Isnt the temp sensor test you did giving you the clues. It is likely to be just a fuel mixture adjustment that is off. If it were the air sensor pot, it would not have worked well when you attached the resistor. And if the coolant temp sensor resistance changes properly with engine temp, and is close to 2.5-3.5K at 20 deg-C, then just enrichen the mixture a bit and most likely you would be fine.

But the fuel mixture is set correctly - in fact ever so slightly rich.
Richening the car up would only get around the problem, not solve it.

This afternoon I tested the air flow sensor potentiometer. It checked out - 0.4V at the specified (near rest) position, nice and smooth up to 5V throughout motion. Also removed ECU and checked continuity for wires to EHA, WOT/idle switch, and air intake temp sensor. I'll check the coolant temp sensor when cold but think it's correct.

So I am 90% certain everything electronic is working correctly.

0.4V is that what you get at idle ? That may be a little low. Can you adjust it to be 0.7V ? Did you remove the sensor at any time ? If you did then, you can adjust the rotation of the sensor a little by loosening the mounting screws to give you about 0.7V at idle.

0.2V to 0.5V is the spec for the 16v engine, according to the US Mercedes-Benz 102.983 service manual.

As it happens on this machine here I've got Bosch's all-informative ESI system which lists this for the 2.5-16 potentiometer adjustment (see attachment). I checked and it shows the same data for the '87 260E.

Bosch's information a bit more circumspect- the base position depends on the supply voltage. My supply voltage was 4.7V so my measured voltage should be approx 0.5-0.8V.

However, it's not quite that simple. These instructions do NOT state anything about moving the sensor plate down to a certain position to perform the measurement. Whereas the Mercedes service manual says to line up the top of the metal plate with the bottom of the shiny cylindrical part of the intake hole and take the measurement from there.

So now I'm confused.....

For information, this is what Bosch ESI says for the position of the air flow sensor (see attachment). No idea how to interpret it though!!

I haven't removed the sensor. I would like to try adjusting the potentiometer perhaps, so long as it won't mess everything up! Is that how you adjust it - just loosen the screw? I can't even see how to get to it, on a 16v it's hard.

O.K., so the range is different for your car. No dont remove the sensor, it seems to be working fine.

I just saw the specs you put, yes the range does depend slightly on the incoming voltage, so 0.4V is on the low side even borderline off, it would not hurt to atleast make it 0.65, but do it as they say, using the trim pot screw on it.

The trim pot screw may be hidden behind some black material, they dont want it to be easily exposed and hence moved. The adjustment by slightly loosening the mounting screw is a very coarse method, only do it if the trim pot does not work for some reason.

The pot seems fine, and close enough in setting I wouldn't bother trying to adjust. The problem is elsewhere.

Steve

I agree, the dynamics of the pot are o.k., however the baseline is 40% off, which in my opinion is substantial. And I agree there is likely some other problem also. Even if the temp sensor comes out o.k., check that its signal is reaching the ECU and ICU, check continuity.

Yep I've tested continuity to coolant temp sensor :/

Today we're testing the ignition timing. My Dad reckons it is off when he had a quick look with a timing light yesterday. However we're being hampered in testing this because the area with the TDC markings on is damaged - half the readings are missing :o. The stud (for some sensor I can't remember which) is there but the TDC marking missing, it looks like it's shattered away!

How could the ignition timing be off, and what could cause that which I could investigate in the meantime?

Did you replace your fuel filter?

Ignition timing was on spec.

No I haven't replaced fuel filter. My Dad reckoned the fuel filter would be unlikely to cause this sort of problem (makes sense... the problem would most likely be there the whole time, or be worse at higher revs/fuel flow). Maybe I should replace it but I'm averse to randomly throwing money at a car to attempt to fix it.....:grim:

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