my car hesitates on acceleration and runs rough - Mercedes-Benz Forum

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post #1 of 7 (permalink) Old 07-22-2011, 10:57 AM Thread Starter
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my car hesitates on acceleration and runs rough

My car will run rough most of the time. I tried fuel additives and spark plug change out with no results.

But it does not do this all the time. When it does run rough, it does so at idle and when accelerating. I'll step on the pedal and it will sputter and then all of a sudden it will take off like a bat out of hell. Weird.

You would think if it were an electrical or fuel problem it would be consistent in that it would always do this. But every now and then the car will run fine, no hesitation or rough running, but again, that is only a small amount of time that the good operation occurs.

Has anyone ever had this problem?

This has been going on for 2 years now. Took it to a gas station mechanic but they could not find the problem. Other than the fact that its annoying it allows me to drive it. Only stalled out once during the whole time.

Oh yes, one other thing,driving at speeds in excess of 50 mph (on the highway) not noticeable. But in city driving you can feel it hesitating up to 40 mph.
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post #2 of 7 (permalink) Old 07-22-2011, 11:13 AM
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Could be a bunch of things, ranging from OVP or fuel pump relay to a clogged filter or a bad pump, leaking EHA, alternator undervolt. Who knows???

What has been diagnosed/replaced up to this point???
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post #3 of 7 (permalink) Old 07-22-2011, 03:08 PM
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Cap, rotor, plugs, wires or the coil? maybe the duty cycle?

I would do a search there is so much it could be you may want to check it all out.

Gotta do a tune up before you can even try and figure anything out.
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post #4 of 7 (permalink) Old 07-22-2011, 03:19 PM
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"Has anyone had this problem" Only a few people. Read ALL of this link. You'll be amazed at the content in the DIY section of this forum or what you can find out by using the search function.

Good Luck

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post #5 of 7 (permalink) Old 07-22-2011, 05:06 PM
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there are three fuel filters in the M104 (in the tank, near the fuel pump and near the engine) i don't know if the M103 has three as well??

check them all..

simple things first

good luck
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post #6 of 7 (permalink) Old 07-23-2011, 10:36 AM
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My 1994 e320 was doing the same thing two years ago, at about 100,000 miles. Uneven idle, hesitation and quirky acceleration on some days ... then run fine the next.

As the macdrone says, you gotta do a tune-up first! You replaced the plugs ... but did you replace the ignition coils? I replaced both plugs and coils and haven't had the problem since, and I now have 126,000 on it.
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post #7 of 7 (permalink) Old 07-23-2011, 01:47 PM
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You need to replace wear parts. Cap and rotor, plug wires maybe, O2 sensor etc.
Then check duty cycle as posted above link.
I would leave fuel filter for last.
Your EHA could be causing this as well, but do the above first. Or just remove the air filter assembly and look/smell the EHA. If you smell gas, could be the 2 small orings for EHA. Maybe even bad unit.


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