All --

I have an 85 no mods. When I first crank up the engine, the car goes through some crazy 300+- rpm jumps and drops. Once I hit closed loop mode everything is all good. It really smells rich when I'm in open loop.

That said, I have changed all of the vacuum hoses aside from those connecting to the plenum directly (the elbows) and those under the plenum.

I haven't checked the EGR, but would that be a good place to start? I'm not 100% on how the EGR works yet.

Also, I don't have my FSM handy at the moment to troubleshoot.

Thanks guys!

And I did do some searching, but I perhaps didn't hit the right keywords up, so if this has been covered I apologize.

 

Is the problem there to the same extent on a cold and on a hot startup??

 

 

My car did that for awhile. What caused it was the 18 year old dysfunctional EVAP system which was nothin' more than a useless smog control creating a vacuum leak.

 

Hmmmm, thinking about it, I would say it's happening irregardless. It just doesn't last as long during hot startups. It definitely lasts longs during cold startups however.

 

Same problem here...driving me crazy!!!

 

The ECM doesn't take kindly to vacuum leaks when cold and in open loop mode. When my problem was a leak from the charcoal canister and misc. vacuum lines, my idle used to surge up and down + - 400 rpms right from a cold start. Would take about 10 minutes and it would go away. Once warm and in closed loop mode, the ECM can conpensate a lot better, therefore less of an idle surge, practically none in my case, about last year.

 

Once an idle oscillation develops, its difficult for the ecm to stop it.

The IAC will slowly close to reduce the integrative error and may eventually settle it down over time, but this seems to take forever and the car may stall during the process.

Check the basics first. Correct fuel pressure, spark advance, tps voltage, min air adjustment, vacuum leaks, condition of plug wires etc.

A small disturbance is all it takes for the rpm to drop a few rpm.

When this occurs, the load goes up increasing the fueling, at the same time the spark advance is going down. The engine is getting more air per rev so the torque is going up causing the rpm to begin to increase again and the pattern reverses load begins to drop, spark advance increases, engine gets less air per rev torque drops....

The IAC generally moves too slowly to force the oscillation, spark and fuel are changing quickly under these conditions.

I believe the intake also becomes resonant at certain rpms which can contribute to this effect. For example, my SR seems to resonate at 1000 rpm and simply doesn't like to idle at 1000 rpm. 1100 is perfect, 900 is perfect 1000 is trouble.

I suspect your somehow operating in either an unstable region of the spark table or encountering intake resonance by idling at an incorrect rpm, so I would suggest to bring things into proper adjustment first.

Then look for things that may contribute to the initial rpm disturbance that starts the process, maybe a missfire.

 

great description of my problem....I've checked all of the above, and have found nothing, so the resonance thing may have some definate possibilities...

 

I have been chasing the same kind of Idle problem since I bought my car. My car has 80,000 miles on it know. Could it be that a sloppy timing chain could cause this? I would really like to know what the experts think about this. When I put a timing light on it the timing is not stable.

 

Mine did the same thing when I had a significant vac leak. Use a neg pressure guage and see what you have.

 

First off, I commiserate and kinda feel better (sorry!!) that I have the same issue as so many others.

Tequilaboy, GREAT write up! I'll poke around those items and see what I come up with.

kopbet89c4, thank you too. Recently a vacuum line got caught up in my headlight cause it got loose There's a really good chance that it cracked one of the lines going to the cannister. Thanks for that suggestion; can't believe it didn't cross my mind!

 

I haven't verified where it's at yet or if this will resolve my idle issue (though I know it'll help out GREATLY), but the other day I was driving home and I happened to turn off the radio when I heard this very obnoxious high pitch whistle. It was pouring down rain so I decided to leave it be for the night. Well, today I heard it again and when I stopped, I immediately checked for a vacuum line that may have popped off.

Well, didn't see any, but I could definitely hear a vacuum leak coming from very close to the EGR or the vacuum lines running right around it.

I figured for this weekend, I'll just pop the plenum and replace all of the vacuum hoses. It's cheap enough and they are 22 years old. But before I do all that work, I'll double check the EGR to see if it's holding vacuum.

Guess my original intuition might not be too far off!

Then again, since my headlights did rip the vacuum hose off the throttle body, I could have just cracked a vacuum line leading off the TB. We'll see!