Stumbling Idle Problem
 

Here's the issue. A couple weeks ago driving while my car was warming up the idle started to stumble at a stop light. It would lug from 500RPM up to 1200RPM, back and forth like it was going to stall. So, I kept the engine revved up around 1500RPM at the stop and by the next light everything was fine.
A couple days later it did the same thing except this time the computer cut in and held the idle speed to 1500RPM. The RPM would drop to about 850RPM only when the car was at a complete stop in neutral. This high idle was a pain because the car would not creep smoothly in rush hour traffic. I read on another post of a similar problem and the solution was to disconnect the battery, so I tried that and it returned the car to normal idle speed and everything.

To the point, now the idle is unstable ALL the time, bouncing from near stall to 1100RPM and always fluctuating. Initially it was only when the car was warming up, and also it seemed worse in the rain, but now it occurs when hot cold anything, anytime it's idling.

Does anyone know what is causing this? It's a '99 with the 1.8L.
If the check engine light comes on while this occurs then goes out does that mean a fault code will be logged?

My thoughts are that a sensor is sending poor quality information but I'm not sure which one. Is there anything I should check right away to narrow things down a bit?

Any help would be appreciated. Thanks.

I know EXACTLY what it is...checkyour vacuum lines....one of them near the intake is leaking or disconnected... I had the exact same prob on mine today looked under the hood and bam!...vacuum line disconnected....hooked it back ub and everything is hunky dory

in addition to what macdaddyslomo said, you could also have a roken intake manifold gasket. the 99'-00' 1.8L motors have a tendency to do this and depending on how severe this is, you could have an idling problem. usually the code it sets is for a lean mixture due to the extra air padd the intake manifold. to check it, take some carb cleaner and spray around the intake manifold where it meets the head, if the vehicle stalls out or the rpms raise alot (depends on where the car is idling) you have a vacumn leak.

Thanks for your help. I checked out the vacuum lines and everything seems to be in the right place. Also I don't here any hissing so I'm not sure it's a vacuum leak. I tried the cigarette smoke test, a vacuum leak should suck cig smoke to the leak, but nothing looked out of the ordinary.

My car finally triggered the check engine light so I took it in to get it checked. The said it was the EGR valve. It was suck open thus causing a poor air fuel ratio, and messing up the idle. My mechanic just cleaned it to free it up and the problem is gone.

However, now the idle does not drop to 750RPM until the car is at a complete stop. Is this the normal operation? When I coast in neutral the RPM's will drop to 1500RPM and stay there until I catch another gear or stop. I have not had the car that long so I'm not sure if what I felt was ever correct.

UPDATE: I disconnected the battery to see if it would stop the high idle issue, and it did. I don't know if it's an OBD II thing or what but it seems the computer gets locked in a different mode when any of the sensors go off line, but then it doesn't go back to normal when everything is fixed and working properly.

Anyway, it seems to be running fine now. Apparently the EGR valves are crap, so if your car starts to idle poorly check the EGR. It usually doesn't need replacing, just a good cleaning.