EspritS4s

I've done a search through the archives for idle problems and have a list of things to check. However, I didn't find my problem described exactly, so I'm hoping that someone has some more insight before I start.

My '87 idles fine when the engine is cold. As the engine warms, the idle begins to fluctuate up and down centered on about 900 rpms. Once the car is at operating temperature, the idle surges whenever the car is idling. It begins by jumping up and down about 200 rpms. After a few cycles of this, it begins surging higher and dropping lower until it's bouncing from only a few hundred rpms up to about 1300 rpms. Then it seems to reset itself and start the cycle all over again.

Any ideas?

Thanks,
Gary

John Speake

Hi Gary,
The correct operation of the idle stabliser is quite tricky............could be many things.

The difference between hot and cold running may be something to do with the O2 loop, which is disabled when the engine is cold.

Is the fault consistant ?

EspritS4s

John,
The fault is very consistent and happens pretty much everytime the engine is warm. occaisonly the idle smooths out, but it doesn't last long.

Thanks,
Gary

John Speake

Hi Gary,
If you can put a meter on the O2 sensor, you should be able to see if the voltage is in the right range.

Also check the throttle closed switch is operating properly, measured at the ECU.

For the idle stabilser system to work it needs the mixture at idle to be correct. Also any airleaks onto the manifold will drive the idle correction loop out of it's alowable range, so it will then "hunt" trying to stabilise.

UKKid35

Sounds very similar to the idle problems I had. On removing the plenum I found that one of the vacuum lines from the throttle body was missing. I haven't put it all back together yet, but I'm confident that this was a major factor.

Randy V

While it could be O2 sensor or ECU-related, the prime suspect is a vacuum leak.

EspritS4s

Thanks guys. I was planning on starting on the assumption that it's a vacuum leak, I was just curious if anyone had run into this exact situation before.

-Gary

Randy V

Gary, your surging symptoms are quite common, and typically the result of one or more vacuum leaks.

Start at the brake booster check valve (verify the valve is operational and installed in the correct direction), and work your way out from there.

There's been some good write-ups with pictures on the vacuum system, but I can't recall right now where they are located. Perhaps someone else can provide you with a link.

Jim bailey - 928 International

That sounds much like a sticky idle stabilizer valve which no longer can open and close quickly enough to maintain a constant idle speed they are sometimes affected by heat . You might also try creating a vacuum leak to see how the engine reacts to that . Removing the oil filler cap allows air to enter the intake , see if the LH computer and idle stabilizer can compensate . You can also try spraying some WD40 or carb spray into the intake with the engine running BUT DO NOT do that above the mass air sensor . Either of the two breather hoses on the right side cam housing make a good place to spray in cleaner engine idling , into the hose . It may take a few shots to clean and lube the idle stabilizer IF IT IS GOING TO CLEAN UP . Things which move always wear the idle stabilizer has brushes (like all electric motors) and it opens and closes .

SteveG

Jim and John, I'm trying to fix rough cold idle. So thanks for the tips. I will try the WD40, but I understand it is temporary fix, so I'm looking at removing the rails, injectors and manifold. I've searched the archives for that process. The question is what to do while I'm there? Replace stabilizer, and vacuum hoses under there. Anything else?

Does anybody have numbers or page numbers for readings on the 02 sensor? I have the WSM, but I've spent hours looking for things that turn out not to be there.

Thanks.

John Speake

SteveG
The nominal voltage is around 0.9v. In typical use, the reading will "hunt" around that value +/-0.1V as the O2 loop corrects the mixture

aaron3

SteveG,

If your looking for things to do with the intake off see:

http://members.rennlist.com/pirtle/svc.html

There's a special section on intake manifold.

SteveG

Mark and John, thank you.