jpyles

The same thing has happened a few times this week:

At idle, the RPMs sometime will slowly drop from 900 to under 800.
Usually, the ECU kicks in and bumps the idle RPM to 1000 or so (but occasionally it doesn't kick in fast enough and the engine stalls).
Then, it idles fine for awhile, but eventually the process starts all over - idle speed drops, and it either recovers or stalls.

The temps have dropped to high 30's - 40's around here, but I don't know if that's a contributing factor or just coincidence. This behavior did not exist earlier in the fall. It happens whether the engine is cold or hot.

Today, I let it idle for about 5 minutes, in which time the idle speed dropped and then recovered about 3-4 times. Then, the idle speed started bouncing between 1000-1500 RPMs as if someone was goosing the throttle. I tried to drive away, but it wouldn't accelerate. I turned the engine off and back on, and everything was fine. That make me believe its an electrical/electronic problem to do with idle control and/or timing.

When driving, the engine pulls strong throughout the RPM range, no misses or hiccups at all.

Any ideas? Are there any mechanical controls for idle adjustment, or is it all electronic?

Could I have a sensor going bad?

Could it be bad gas? I'd think a bad tank of gas would also cause problems at higher RPMs too.

Thanks!

ron mcatee

It would help to know the year of the car.

jpyles

1988 3.2 with Autothority MAF and chip. No other mods.

old man neri

I would start with the easiest thing first....clean out your ICV. Have you checked for vacuum leaks?

ron mcatee

Remove and clean the Idle Control Valve.

jpyles

I have not checked anything yet. I found some instructions on idle adjustment, which seem easy enough to do. Here's a similar problem posted on pelican:

http://forums.pelicanparts.com/showthread.php?p=4133721

Are there instructions somewhere for removing / cleaning the ICV? I haven't found any yet.

rusnak

I agree with the above. On mine it turned out to be a vacuum leak at the back of the throttle body there is a very small hose going to the ambient air valve on the oil tank. On mine that was cracked. If you have vacuum leaks, chances are you have several. You need a small inspection mirror, strong flashlight, and a can of starting fluid, not wd-40 or anything else.

ron mcatee

jpyles, I tried copying the picture from my manual, but I couldn't get it to work. The Idle Control Valve is directly in front of the throttle body. On the left side it has an electrical plug. Top and bottom it has a clamp holding on 2 hoses. When removed, put it in some solvent to free up the mechanism inside. I always use a little air to blow dry it. Don't use full force of air, only a little to evaporate or move the solvent out. also as stated, check for vacuum leaks. Good luck.

theiceman

I flushed out mine on he Audi with WD40 ,. the stuff came out filthy black at first then cleaned up , I had realy eratic idle as the ICV struggled to compensate. After the clean worked great( 3 years now )

rusnak

jp, the Bentley manual has a great section on testing the ICV. I bought a new ICV for mine thinking that was the problem, only to find out it was some inexpensive vac hose.

I think you should buy the Bentley manual. It gives you a procedure for cleaning and checking the ICV valve.

KC911

Based upon personal experience, you've been given some excellent advice regarding cleaning (or replacing) your ICV first, and then checking for vacuum leaks around the throttle body. If you start with the idle adjustment first, all you'll potentially do is get you "baseline idle" out of whack (it didn't just suddenly change on you) and imo, you've got something else that's entered into the equation causing the fluctuations. Check on pelican, lots of threads regarding cleaning an ICV. I cleaned mine several years ago, it helped for a while, but I ended up replacing it (it was nearly 20 years old), and also tightened up the hose clamps around the throttle body just a bit, and my idle has been perfect ever since. Good luck!

Keith
'88 CE coupe

jpyles

Thanks for everyone's response on this thread. On the recommendation of most of you, I did some research on the ICV and finally got around to working on it this weekend. Here's what I did:

Warmed up the car with a 10-minute drive. Engine was idling decent, but RPMs were lower than I'd like, around 700-800. Still would occassionally drop and surge back.

Removed black cover on left side of engine bay and put in piece of wire to bridge B and C on test port. Idle increased slightly and seemed to smooth (no surging).

Used 7 MM socket to adjust idle screw. Brought idle to 900 RPMs. No surging, smooth idle. actually I could change idle from 1000-800 with the screw adjustment pretty easily. Does this take a vacuum leak off the table?

Removed bridge wire. Idle dropped from 900 to 800 and engine started to struggle to keep RPMs again.

Shut down engine, removed ICV. Checked condition of upper and lower hoses, didn't see any cracks. The ICV did not look too dirty on inside. I cleaned it out with some brake cleaner and let it dry.

Replaced ICV. Started car and same behavior exists. The idle is 900 when test port is bridged, otherwise around 800 and struggles to keep RPMs.

This points me to the ICV again. I did not check the operation of the valve because I couldn't find any directions for what to do, and I didn't want to damage it by sticking a screwdriver in there. Could it be electrical? I did not check volts/ohms on any of the wiring. My gas gauge has also started to act erratically, the needle bouncing up to full - in case that could have anything to do with it (but probably just a case of when it rains, it pours).

Thanks, Jason
1988 Carrera with Autothority MAF and chip

theiceman

May be time to go back to basics and check and clean all the grounds in the car. Still sounds like an ICV issue to me if your base CO adjustmwnt is correct.

rusnak

I agree with that. Always a good idea to check the obvious stuff first. I still say it sounds like a vac leak to me. You can plug the ICV back in (warm engine) and see if the valve is working by turning it toward you and looking inside the valve. If it is moving freely, then chances are it's ok.

At this point the diagnostic section of the Bentley manual is going to help you the most. You can also (carefully) try a can of starting fluid around the intake manifold gaskets and injectors to check for vac leaks.

Re-check for loose clamps or broken hoses, cracked hoses etc. You need a strong flashlight and a small mirror.

theiceman

..and some patience