89 300E runs rough at start-up
 

After sitting for awhile or over night, sometimes it will be so rough it will shutoff and I will have to restart and feather it for a minute or two to keep it going. Once going it only takes a mile or so and runs fine. Rotor was replaced, cap good, plugs good, decarbed MAF.
Thanks

What kind of bad running?
Is it a chugging sound? or low revs? or both?

A chugging (solved by revving) is a symptom of your big-ends being warn.

It sounds like it is not getting enough fuel, like running out of gas. Spitting and spudder I guess would be a more technical terms to descibe the sounds and actions. When it is warmed up the idle does fluctuate every now and then.

I have the same problem on my 88 TE (185k). At start up after sitting overnight it fires right up but runs as if is only firing on 5 cylinders then smoothes out after a few seconds. New plugs, wires, rotor, cap with no change. Treated with Techron with no change. I am thinking a fuel injector problem but would like to make a better diagnosis prior to replacing all the injectors. The coil is original, could a bad coil cause this type of problem?

Doing some front suspension work now (tie rods & struts), this start up situation is next on my list.

Any ideas would be welcome

Ed

Yes. A bad coil could lead to something like this I suppose.
But with that kinda mileage it does sound to me like both of you could have slightly warn big-ends.
The good news though, is that on a Merc its not necessarily too much of a problem. You'll still be going on for another 100,000 miles with it..

With warn big-ends, when you start the car you'll get a chugging as if the revs aren't high enough for a few seconds whilst everything gets chucked around the engine and sorts itself out.
If you gun the accelerator on start-up (not recommended), then the chugging goes quicker, but does more damage.

I have the same problem - rough idle at start up and then smooths out. In searching this forum the majority of responses to this problem are that the over voltage protection needs to be replaced. Evidently the original supplier of this part did not make a "quality" product and many problems can be traced to this part. It is fairly cheap to replace (around $50 Fastlane - I think) and is an easy DIY project.

Mail2neil,

Don't want to sound igorant here but what do you mean by "big ends"?

Thanks for your input by the way.

EJC

I found a fix on the Mercedes club website that I tried this past weekend. If you've tried other things (plugs, cap rotor, etc.) on a 300E and have not fixed a rough idle at cold start, you can trick the computer into making the mixture richer at cold start up (which smooths out the idle) by splicing a 470 ohm resister (5 for $1 at Radio Shack) in-line with the engine coolant temp sensor. I tried this and it really seemed to work for me. You can find the directions on the MBCA website. The wire you need plugs into the rear of the cylinder head on the driver side, it is a green wire with a red stripe (it is covered in a hard black plastic sheath for much of its length, but you can see the wire where it plugs in; on my car the plastic sheath was very brittle and broke apart, and the colors on the wire were faded).

Here is a link:

http://www.mbca.org/pages/tech/MBCA_Rough_starting.htm

He meant worn conneting rod bearings. The "big end" of the rod is on the crank, the small end to the wrist pin. It makes a fairly distinctive "rod knock" sound until the oil pressure is up. Wouldn't cause a rough idle, though.