Hi

I have a 1986 560SEL that has the following symptoms:

1) Upon start, warm or cold, it runs for a few seconds like it is missing on one or two cylinders, then smooths right out. This is only about one or two seconds (maybe three). But it is rough.
2) At idle (about 900 RPM), warm or cold, it has a very slight surging (perhaps 850 to 950) that is noticable only by ear with the hood open.

I've had the following "professional" opinions thus far:

1) a vacuum leak
2) a dirty fuel filter
3) unadjusted idle "something or other"

All comments are greatly appreciated
Thanks

Well yes, it could be a vacuum leak. One should always look for obvious ones. The minute a mechanic tries to sell you new manifold gaskets though (a reasonably big job), tell them to jump in the lake until you've exhausted every other possible answer.

On your care there are several items that affect this. Also note that the idle surging and the startup issue may be two different issues.

1) Cold Start Valve. This is a ~$100 part that is easy to change.

2) idle control valve may be dirty or faulty. Bought aftermarket (VDO), I don't remember if FastLane here has it but it should be your first shot if replacing it, or autopartsauthority.com for $240. Easier to change than oil. Cost $750 from MB though so must be bought aftermarket. It is cleaned-out with carborator cleaner, search on the forum here for any tips on that.

3) Yes indeed look for all oil leaks

4) The fuel filter may indeed be dirty and require replacement but I doubt that all of these problems are caused by the filter. The oscillating idle is probably not caused by fuel delivery from the pump/filter but instead is related to control of the fuel injection system. I assume performance is good anyway?

5) So you should look for vacuum leaks in all places, including around the fuel injectors. I have some past posts on that (click on my ID) and also search for vacuum leaks, finding, etc. Be very careful if you use anything combustible. Never ever spray anything on the manifold you could burn-up the car. At the same time, using this method I found a large vacuum leak left by an inept mechanic after work done to solve some other issue.

Thanks - cleaning the Idle Control Valve seems to be an easy (and worthwhile) task - I will start there (after Santa leaves me some carb cleaner under the tree ... in fire-proof wrapping, of course )

Note my reference to looking for oil leaks was a typo, you should look for vacuum leaks, oil leaks won't relate to your problem. Vacuum leaks can be everywhere, from around the injector seals, under the fuel distributor, hoses, etc.

In addition, you could be losing fuel pressure when you shut down (I.E. the pump one-way valve or accumulator could be leaking). Start by running a premium grade fuel injector cleaner in your gas (something like Chevron Techron, 20oz). Also unplug your cold-start valve before startup; if your cold-start miss goes away, it could indicate a vacuum leak, possibly the auxiliary air lines.

Surging idle could be vacuum leak, defective k-jet componant (O2 sensor, lambda valve, etc), faulty throttle switch or sticking idle air valve, or faulty idle control unit. You can short out the terminals on the throttle switch to see if your idle changes; when the appropriate terminals are crossed, it causes your car to idle down.

__________________
Don't ask me, I'm a shop-owner by default

Hey, thanks - this will keep me busy for a day or so, tinkering, but then I'll have to wait until next week when I can also buy some parts.

__________________
-----------
1986 560SEL (just sold)
1990 300E (sold)
1987 190E (sold)
Off to China!