Car experiences surges as high as 1400 rpm, then will drop down to 500 rpm. Runs fine under power. No mods to speak of. Have replaced O2 sensor, IAC, TPS, coolant temp sensor and had TB rebuilt. Blades in TB are NOT hanging. Timing is fine, new injectors, too. Seems to be a sensor or computer driven problem, like something clicks and it revs high, clicks again and it drops back down. If I let it sit and idle for a while (few minutes), it will eventually land at about 700 rpm and pretty much stay there. Drive it though, and you never know what you'll get if you have to stop. Seems to chuggle when it's rainy more than if it's dry.

What on earth could it be? Any ideas?

 

Anyone willing to even hazard a guess? I've had it in to the shop several times; one time I was told I needed a new EGR, but another mechanic says no, EGR works fine. I've occasionally thrown a code 32 (EGR), but usually only when hot in the summer. Turn the car off and the check engine light goes off. That made me wonder; is it the EGR or the EGR temp switch? I read the switch is the most common reason for a code 32.

 

when you replaced the IAC and TPS did you go through the proper procedure?

 

Stock PROM chip? The idle speed on My '85 (with very little of it OEM, but still using the '85 ECU and harness) is very sensitive to ignition advance - the idle speed will fluctuate/hunt/surge if the advance isn't just "right". I've "flattened" the advance in the PROM chip from the bottom of the table up to about 1200 rpm to try and help, which it does, but changes in the baseline (distributor) advance will still change the idle characteristics quite a bit.

It's probably a MAF table thing - small change in rpm = more or less airflow which the MAF sees then the Chip tries to adjust fuel flow for which = more change in rpm, etc... Playing with the bottom few lines in the MAF tables might take some of the overcorrection out of the process and damp down the problem. Maybe. Someday I'll play with it and see...

Another thing that made a big difference was the worn out (lot of play in the shaft bushings) throttle body. Swapping on a new BBK TB made a big improvement in Consistency, at least - with that it does the Same thing Every time it comes back to idle. Worn out or "sloppy" throttle shaft bushings are a pretty common deal if the thing has any mileage on it at all.

The only EGR code you can possibly see (in the '85) is for the temp switch. That's your code 32. Usually it's a toss up weather it's because of a stuck/sticky EGR Valve (or the control valve/solenoid - the more likely failure) or just the temp switch itself dying. Replace the switch first - it's the cheaper part. Actually - first try just disconnecting the valve (pull and plug the vacuum line at the valve itself) and see what happens - won't bother anything except Maybe the emissions tester. If the code still comes up then it's a temp switch problem. Assuming, of course, that the valve is actually Closing at all. That can be checked with a hand vacuum pump and the tachometer...

 

A simple test to run, is to test the resistence in your fuel injectors. Solvent type fuel injector cleaners will desolve the coating on the coil wires in the injectors. This will cause the coil to short and change the flow. If your injectors are old, it would be a good idea to test them. Just pull the connectors and you should read 16 ohms across the two terminals. ( L98 Engine ) Hope this helps.

Mickey

 

Dave, if you figure out what it is please let me know -- my '85 is doing the exact same thing.

 

Vacume leak?

 

Make sure your have a correct (ac delco) 195 degree themostat. Put a 160 degree in mine years ago and had the same problem.Thought it was a coolant sensor. Turned out to be the themostat. Good Luck. Let us know what happens. Save the Wave.>George