Hi: My Mercedes 94 E420 has a bad engine miss upon acceleration and then it runs bad. I first thought it was a transmisson problem. I had a diagnostic check run. It said

Removed both caps and checked. Ok and rotors okay. Checked codes, none in engine. Checked fuel pressures 3.7 bar. Checked fuel out of injector rail, perfectly clean. Getting price of inj. module and m16 throttle actuator. could be either one at this point. module easier to replace and remove if not the problem.

The cost of the first guess will be $1,100. If not the problem second guess is $2,450.

Advice would be appreciated.

Wiring harness been done?

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John Shellenberg
1998 C230 "Black Betty" 240K

http://img31.exs.cx/img31/4050/tophat6.gif

Is there a check engine light on? Whenever these have a ignition fault it shuts down the injector for that cylinder. I would really get a technician to look at this problem instead of guessing at units. It can be expensive throwing parts at cars especially since most electronic components are not returnable. It isnt hard for a technician to determine what cylinder is not firing. From there you either have no fuel,no fire ,or no compression. Pinpoint the cylinder and then backtrack the problem using basic diagnosis skills.

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euro 287
Mercedes Technician 7 Years (retired to Hyundai)
2000 Dodge Durango
98 Mazda truck

What do you mean about the wiring harness? Thanks

The engine wiring harness and cars with throttle actuators have harness' that fall apart.

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euro 287
Mercedes Technician 7 Years (retired to Hyundai)
2000 Dodge Durango
98 Mazda truck

Thanks Euro. The diagnostic check was done by the only Mercedes dealer in Des Moines, Iowa. Should they have known to check what you said? It just seems to me that they are guessing with these two parts. He said he would try the $1,100 part because it would be easier to replace if it didn't fix the problem. Maybe I should take my business elsewhere?

Much appreciated.

Had the same problem with my Toyota recently and after checking just about everything it ended up being bad spark plug wires. The wires can be checked easily by measuring the resistance.

Personally, I would have a second opinion. Why start replacing (the easiest ) first .Personally I like sure diagnosis, know the answers upfront and not guess at stuff like this. It is your option but common sense says something wrong here.

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euro 287
Mercedes Technician 7 Years (retired to Hyundai)
2000 Dodge Durango
98 Mazda truck

That particular model (all 92-95 W124 and W202 cars) have wiring harness problems that can cause all kinds of wonky symptoms, including bad engine miss. Often, as the engine rocks slightly under load, the harness is moved and the problem occurs.

The insulation on the harness fails, and the wires become exposed and frayed. As they short, various problems occur. It can be inspected visually, and the dealer should have done it, but some do not perhaps hoping not to admit that this is a pattern failure on these cars.

I'm not saying this is what the problem is, but check it out before throwing parts at it. Also, the dealer is "guessing" that one of those components is the problem? That's NOT the right way to do it. They need to FIND the problem first. Eliminate the easy stuff (visual stuff like the wiring harness) and then really get into the problem.

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John Shellenberg
1998 C230 "Black Betty" 240K

http://img31.exs.cx/img31/4050/tophat6.gif

I will check out the harness and spark plug wires first! BTW, the check engine light doesn't come on.

This board is great. Thanks for your advice.

I find it is cheaper to spend the money to diagnose the issue properly. Drive that Chevy 4x4 till the mercedes issue is properly diagnosed. There has to be a better benz mechanic in Des Moines right. - let him diganose then if it is a warranty related issue let the dealer do the work. Throwing parts at the problem is ridiculous - get a second opinion or make the dealer only charge you for parts and labor that fix the issue properly. Best of luck and keep us updated.

Where is the wiring harness? Is it under the passenger floor mats? As you can tell, I'm not very mechanically inclined, but was going to check for corroded, loose wires first. Thanks again.

Under the passenger floor, there is a yellow wire, not corroded, and not connected to anything. Is it supposed to be? Thanks

From what I've come to learn on this website, I thought the wiring harnesses were bad only on the M104 engine from 92-95.

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Paul S.

2001 E430, Bourdeaux Red, Oyster interior.
79,200 miles.

1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".