The car is a '91 300E 2.6 with 150k miles. Recently had the fuel filter clog up and had to change it. Afterwards, she ran better than ever to be honest. A couple of weeks ago, however the car developed a serious stumble, hesitation and misfire. I thought it might be the fuel filter again since that last one was so clogged, so I changed it again. No better. The car starts with little to no excess cranking every time. There is however no power. Low idle speed, it’s barely 500rpms. When you go to accelerate it will almost stall, jerk and shift hard with sputtering and bucking. Coming to a stop the car will try to stall out. Little to no power.

The check engine light had come on when the original fuel filter failed and was giving me code 17 for O2 sensor heater. Since the car had been running ok after the filter was changed I had taken my time getting to change it. After the car started acting up, it also threw a Code 22. Thinking this may now be the cause, I replaced the sensor. Had to go with the Mustang sensor route because no one had the right one here on the island. Even the dealer was over a week out and at $300. After installing it the check engine light did not go out and the codes remained. Car also ran the same. Went back and resoldered the connections and reversed the white leads. No check engine light, no codes, but car runs the same.

Pulled the plugs. They were fouled with carbon. Cleaned and reinstalled. Still the same. Checked the wires at night and saw no arching. Checked the lead from the coil, blue spark up to 1/4 inch. Replaced the distributor cap, it was toast. Car is still the same. Went ahead and put in new plugs, wires, cap and rotor too, but no better.

Replaced hose to idle control valve because of a leak there. I have a looked for other vacuum leaks, but not found any so far. I did pull the idle control valve and sprayed it with cleaner. This actually gave me the most improvement so far, but only marginally. The car is still not drivable. A few days earlier I had to take it a few blocks to get gas and it was pretty scary. The check engine light came on while driving again, with a code 17. Cleared it and it hasn't returned, but I haven't pushed it either.

Went back to the O2 sensor and checked it following Ps2cho’s guide. I got correct readings for the heater wires, but when I probed the sensor wire, things didn’t look right. I got .45v before starting the engine, but once started; voltage ran slowly up to .8 and stayed there. No drop, only a little fluctuation of a couple hundredths. So I figured that the new sensor was shot and ordered the correct Bosch one. Just installed it, and no difference, and getting the same readings on the dmm as before. Runs up to .80 and holds there.

So, has anything come up that point to a specific direction to follow up on? The one thing that sticks out for me is that the car starts with just a turn of the key. No excess cranking even if it stalls out. When it first starts up, and you put it in gear, it starts to move as if nothing is wrong. But almost immediately it starts to bog down and jerk into the next gear. After that it consistently stumbles on take off and stutters at high RPMs. Seems to be running very rich. The plugs were carbon fouled and the o2 sensor I had first replaced was completely sooted up after only running while I have been woring on it. Parts are starting to add up and each round takes a week or more to get here. I’ve been chasing this for over a month now. Any help will be greatly appreciated.

Just took the car out with the dmm hooked up to the sensor wire. Still went up to .85 with fluctuations of .05 plus or minus. Still no real drop. I don't know if this means anything or not; but with the key off completly, it still reads voltage, but slowly drops. Anyone got a clue yet?

I believe your car has a ECU unit which affects most of the issues you are having. From description "Controls the fuel pump, RPM control, idle, A/C compressor, oxygen sensor, heater and onboard diagnostics". Just a thought.

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Good luck.






1998 E320 Wagon

What I was thinking as well....

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1991 300E - 212K and rising fast...

Thanks for the responces. Any way to verify this without just swapping the ecu. There's no boneyards around here and used parts are hard to come by.

Try to find another coil and swap it. I had the same problem on a '87 300E.
Anziani

90-up uses a different ECU than the earlier W124. I have a spare ECU that will fit your car, that you can try if you pay for shipping. I believe I have a spare coil I can include as well for testing. PM me.

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1991 300E - 212K and rising fast...

Thanks JohnM, check you inbox.

You need to get the repair CD. You are probably looking at the fuel distributor having problems $$$. The o-ring in the plunger goes bad and starts dropping raw fuel into the intake. When the computer can no longer compensate, you get the erratic O2 sensor readings that you are seeing. When all of this happens, plugs carbon quickly and O2 sensor gets coated very quickly The service CD will show how to test the KE jetronic components using a multimeter so that you don't go throwing parts at the problem. Probably NOT your ecu! Your key symptom is the plugs fouling quickly.

Have the CDs, started reading. Looks like I have a lot to learn. I have taken some readings already. Don't really know what they mean but will post them when I get home from work tonight.

500rpm from a cold start with stuttering and stalling sounds as though there is no cold start enrichment. A failed OVP relay gave those symptoms on our Euro 300E (1992 with cat).

Finally got some progress. Replaced the fuel distributor with a used one I sourced. It made a whole lot of difference, thanks rickhall. It still runs rough, but it runs. I will work on it later this week and post the readings from the x11 port. My multimeter doesn't do duty cycles but originally the reading was steady at 6.9v (iirc) once at temp with no change at 2500 rpms. Now a quick reading before my son took the car to work showed it starting at 6.9 and slowly dropping to .9 v at idle. Spiked back to 6.9 v as rpms dropped back to idle. I need to read a lot more of the manual, but that sounds like a lot better response than what I was getting.

Still need to pull the plugs and clean them up again too. That may smooth it out a bit as well. Thanks for the help so far.

I think I have solved the rich running problem and wanted to bring this thread to an end for anyone looking for a solution. Checked duty cycle and it indicated 20% pointing to a faulty throttle position switch. Took readings and it did seem buggy. Found one on ebay for less than $20 and that seems to have brought things to a point that is at least drive-able. I know there's a second vacuum leak at the idle control valve now that I messed in that area. Most of all just want to thank those that have helped on this issue. I have to admit in the past when I read about duty cycles and Lamba stuff I was very intimidated. Still am, but it has gotten me this far.