97 Chevy Cavalier Cold Start Idle Problem

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I have a 97 Chevy Cavalier 2.4L manual trans. Sometimes when we start it the car idles rough and will stall if the gas is pressed. If left alone, the idle will go up and then all the way down until it almost stalls.

This seems to happen more when the engine is cold (in the morning) and happens when the weater is cold or hot.

No codes. My dad checked for vacuum leaks. We replaced the O2 sensor last year.

The mechanic replaced the spark plugs and said he fixed a leak in the fuel "rail" or something. It still happens. He said he never saw the behavior exibited.

If we can coax it to rev high RPM and slip the clutch it stays running.

Any ideas? Thanks in advance.

posted by  sdmcnitt

Well, based on the fact that you have done some work to the car and weeded out some of the possibilities, idling problems are very likely caused by the IAC system on the intake. I have written up similar postings on this that you can search for, BUT, on a high level the IAC system is the "idle air control" system. Some manufactures like Chrysler call it the AIS system (air injection). This system injects air into the intake to mix with the fuel to obtain the right proportions of air and fuel PARTICULARLY on cold starts. If this system is not working, you will have rough idles, possible backfires, and stalling. As well all know, for an explosion to occur, you must have ignition, fuel and a supply of Oxygen. This IAC system is one of the reasons that when you start a fuel injected engine for the first time when it is cool, it should run at an elevated RPM for a few seconds and then gradually slow down to normal idle around 750-800 R's. This is because the IAC system is injecting more air and naturally...speeding up the engine rpm's.

I would hook up my OBD II scanner to the car and watch the IAC system as I fired the engine to see if you have any activity. If she won't idle, there is a very good possibility that you have an issue with the IAC system. If that is the case, sometimes you can pull off the sensor/motor from the intake and clean it really well and it will work again. Othertimes you are screwed and need to replace it. Before you remove anything, take some really good intake cleaner (berkibile 2+2 works REALLY well) and run it all through the intake and into the ports on the intake that the TPS(throttle position) and IAC sensors are. See if that does anything. If not, pursue other venues, good luck.

The mechanic probably replaced the fuel pressure regulator on the fuel rail, those things will leak when they start degrading. I had to replace that on the fuel rail on my '96 Grand am. I think that cavalier you have has the same 2.4L engine with DOHC. Let us know what happens.

posted by  cmeseadoin

cmeseadoin: Thanks for some leads. My dad thought the same thing and explained too how the "idle air bypass" and other stuff worked(sorry but I am not good at remembering).

We looked at the procedure for testing the IAC in our aftermarket manual. I seem to remember that I needed some testing equipment that I did not have.

Your steps sound real good though. Will do and let you know. My dad has the OBD II hooked up so will try to look for some readings for the IAC. Lots of cleaner stuff too!

I only get to work on cars on weekends. Talk to you next week.

Thanks again. :thumbs:

posted by  sdmcnitt

Sounds good, make sure to come back with any other q's or results from what you find! :thumbs:

posted by  cmeseadoin

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