Problem just occurs today. I come to a stop light and it would idle up and down. Say 1500 then to 500 then up and down again, non stop. I does not settle into the 750rpm range. It sounds as if the computer is compensating for an air leak at the intake or something.

If I am trying to follow an air leak problem where would a leak normally occurs in the intake path? I am suspecious of airleak or some worn rubber hose because car overheated over last July/August and something must have melt in the engine. During that time I had a fanclutch that went and the engine temperature went up high for a few weeks until I had the clutch replaced. Now this is probably the result of that incident.

Hope someone can help me pin-point the problem.

Air (vacuum) leak most likely at the idle air intake hoses, or the various lines that connect to the intake. Inspect/replace them all. Even the rubber o-ring on the dipstick can leak.

A hunting idle can also be due to a work out air meter potentiometer. This is a pricey part and not readily available separate anymore. Try all the hoses and such first.

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What about sensor wires (to the computer). Can those melt out and short intermittent sensors? Would an engine overheat generate enough heat to melt wires? If so what tapes can be used to tape it up and isolate the wires again? I can only think of heat shrink tubing but the problem is how to slip those onto the wire.?

Doubtful. I believe your airflow potentiometer is the cause. Vacuum leaks won't cause idle fluctuations that extreme.

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Where is the airflow potentiometer located? I just now pulled off the air filter box and started the car for a good half hour to see whats going on. This means I can directly see the flap (butterfly) on the airflow meter. I was just pushing on the throttle body arm to up and down the RPM. It seems that the thing runs normal until the engine heats up around a half hour or so later. Then taht's when the idle searching begin and the rpm goes up and down.

One other thing that is absolute amiss is that there seems to be some minor exhaust leak coming up the engine compartment close to the firewall on the driver side. It smells like exhaust and not gasoline.

Also there are lots of black smoke out the tail pipe. I had the car parked in a new spot and have the engine running and the ground below where the muffler exhaust output is just scorch black where it was clean cement before. OK I will go grab a camera for a photo now.

Here it is, black smoke in the tail pipe.

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1987 260E 2.6L

I pulled out the plugs to check and they all look quite dirty (see photos). Anyway, I sprayed some WD40 on the tip to try clean them up a bit. However now the car wouldn't start up anymore. It will crank and crank and I can confirm that there is voltage going to the plugs, but nope, engine won't start up. Is it a coincident or is it because the WD40 has basically ruined the plugs? Are plugs designed in a way that you can not try to clean them up at all? I am going to get a new set but just wanted to start the car up.

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Holy cow, Throne, I don't understand why the car was running before you sprayed the plugs with WD40. I'm going out on a limb here and assuming that maybe the car hasn't had regular maintenance over its lifetime. So I think you're right to replace the plugs ASAP, and probably the rest of the ignition consumables (wires, cap, rotor, maybe even coil, air filter), before trying to figure out the idling issues. It appears to be running waaaay rich. Have you tried testing duty cycle?

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1988 California version 260E (W124)
Anthracite Grey/Palomino
Owned since new and still going strong and smooth
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Past Mercedes-Benz:
1986 190E Baby Benz
1967 230 Inherited from mom when she downsized
1959 220S Introduced me to the joys of keepin' 'em goin'

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Get new plugs, those are done for.
H9DCO from the dealer. $3/each.

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It looks to me like the valve seals were leaking, it was fixed, but the plugs were never replaced. Do you have any oil consumption or history for the car?

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OK, I got new plugs and the car started up. However when I step on the gas to rev up the rpm and then release teh pedal, the car just cuts off. Totally weird!