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Old 08-25-2007, 04:25 PM
Dead Man Driving
 
Join Date: Aug 2007
Location: Newbury, UK
Posts: 6
Another rough-idling W126 - '89 500SE

Apologies for being a newbie with an olbie problem, especially one that's been discussed many times previously. I have read many of the relevant posts, and have been taking away as much of the wisdom as I can.

I picked up this car a few months ago, and it's become clear that it's been worked on by gorillas in the past. The more I search, the more tacky and badly executed work I find. I've fixed many things already, each of which has improved the operation of the car. But so far the weird idling - high at about 1,000 rpm in N or P, and going [bomph] every few seconds - hasn't responded to my threats or labour. I'm trying to avoid the FD replacement till I'm sure, though.

My question is peripherally related to this: I'd like to know the actual detailed operation of the idling mixture adjustment screw, which in my case appears to have been tweaked waay out of position.

As I have no manuals on the car yet, the following is a slightly fanciful diagram I put together of the mass-flow device as far as I can work it out. The details of how the arm and plate actually change the potentiometer probably don't matter yet, but I'd like to know what the screw actually does.



From what I've read and seen, it sounds as though the screw actually contacts the arm, and limits its upwards travel, as shown. Is this the case?

That doesn't happen in this car. With the engine off and the plate & arm as far upward as it can go, the screw is still at least 5mm from touching the arm. If I push down on the screw against its spring, I can just get it to touch the arm.

Is this a red herring? Is the actual screw operation not related to holding the arm down? Does it control a jet or something?

Furthermore, the bolt in the centre of the plate, at the top of the arm's travel, is about 10mm from the rubber buffer on the crossbar. I assume that they wouldn't normally touch, but the distance seems excessive.

I guess my dream scenario would be to get a longitudinal cross-section drawing of one of these devices, but a description would do just fine.

Any info most gratefully appreciated.

CD
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Old 08-26-2007, 05:03 PM
jdc1244's Avatar
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Join Date: Apr 2003
Location: Lakeland, FL
Posts: 170
Are you having a problem with high idle or rough idle? I assume both can be present at the same time but often itís one or the other. Unfortunately Iíve had experience with both.

You seem to be referring to a high idle so Iíll address that.

Yes, the idle adjustment screw does contact the sensor plate lever but it does not have any contact with the fuel distributor; the FD is effected by the position of the lever which indicates the amount of air entering the engine. The adjustment screw is more for Ďfine tuningí than addressing a major high idle problem - which would explain why your adjustments of the screw have not solved the high idle.

I suspect you have an air leak. The idle control valve may have failed and is allowing excessive air into the system resulting in the high idle.

The best scenario would be you have access to a good functional ICV from a 116/117 to test in your engine. Failing that a 24-hour soak of the valve in brake cleaner may free the mechanism. My ICV eventually had to be replaced which solved my high idle.
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Old 08-28-2007, 04:04 PM
Dead Man Driving
 
Join Date: Aug 2007
Location: Newbury, UK
Posts: 6
Talking

Quote:
Originally Posted by jdc1244 View Post
Are you having a problem with high idle or rough idle? I assume both can be present at the same time but often itís one or the other. Unfortunately Iíve had experience with both.

You seem to be referring to a high idle so Iíll address that.
Yes, the idling was both high and rough. On no load the car would idle at about 1200rpm. And with a miss every few seconds at most.

But I fixed it! Turns out the throttle switch - which I hadn't even known existed until I saw a photo in another thread - was not closing when the butterfly was completely shut. I expect that the coupling slot is just worn, because I whacked it, hoping to realign it slightly, and it started functioning again! Finally the engine was aware of a no-throttle situation, and the ICV and idle jet (which I'd previously cleaned and checked) started to operate, and the idle dropped to 600rpm.

But the roughness and missing was still very much present, and in the course of fiddling with the distributor I noticed faint signs of arcing around the cyl 2 contact inside the distributor cap. Checking showed that the #2 spark plug cap was open circuit, so I jerry-rigged a replacement. Perfect, smooth, clean 600rpm idle, and oodles of power!

Everything is massively better, but I still have this weird situation with the idle mixture control. I'm very grateful for the information you supply:

Quote:
Yes, the idle adjustment screw does contact the sensor plate lever but it does not have any contact with the fuel distributor; the FD is effected by the position of the lever which indicates the amount of air entering the engine. The adjustment screw is more for Ďfine tuningí than addressing a major high idle problem - which would explain why your adjustments of the screw have not solved the high idle.
As far as I can see, even with the engine off, the idle screw is a long way from contacting the plate lever. This is why I was wondering if the screw did its work some other way, and it's really useful to be put back on the right track.

I suspect the screw itself may be busted off somehow, and I'll cross-check it against the 420SE I have here and see if I can make sense of it all.

All things considered, though, I'm really happy to have this engine delivering the right stuff at the right revs. I think it's a useful lesson for others with rough idling and/or running: don't forget to check the HT cables and caps! Those kilovolts will try very hard not to go where they should, and if they get loose they'll burn things! New HT leads all round after a few 100kmiles could pay for themselves in fuel very quickly.

Many thanks for the invaluable info.

CD
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