topdownchevy

Hi
My 92 Stanza has an idle question I have not found here.

111,000 miles, automatic tranmission. Tranny service about 20k ago. Timing belt at about 70k. New plugs, wires. Original owners.Typically runs great.

O.K. now that that is out of the way, here are the symptoms....

Car starts good. Idles good untill you put a load on it.
A/C on, in park, idle drops down to about 650 rpm, car shakes a bit. Put car in reverse, no change. Put car in drive and idle drops to around 500 rpm, then acts like it is being intermittantly starved for fuel. Engine sounds like it may stall, but does not. Rpm drops to about 250 then back up to 5-600, then back down again, etc...until A/C compressor cuts off. Then idle shoots to about 1500 rpm, then settles back down to around 800 rpm or so after about 10-15 seconds. As A/C kicks on and off, repeat symptoms.
Problem occurs mainly in drive. While driving around, can just barely detect an intermittant slight drop then increase in power, up & down, up & down, at about a 5 second interval. Vaccuum sits at around 20 inches at idle in neutral, 15 inches in gear with A/C OFF. When in gear, fluctuates wildly.

There is nothing even coming close to this in the Chiltons manual. That book reads like stereo instructions! Very hard to follow or look stuff up!

Thanks for any help!

PapaCap

So does the car idle more normally when the AC is completely turned off?

When the AC compressor kicks on, it sends a signal to the ECM to increase idle RPM to make up for the additional load on the motor. For some reason, it's causing your ECM to bonk out. Which could be related to a mechanical problem.

First, keep the AC off, drive it around, and let us know how the idle is.
Second, with the motor off, try spinning the AC compressor with your hand (the front plate). It should spin with little resistance. If the bearings are starting to freeze up, it could be putting a huge demand on your motor causing the erratic idle.
Third, have you replaced the vacuum hoses? A car this old, all the vacuum lines should be replaced. It costs about $5 to $10 to do almost all of them. Most people do the old 'if I see a crack, then it's bad' test, but the only way to be sure a line isn't leaking is to perform a leak down on each hose (yes, the ends of a hose can stretch out and not be cracked, still causing a poor seal). It's worth the time just to replace all of them and eliminate a vacuum leak.
Fourth, have you timed the motor? An ill-timed motor can causing idle surging.
Fifth, are the fan motors kicking on when you turn the AC on? If you run a jumper on the fan relays with the motor off, you can tell if a dying fan is causing the 'shakes.'

What's interesting is that your car shakes when you turn the AC on. I don't know what you mean by 'shakes,' as a bad torque convertor can cause this. But like I said, if the AC compressor bearings are beginning to freeze up, it can cause this as the belt has to spin the motor over harder to keep it running.

There's ya some homework for the week. Let us know what ya find out.

pc

topdownchevy

O.K.
All vacuum lines were replaced about 4 years / 10k miles ago along with all the injectors & connectors.

Engine shakes car when idle comes down too far. Motor / transmission mounts replaced about 5 years ago.

The engine rpm drop occurs with the A/C off also, but not as pronounced.

Upon a suggestion from my transmission shop yesterday, I cleaned the Air Idle Control Valve.

This has helped the engine run much better at idle and WOT.
He said he has customers come in with this same complaint a couple of times a week with Nissan and Toyota vehicles. 90% of the time just cleaning the Air Idle Control Valve does the trick.

Thanks for your help!
Nathan