86 Carrera ICV Questions
 

Hi Everyone,

I am a new owner of an 86 Carrera and a long-time owner of a 67 912. What a difference!

Since I took possession of the Carrera, it has suffered from the apparently common idle hunting issue, and I have researched the various possible causes. Checking the resistance in Ohms at the ICV seemed the easiest to check, so I did that.

I was supposed to get 20 Ohms from the center terminal to each outer terminal and 40 Ohms between the outer terminals.

What I got was 23.4 Ohms between the terminal closest to the front of the car and the center terminal, and 24.8 Ohms between the other outer terminal and the center terminal. Between the outer terminals, I got 47.3.

Now, please remember that this whole electronic realm, this Ohms thing, is a new experience for me.

My first question is this: Have I found the cause (or one cause) for my Carrera to hunt for the right idle speed? Another way to ask this is: When someone--the Bentley Manual, for example--says resistance should be 20 Ohms, do they mean 20 EXACTLY?

Even though I got the above "wrong answers" to my ICV test, I cleaned the thing with isopropyl alcohol as others have suggested. This did change things a little; now the distance between the high idle speed and the low idle speed is less, but it still hunts between 950 rpm or so and 1100 or so.


Thanks everyone!

Jon

Jon,

Unplug the Idle speed control and plug the air hose for good measure.

If this steadies the idle you will know for sure that the ICV causes the erratic idle.

Thanks Ned!

I did as you suggested. When I unplugged the electrical connector to the idle speed control valve, nothing changed. Undaunted, I unhooked the air hose, which of course made the engine die. I put it back on without the clamp, started the car, took off the air hose and thumbed the intake side of the idle speed control. The idle smoothed out immediately.

I think all of this is great. I can replace this gizmo myself. You 911 guys are probably used to that fan spinning after your chin whiskers!

Thanks again for your help.

Jon

Jon,
Do a search on cleaning the ICV. I would zap it good with carb cleaner or brake cleaner first to see if dirt/goop is affecting operation.

Ned,

I did do some searching about cleaning the ICV. When I shake the thing, the piston does rattle around in there. The travel is very short, but it also sounds like it's hitting the stops pretty firmly on both ends.

I can also see some oily gunk in there, or I could before I poured rubbing alcohol into the thing and shook it. I also twisted the thing back and forth, and this is where there may be, or may have been, some hanging up.

But wait: Does your suggestion to clean the thing out mean that the Ohms of resistance might be within acceptable, or working, parameters?

The only cleaning method I found on my search was to pour the alcohol in it and shake the thing. My plan for tomorrow was to order a new ICV and spray some brake cleaner in this old one, something with a bit more force behind it than just pouring alcohol in it, and maybe even to take the thing apart. I just love taking things apart! However, I was only going to do all of this cleaning and dismantling because I thought the Ohms readings meant the thing was no good, anyway.

I guess I'll clean a little more carefully . . . .

In the interests of full disclosure: When I picked up the car from the PO, he said: "It's at least two quarts low on oil, so put in a coupla quarts and see where you're at." You guessed it; about an eighth of an inch (or less) ABOVE full. The car has smoked like a train on a few occasions, mostly going up really steep hills.

Could the overfill have pulled oil into the idle control valve?

Thanks again,

Jon

If it is the original ISV it is probably ready for replacement anyway. Cleaning it with carb cleaner will sometimes get you a little more use but eventually the motor wears out.

That said, I suspect that your erratic idle is not caused by the ISV.

-Chris

Idle problems can be caused by many other things.
Cylinder head temp sensor is another part that goes as well.

Jon

Start with the easy stuff. Check your vacuum hoses. There are a couple that attach to the back of the trottle body and at the fuel pressure regulators.

I had a similar problem when I first got my '88 Carrera and I found one of these line disconnected. Fixed the idle hunting when I reconnected it.

Good Luck
Andrew

I had a flucuating idle on my '88 and someone suggested changing the O2 sensor which I did and the problem went away.

Jon: In my 84, I had an idle hunting problem. I replaced the ICV and O2 sensor, but the problem persisted. Then I had my mechanic adjust the mixture. Problem solved.

Sometimes the simplest solutions are the best.

Scott

Clean the AFM and the ISV with carb. cleaner.