Erratic idle
 

I have a '92 with the 350, everything is stock. When coming to a complete stock, like at a traffic light, the idle drops down to about 400rpm, jumps up to about 650rpm, and then levels out at just over 500rpm. The 500rpm idle is rough and feels like it is about to cut out. It runs fine at anything above idle. The IAC is new and I adjusted the idle according to the factory service manual, the EGR is clean, ignition system is fine, there aren't any vacuum leaks, and there aren't any codes. What could be the problem?

Re: Erratic idle
 

I had the same problem, I took my car down to get a diagnostic test but nothing came up, just a small knocking....I would suggest changing your spark plugs, wires, cap n rotor, clean the distributer lobes, check your fuel system, any vacuum leaks.

Re: Erratic idle
 

Did you try cleaning out the passages where the IAC screws into the throttle body? I've seen it happen where carbon builds up in there an wrecks havoc with the IAC pintle. It never hurts to keep a clean throttle body and is a cheap easy thing to try. As a fellow TPI owner, I've learned when it isn't the obvious, it's usually something stupid!

Re: Erratic idle
 

I did clean the IAC passage. With the engine warmed up and idling the fuel pressure is 33psi, pulling the vacuum line to the regulator makes it jump to 44psi. It seems that the regulator is keeping the pressure to low, right?

Re: Erratic idle
 

Pull a couple plugs, i start having this problem when i fill up with new gas, and the car starts to act like its going to die and then fix itself and then tries to hunt again.
I always have a plug on the passenger side, the one closest to the radiator, clog with deposits for some reason. I clean it with intake cleaner and file the deposits off and it runs like a champ.

Its good that you cleaned the throttle body.

Re: Erratic idle
 

When you adjusted the idle, I'm assuming you probably mean that you turned the torx screw to get the right idle speed. Have you checked the TPS voltage? When you adjust the manual screw, it can change the TPS voltage at idle, so you might want to try looking into that.

Re: Erratic idle
 

I pulled the plugs, and they all look good and clean. After adjusting the idle (with the torx screw) the TPS is reading .56 volts at idle and 4.22 at WOT.

Re: Erratic idle
 

Is the SES or check engine light coming on?

I would pull the ECM fuse for awhile or disconnect the battery and relearn the ECM before going more in depth.

Its a little difficult to diagnose without a scanner to see what the sensors are telling the ECM. Coolant temp, throttle setting (TPS) and feed input to the ECM and it controls the idle speed.

Check for a vacuum leak or a plugged PCV valve ...

Re: Erratic idle
 

:iagree: Disconnecting the battery to reset the ecm is a good idea. Sounds like your TPS is reading correctly. Did you have the pintle on the IAC retracted into the housing all the way when you installed it? They can get damaged sometimes if the pintle bottoms out in the TB before the IAC is tightened down all the way. After resetting the ecm, try taking it for about a 10 mile drive to see if the ecm will re-learn the IAC settings. Like Lucid said, you're kinda shootin in the dark without a scanner. One other thing you could try is taking it for a short drive with the MAP sensor unplugged and see if it still hunts at idle.

Re: Erratic idle
 

I checked the PCV and I checked for vacuum leaks. The IAC pintle was retracted before I installed it. Also, when stopping hard the idle drops way down and feels like it is going to stall. I have this problem with carburated engines, but never with efi.

Re: Erratic idle
 

When the car is running does the check engine light flicker really fast? i had a problem with two of my camaros when the ECM went out. The ECM would flicker repeatedly very faintly... not the usual way it comes on and pops off, i mean it would flicker like 30 times per second.

You might want to check the wiring to the TPS, and the connector for shorts or opens. This is where a scanner would come in handy. Also make sure you check the TPS with a volt/ohmmeter to ensure that there is a smooth transition from idle to WOT. If you have a dead spot in your TPS, that would make the ECM hunt. (I would start here first, the wiring and the connector, then the TPS)

Is your car a MAF or speed density setup? If its a MAF, check the MAF power relay and clean the terminals and put dielectric grease on the connector. I would also recommend spraying the MAF sensing element with TV tuner cleaner (also called Electronics Cleaner) specially made for cleaning delicate electronics without leaving residue.

Its got to be something simple.

Re: Erratic idle
 

The check engine light works the way it should, no flickering. I will check the TPS and the wiring. This is a speed density engine. I think I am going to have to bring the car to a shop so they can hook up a scanner, unless I can find a place that rents scanners.

Re: Erratic idle
 

I'm getting the same thing on my newly installed Speed Density Set-up, I haven't driven it yet but my engine seems to be trying to seek a idle position.

How long does it take for the ecm to learn the settings?

Re: Erratic idle
 

Did you resolve this issue? I have a similar problem with my 92 350 GTA and I'd be interested in what you found out.

Thank you,
Harry

Re: Erratic idle
 

It ended up being two bad injectors, even though they had the correct ohm readings. To check for bad injectors warm the engine up, disconnect the IAC, and then unplug the injectors one at a time and take notice of how it affects the idle. The idle should get a bit rougher, if it doesn't you most likely have a bad injector.