
12-29-09, 01:55 PM
I agree it's a vacuum leak. Easy enough to find by checking hoses/hose connections. If the hoses/connections are good, a Mity-Vac hand pump would be my choice of tool to check the vac systems. That inexpensive little tool is very handy for many jobs.

12-30-09, 10:28 PM
Based on our two conversations this evening I think we have narrowed it down to the IAC valve. Let me (us) know it that was it.

01-06-10, 02:54 PM
Sure enough, the IAC. I basically let the car idle normally (as per your instructions over the phone), and then unplugged the IAC, thereby leaving it fixed in that position.

Since then it kind of wants to stall when I turn it on, but the PCM adjusts something in order to leave it going. Otherwise it hasn't thrown any codes. I've been driving it for one week with the IAC unplugged (still in place).

The only difference now, is that the car does not rev high while the scroll compressors heat the catalytic converter for 40 seconds after turning on the car. It idles at 650 instead. Am I breaking anything by driving it like this?

01-06-10, 04:41 PM
Nah, you're just no compensating for additional load like A/C and cold start fast idle, etc. That's why it wants to stall. I'd still replace it when you get a chance.

01-06-10, 04:44 PM
Cheapest I've found it is $215 or so at RockAuto. ACDelco version (non-OEM are more!)

01-06-10, 08:59 PM
Yikes! Expensive little bastard. They are pretty reliable so a trip to the junk yard might be in order.