Rough Idle 190E 2.6

Would really appreciate any help with the problem of the very rough idle on my 1991 - 190E 2.6, It does not do it all of the time. It starts the very
rough idle only at a stop light or as I should say a real low idle of 500 RPM or less. I can give it a little gas and the spitting and sputtering gets worse till it reaches about 800 - 1000RPM, and then its smooth as silk. As I say it only does this maybe once a week or so. I was told by the Mercedes Specialist to run some Marvels Mystery Oil in the gas tank, but it does not seem to help. They say it is a injector that is stopping up, not spraying the fuel right. I have done this with several fill ups. I have used 4 oz. of this per 10 gal. of fuel. Could it be water in the gas? I have also used other additives, Injector cleaners, Heet, Etc. Please Help. Ken
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Ken, join the club. I've had the identical problem for a while now and tried numerous things all to no avail so far. Suggestions for the cause have included: - the OVP relay. Not so. - ICV, Not so. - vacuum leaks. Not so, judging by spraying Butane over engine without a change in it's idling character. - incorrect idle RPM (ie it's 'supposed to be 7-800 rpm). Not so. A 500 rpm is perfectly normal when under load (D) if nice and steady. - rotor and distributor cap. the former was good and the latter was a bit chewed up at the points, but - give me a break - it's a kV spark jumping across there. - wires. Apparently not, moved them away from engine with no difference in performance.
- I noticed that maybe, just maybe, it gets a bit worse on foggy days and a bit better on cold and dry days.... could be a clue.
- My best guess is old age and a whole bunch of things are not perfect and add up to a 'sometimes' rough idle. In addition mine sometimes refuses to accelerate. If I ease of the pedal and apply it gently it then seems OK. Then almost like a switch 'clicking' in the car will drive perfectly after acting up in this manner.... go figure.
cheers and good luck, keep us posted re what you discover
guenter

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I'd suspect the MAS.
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I would check a couple of things.
1. Spark plugs to see if it is clean or you got some odd symptoms.
2. Resistance of each spark plug wires... They should all be below 1300 ohms.
3. Distributor cap and rotor... make sure they are clean, not cracked and if you see black oxidation, use sandpaper to clean them off both items.
4. Vacuum leaks. Check every connections... Clear hose to the rubber boots. Make sure they are tight and grabs. If not, trim the rubber boot back a bit more to new portions and reinsert clear tube. Test all connections by spraying carburator cleaner on it... if engine acts different, then you still got leak.
5. Spray carb cleaner on intake manifold to see if it is affecting engine.
6. Check EGR... be careful as it can be very hot around there...Do this when engine is cold. There should be a hole on the bottom where you can push up the internal diaphragm... Start engine... and push up the diaphragm... if engine stalls or runs very rough, it works.
What mileage do you have on the car now? Most people don't realize that timing chain need to be changed when you got high mileage... because it stretches and change the timing. You can try to compensate for this by changing your timing a little less... for example, if it is supposed to be 5 BTDC, you can go lower to 2 or 0. But other than this, you really need to change timing chain if engine has more than 150,000 miles. Most mechanic suggest you change at about 100 to 120K to be on safe side.
Timing chain also affect emission too.
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The car is showing about 85K, this is one reason I purchased the car, seemed like low milage, of course who knows because I just bought it last summer. I am sure it has been through several owners. The car just started this about 2 months ago. by the way the spark plugs were changed out about 4 months ago. Thanks so much Ken

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I do want to add that the motor surges at a slow pace up and down when this rough idle is going on. When at its slow idle is when this is going on, but when it surges to a little faster idle, its smoothes out, and then starts going down to a slower rough idle. and this is just a repetitious mode for a few minutes. and sometimes the engine dies. And then for some odd reason, it starts running like a dream. I hope I am explaning this were you all can understand this and I am not sounding too nuts. Thanks Ken

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Just wanted all to know, the car finally died and I could not keep it running. come to find out it was the distributor cap and rotor. both in bad shape. Replaced both and it runs like a dream.

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Outstanding... you figured it out. So what was the date stamped on that distributor cap? Did you ever change it?
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Really, I was not the one who figured it out, because I had to get it towed to the garage. I ask all along, if the Dist. cap and rotor could be the problem, and the garage said a MB cap and rotor are hardly ever the problem, they said they are built so tough. Like you said Tiger, the cap was a mess of moisture, looked like a film of oil, and carbon. I'm sure it was shorting out. Tiger, I noticed one of your suggestions was to check this, guess I waited too late, before it got me stranded on the highway. Thanks for all who responded. Ken

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