Bad sensor?

Maintenance/Repairs
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1

Hi all. Here’s my elusive problem. I’ll try to be brief. 93 Benz 400e. 140K miles. Problem happening for the last 40K miles. Issue is extremely intermittent hesitation/stuttering. Totally at random, it will start to miss badly, bog down, try to stall, etc. Heavy throttle does not help - if I feather it, I can coax it to my destination. Next time I start it, the problem is usually gone. While in this condition, as I feather the throttle, it will sometimes fire properly, the car lunges, then bogs down again. This lunging/hesitating could happen several times per second, then bog again for a couple seconds, etc. Makes for a super jerky ride. If I hold it in a low gear to get the revs up, it usually improves some. Seems like ignition trouble to me, but it will go months without displaying this behavior, so the local MB repair shop (very reputable) can never duplicate the problem. Usually happens during warm and humid weather, but not always. I changed the plugs (not the wires - they look fine), which SEEMED to help a little, but not much. Air filters are good, I replaced the oxygen sensor, but no improvement. This car has no check engine light, and one can’t pull codes off it. It’s not bad gas, because it happens maybe 2-8 times per year. 99% of the time it runs fantastic. Can’t pin this down, and all I seem to get from people I explain this to are dumb looks. The shop said maybe the traction control is getting confused and cutting the engine, but to me it acts more like 1 or more cylinders just stop firing. Someone else said maybe fuel pressure, but the problem is very infrequent and has been going on for years - most of the time it runs great. Could this be a bad sensor? Maybe mass air or throttle position or something? Is vapor lock a thing of the past? I’m stumped. Thanks for any suggestions.

2

Your problem sounds interesting, but frankly I could not read all the way though that block of type. A few paragraph breaks where appropriate could help.

I suspect you will get more help if you can reformat your message, maybe using paragraphs or bullet points etc.

Good Luck

3

Any chance it’s your Over Voltage Protection Relay? Also called Overload Protection Relay. When these go bad, they act up randomly. It basically drops out your electric power from the alternator temporarily. The dealer should be very familiar with these.

Another part that goes bad on MB’s is the EHA valve (Electro-Hydro Actuator or something like that) that mounts to the fuel distributor. It also may be know as a fuel governor.

I’m no expert, but I have a 1989 MB and have replaced both these parts. I bought both parts off the internet, Ebay I think.

4

Maybe this might be a bit easier:

Hi all. Here’s my elusive problem. I’ll try to be brief. 93 Benz 400e. 140K miles. Problem happening for the last 40K miles. Issue is extremely intermittent hesitation/stuttering.

  • Totally at random, it will start to miss badly, bog down, try to stall, etc. Heavy throttle does not help - if I feather it, I can coax it to my destination. Next time I start it, the problem is usually gone. While in this condition, as I feather the throttle, it will sometimes fire properly, the car lunges, then bogs down again. This lunging/hesitating could happen several times per second, then bog again for a couple seconds, etc.

  • If I hold it in a low gear to get the revs up, it usually improves some. Seems like ignition trouble to me, but it will go months without displaying this behavior.

  • The local MB repair shop (very reputable) can never duplicate the problem. Usually happens during warm and humid weather, but not always. Air filters are fine.

  • I replaced the spark plugs and the oxygen sensor, but no improvement.

  • This car has no check engine light, and one can’t pull codes off it. It’s not bad gas, because it happens maybe 2-8 times per year. 99% of the time it runs fantastic.

  • The shop said maybe the traction control is getting confused and cutting the engine, but to me it acts more like 1 or more cylinders just stop firing. Someone else suggested maybe fuel pressure, but the problem is very infrequent and has been going on for years - most of the time it runs great.

  • Could this be a bad sensor? Maybe mass air or throttle position or something? Is vapor lock a thing of the past? I’m stumped. Thanks for any suggestions.

5

Thanks for the suggestion. I’ll look into each of these possibilities.