Potentiometer on Idle Control Unit

[daimlerdude]

A 380sl I've been restoring had a surging idle. So I had Programma rebuild the idle control unit, which cured the surging.

However, the car is now idling in gear at around 1000-1100 RPM's, rather than in the normal 600-700 range, and tweaking the potentiometer on the unit doesn't seem to do very much.

So I was wondering if anyone here could tell me what that potentiometer is supposed to do, if not somehow increase and decrease the idle.

Thanks very much in advance.

[rowdie]

What year? I do not understand the question about potentiometer. The ICU #006 545 85 32 (1984) is not adjustable AFAIK. The most common cause of high idle is vacuum leaks. The idle control valve if not closing properly can also contribute. Then there is the air/fuel mixture adjustment that I do not recommend adjusting without an exhaust gas analyzer.

[daimlerdude]

Originally Posted by rowdie

What year? I do not understand the question about potentiometer. The ICU #006 545 85 32 (1984) is not adjustable AFAIK.

The ICU on my 81 is 002 545 0132. It is unlike every other one I've seen in that it has a potentiometer on it, presumably to allow the idle to be adjusted. According to the people at Programma, MB made some of these with a pot, but stopped for some reason, maybe because they didn't want guys like us to adjust them?

Originally Posted by rowdie

The most common cause of high idle is vacuum leaks.

I sprayed brake cleaner on every vacuum tube and connection, but didn't notice any difference. I recognize that the best way to check for leaks is with a Mity-Vac, but I've never used one. Do you just connect it to each hose, pump it up, and see if the line holds the vac? Or is it more complicated than that?

Originally Posted by rowdie

The idle control valve if not closing properly can also contribute.

Cleaned that out with brake cleaner, and it seems to be working properly.

Originally Posted by rowdie

Then there is the air/fuel mixture adjustment that I do not recommend adjusting without an exhaust gas analyzer.

I did have to adjust this a while back to cure a slight misfire, and the car was running smoothly until recently. But I do suspect it may be running a little rich.

[rowdie]

Thanks for the explanation. Always something to learn on these.

I have heard a smoke generator is the best way to test for vacuum leaks. Shops use them but they are expensive. I built the one shown here DIY How-to: build a vacuum leak detection machine - Pennock's Fiero Forum using a 12v compressor. There is another one that uses a cigar but I found the cigar hard to keep lit and it did not generate enough smoke.
I connected it to the main vacuum line that comes off the rear of the intake plenum.

[abiby]

As Rowdie says, a vacuum leak is still a good possibility. I am of the opinion that replacing the vacuum hoses and especially fittings is probably easier and faster than finding a leak. Often the issue is numerous tiny leaks from small line cracks and hardening rubber on connectors.


Also, even after cleaning, the ICV may not be closing down as much as it should. They do have a limited lifespan.

You can try clamping the large hose going to the ICV closed, which should drop the idle down to the point of almost killing the engine. If it does, your ICV isn't operating properly. If it doesn't, it's probably the vacuum leak(s).

[daimlerdude]

Thanks for the replies. I just finished building a variation of the smoke generator discussed on the link that Rowdie provided. So I'm going to give that a shot, in addition to closing off the hose to the ICV.

If it appears to be the ICV, I may try adjusting it per the instructions Dave Petryk provided here:
http://www.peachparts.com/shopforum/sl-forum/243090-idle-speed-air-control-slide-valve-confidence-check-2.html

And I'll post my findings as I get them.

Thanks again for the help.

DD