Mercedes 320E running roughly and stalling - Martin
I bought a 1993 320E just over 2 weeks ago. Occasionally the engine idles roughly -- the normal idle is very smooth but at times there is a noticeable shudder, and when it does shudder I've known the engine speed fluctuate as well (cold idle's about 1000, warm is about 600, it can hunt over that sort of range at maybe 1 cycle/second). A number of times it has actually stalled, which it being an auto 'box you'd not expect to happen.

If it does stall, it won't start unless the accelerator is pressed (it doesn't need much). It needs the gas even if it's stalled when warm, so I don't think that's the catalyst protection that's been discussed here. It seems a lot happier starting from cold if you give it a bit of gas too.

This doesn't only occur when the engine's idling but also when the accelerator's being pressed, whether you're already moving or pulling away from rest. The observed symptom when the car's moving is of it being jerked backwards and forwards, a bit like what you'd get from rapidly altering the position of the accelerator, only it's quite a bit sharper. Rev counter stays steady in this situation. Sometimes bringing the accelerator up or down a bit sorts it out.

Has anybody else experienced similar symptoms, particularly with this model?

I've not yet managed to get the car to misbehave on demand, but circumstances in which I've known it to happen include coming off the accelerator to let the car slow down by about 10mph (usually from about 70 down to 60) then going back on to maintain the lower speed: when you go back on the accelerator it starts jerking. Also slowing the car down to walking pace to manoeuvre into the office car park, then coming to a halt, is a good way to get it to act up.

I don't think it's engine temperature-related ... it'll happen whether I've just started the car or if I've driven 150 miles. I have a gut feeling that it might have something to do with the outside temperature: it seems to be more prone to misbehave the colder it is.

I've been running the car on regular unleaded (usually Shell but the occasional Esso and one Total) and have had the same symptoms with all the tanks. If it was a problem with the fuel itself I'd expect to be seeing the symptoms much more frequently. It's set up to run on 95-octane ... again I'd have expected more regular complaining if it wanted 98.

A possibly related observation: when the engine's cold there is a noticeable and not particularly pleasant growling/whining noise when it's revved much above 2000rpm, and it feels a bit reluctant to change up through the gears. The whining isn't the gearbox: I've confirmed that by gently revving the engine with the 'box in Park. This noise goes away quite suddenly once the engine's been running for a minute or two.

Any ideas would be much appreciated! Many thanks.
Re: Mercedes 320E running roughly and stalling - Cliff Pope
I've had the symptoms on a different car, but I believe lots of cars have a similar system, and that is that the solenoid idle speed valve gets sticky. I noticed too that it depends on temperature. If your car has one (possibly an alloy casting about the shape of a Brasso tin, near the inlet manifold), try cleaning it out in petrol and checking with test wires that it snaps open and shut.
Hope this is relevant and of some use.
Mercedes 320E running roughly and stalling - David Lacey
HT leads perhaps?
Re: Mercedes 320E running roughly and stalling - David Moore
New leads and plugs cured the Polo
Re: Mercedes 320E running roughly and stalling - Andrew Moorey (Tune-Up Ltd.)
Had a similar fault on a '94 280, turned out to be a faulty oxygen sensor (lambda). Get it tested before shelling out on a replacement though!
 

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