I am new to this forum and I hope I've posted in the proper section.

My wife has a 1992 190E with a 2.6 engine.

It runs great, but occasionally it will, all of a sudden and for no apparent reason, briefly run rough and stall.

When this happens it will NOT start again. When you attempt to start it, it makes a "WOOF" sound, sort of a muffled backfire. It seems to come from the drivers side of the engine compartment.

If it is left for an hour or two it will again start and run fine. . .until it does it again!

She does very little driving since she is handicapped and does not work; maybe once every other day, less than 10 miles per trip. Most days it does not misbehave.

I've replaced the plugs, plug wires, and switched to another brand of premium gas. I was formerly using unleaded 87 octane. I also added a can of octane booster.

The mechanic I've been using for years claims that unless he can test the car when it is misbehaving there is no way he can diagnose the problem.

Do any of you have any ideas?

 

Sorry about your problem. My wife had a 87 190E with virtually the identical problem. It was like it had a brain fart and would suddenly stop on its own. We ended up trading the car in and even MB wasn't able to find the solution. We used only Premium 91 Octane. After 15 years the car only had 100,000km (62,000mi) which were mostly short city trips.
Maybe someone else will be able to help.

 

Here's where things with this problem, for anyone who is interested.

My mechanic, who is also a personal friend, has been working on European cars for more than 30 years. He has suspected from the beginning that the problem has been caused by vapor lock, brought on by high summer heat and the fact that my wife has been fueling the car with regular 87 octane unleaded. A few days ago we syphoned the fuel tank and filled up with high test. We also added a bottle of additive which is supposed to increase octane. While test driving the car on a 94 degree day, it failed after about 10 minutes of driving.

We both noticed loud belt squealing, and when we popped the hood saw that the belt was loose. Very loose. The belt tensioner had failed. Also, I noticed that when I had replaced the radiator six months ago I had failed to replace the cowling that went around the fan. It was in the trunk. My friend surmised that these factors had caused a heat buildup, which brought about the vapor lock which he still believed was the cause of my problem.

The new tensioner is on order and is supposed to be here today. When we get it all put together properly, including the fan cowling, we'll see if his theory is correct.

I'll post again when we do.

 

Hope your theory pans out. We owned the car from new, only used Shell V-Power 91 octane, never let the tank get below half full, never replaced the radiator, or even the belts and tensioner during its lifetime with us.
I must admit I don't recall if the problem only happened in the summer although we typically did NOT drive the car much in winter (no snows) so it sat covered in snow for 15 winters. It was serviced regularly only at MB. We could never get it to fail on command like you can. It probably happened less than 10 times in 15 years but still made my wife uneasy, with good reason.
GL

 

rotating burned valve from low octane maybe. runs rough when its lined up on hole. fuel pump relay throw a used one in there from a salvage yard

 

After removing the radiator, fan, and belt tensioner as well as all associated hardware, we discovered that the tensioner was defective. No local parts stores had it in stock, and we had to wait an extra day when the store we used ordered the wrong part.

The part came in about noon yesterday, an overcast, muggy and hot day here in Northeast Ohio. We set to work in my driveway. It was sweaty labor.

We finished this morning. Around 10:00AM we tightened the last bolt and filled the radiator with fresh coolent.

We hit the ignition and. . .nothing! It cranked and it woofed, just like it did all those times when it stalled and would not start.

We checked the coil wire and saw spark; excellent spark.

We used starting fluid thinking that if the problem was no fuel it would start with the either. No luck there.

My mechanic buddy then checked the spark at a plug. There was spark, but it was very very weak. He concluded from that the problem was a defective ignition rotor.

I ran up to autozone where I bought a rotor. After installing it the car turned right over and the engine purred contentedly.

Think we were very fortunate that our intermittant problem showed up when it did, allowing by mechanic find the answer.

Hopefully, that's the end of this problem.

 

Thanks for the update. Hope it continues to work.
I located some notes from my '87 and found that MB had replaced the fuel pump and fuel relay which seemed to fix the problem but I had a further note that indicated it did happen once more (in 2 years) before we traded the car.
GL