Hello All, First time writer, long time reader here. I recently picked up a 1991 3.1L Firebird... off the lot it ran great, lil rich but I figured that was from sitting around. Got it home, cleaned it up some, ran it around for a couple days... did just about every repair it needed myself (under the guidance of an experienced mechanic). One problem I couldn't fix though, was my Driver's side power window. I took it to a local garage and they got me a new motor and regulator. I installed it... and now I am having idle issues. I can barely pull out of my driveway, and my idle is under 700. If I don't feed it gas I stall out, then I need to give it gas while turning ignition or I get no fire. I used my MODIS DTC scanner and read a trouble code with the IAC motor, replaced it and the valve housing it was with... (oh yeah, the moron that owned it previously thought he was jim dandy with cars.. welded all tons of crap together that needed simply a bolt and a nut... he stripped out the IAC motor first and had to use a couple gaskets to get a vacuum seal on the housing, then used oversized screws to lock it in place, albeit cock-eyed) and I'm still getting the idle issue. I let it idle in my driveway in P, the idle jumps around between 1500 and 500, and when its finished doing that for a while it stalls. It's had a basic tune up, new Dist. cap and new plugs, new rotor... no vacuum leaks that I can find, no liquid leaks, fresh oil, fresh coolant... I'm kind of at a loss here. Dealership wants 90 bucks to run their scan on it, and I'm hesitant to let them at it... Any ideas?

Whenever the battery is disconnected or the IAC changed the 3.1's need to have an idle learn procedure completed. Try this first and see if it clears up the fluctuating idle for you.



Idle Learn Procedure for 3.1L V6 (auto only):

Disconnect battery, wait one minute, and reconnect.

Turn off A/C.
Turn off Backlight Heater (rear window defroster).

Do Not Touch The Throttle at Any Point.
Do Not Touch The Steering Wheel at Any Point.

Set parking brake and chock tires.

Start engine, let settle a bit and put trans in drive.

Let engine run until fan cycles on then off.

Turn off engine.

Wait five minutes.

Start engine and place in gear.

Let idle for five minutes.

Shut off engine.

End of procedure.


If the throttle is touched at anytime during this you have
to start from the 'disconnect battery' again.

If the steering wheel has pressure applied you have
to start from the 'disconnect battery' again.

If the engine stalls at any point just restart and place
back into gear.

--

RBob.

weird procedure... think that works for other GM V6 motors Rbob?

I have heard about the Idle learn procedure protocol, and my Chilton describes what to do. However....

I had taken it out as requested by a dealer mechanic I know, and drove it above 40 mph. After that it seemed to run and idle ok until I stopped the car and cut off the engine. It wouldn't start after that. Had to manually get the engine going, and stay running by increasing the idle speed.

It now idles at 1k, steady. If I decrease the speed manually, it dies. I've had three differing opinions from what I would assume are reputable sources. Pontiac Dealership says Fuel Injectors or TPS sensor, Local Garage says TPS or EGR valve, and another friend (mechanic) says Injectors and O2 sensor. I may try replacing the valve and the sensors, or at least taking a look at them... I don't believe my injectors are bad, there doesn't appear/sound like there is a misfire or a struggle, just more like not knowing what idle speed to properly set itself at... I would assume there's a more telltale sign if my injectors are bad. (not used to working on MPFI cars, the only one I've really been hands-on into is carbureted...)

These new dumb cars with chips in them really annoy me. I'd rather set my idle manually by the pin and have it idle at that speed no matter what, and if it didn't it was a little easier to figure out.. I'd rather just bypass the ECM altogether and do THAT.

Well surging or a choppy idle is typical behavior of either running rich (possible faulty fuel pressure regulator, or broken injector), or a stuck EGR valve (flowing at idle, EGR should never flow at idle).

All of your sources have reasonable suggestions. A hot shutdown and then issues after re-start, but no starting issues when it's cold, points a finger to a leaky injector or two. Once the rail is pressurized, car is shut down....injectors bleed off fuel pressure into the cylinders. Restart, several cylinders are partially flooded, motor goes rich, and then the computer tries to find an idle with the IAC valve while it burns off the extra fuel, resulting in strange up and down behavior.

Use your Chilton's or talk to someone, find out which wires to backprobe on the TPS and the normal idle voltage. Rock the key to 'On' and backprobe the wires with a meter for voltage. Make sure the idle and WOT numbers are correct, then do a sweep test...look for jumps or peaks. It should be a SMOOTH transition between the idle, and peak voltage. You can do it with any meter, but the transition part is much easier to measure with an old-school analog meter. Most newer volt-meters are digital, which are better for calibrating and setting things, but it's a good idea to own an analog meter as well for sweep tests, since they go for so cheap now.

Yes it does. Not sure exactly which ones, but the Corsica's and Beretta;s come to mind.

RBob.

hmm, thanks, I may have stumbled upon a solution to the funky idle in the 3.4L DOHC grand prix...