Mattman928

I'm having a problem with the engine surging at idle on my 1990 GT. If you listen to it, at idle you will hear what seems like a cyclical dip in the RPMs. Occasionally, the engine will stall.

Any thoughts on what I can check or test???

Larry

I'd check these in this order:

1. LH brain - have it tested at any of the Big 3 or just swap it with someone whose car is running fine (Some on this list have suggested that a bad unit go into a good car rather than vice versa to avoid the chance of something in the bad car frying the good unit.)

2. MAF

3. False air/vacuum leak

You can find out more on procedures for checking each of these by doing an archive search. Also, you might check the "new visitor" post at the top of this forum for helpful websites.

Good luck and let us know what you find.

Randy V

Larry's pointed you in the right direction. I'd focus on vacuum leaks first as they are a common culprit and often (relatively) simple to correct.

An easy way to check for vacuum leaks is with a cheap spraycan of brake cleaner. With the engine running, spray the cleaner around all vacuum lines, concentrating on the brake booster and the vacuum-controlled fuel management components.

When a vacuum leak is encountered, the brake cleaner fluid is sucked into the engine and combusted, resulting in an increase or decrease in engine idle, depending on the solvent blend of the brake cleaner you are using.

Jim bailey - 928 International

All the 1985 and up 928s have an idle stabilizer valve which is controlled by the LH comuter . while the 85-86 has an idle speed adjustment screw the 1987 and newer relies completely on the LH brain to tell the stabilizer valve to maintain the correct idle speed . I would start with the idle stabilizer valve which has internal moving parts odds are it is sticking responding too slowly and the car stalls . Some have reported they shot lubricant /cleaner into the intake and the valve worked better . DO NOT SIMPLY lift the aircleaner and spray or you will probably DESTROY the hot wire mass air sensor that would be bad. You can use the breather hose on the rightside cam cover to introduce what ever you want to try below the hot wire but above the idle stabilizer valve . Do this with the engine running at idle so intake air and your cleaner are being pulled through the stabilizer valve . Carb cleaner / WD 40 are possibilities note that it will be burning in the engine and may cause interesting smoking too much will flood the engine .

Nicole

If the engine stalls, I would agree with the guys that there is a problem with the idle regulator valve. However, why is your idle dipping in the first place? You call it a cyclical dip. Does this mean it is regularly, every few seconds, or every few rpms?

Does your car get driven regularly, or does it sometimes sit for more than a week or two?

If you follow this board, you might have read one of my earlier posts. I had my car pretty much completely refurbished, and while it never ever stalled, the idle still kept dropping randomly. Then a compression test came out with significant differences between the cylinders.

Running intensive valve cleaner, and then performing a Rislone treatment with an oil change fixed the problem. But the car had already gotten a new idle regulator valve.

If you end up having to replace the valve, make sure that the following parts are replaced while everything is apart: knock sensors and throttle position switch. My "tps" was diagnosed as faulty just a few months after the other two had been replaced...

Mattman928

Hey everybody! It turns out it was the ICV (Idle control valve). As per the recommendations, I sprayed WD-40 down the vent hose (downstream of the heat sensor wire) and voila, it worked. The eninge purrrs, and the surging is gone.

Thanks everybody!