Kickdown problme.

[carman850]

Hi everybody,

Just bought a 92 400E with 200K km, and found this wonderful website.

This is my first Mercedes, I love it very much, it handles so well at highway speed, I feel sooner or later I will get a speeding ticket

However I am experiencing some strange problems when I kick down at highway speed. It happens when I want to experience its acceleration at highway speed, at 120kmph, if I floor it, it downshifts as expected but as soon as rpm reaches 5000, I feel that engine bogs down and car actually slows down, if I keep gas pedal floored, it will get into a kind of oscillation: bog down and come back, car just shakes back and forth violently.

Has anybody experienced the same symptom? Does this just mean air filters, fuel filter or plugs need to be replaced? or some serious power train problems ?

Thanks.

Hi Carman And welcome to the forum I am new here also .It seems like a pretty god place for gathering info.Unfortunetly I dont have any experience with the car that you have .But the first thing that came to my mind was I wonder If there is a govener on your car ? I would guess not but you never Know .Hopefully someone will jump in here soon and help you with your problem.

[David C Klasse]

I don't think this is governor related.
But your rev-limiter might be acting up. There have been a few people with W124s who have had that problem lately.
Does it only do this in 3rd gear? Does it only rev to 5k in 1st and second gear also? And is it exactly at 5k or whatever? Or is the "bogging" down gradual? (IE, starts at 5k, and goes to 5500 when it's really bogging). And when it hits that 5k mark, is it a pattern "bogging" down? DOes it let it rev and then cut, rev then cut? I am curious because this may help determine if it is your reve limiter.

But I don't think it sounds like a serious powertrain issue or anything like that.

Good luck and welcome!!

[carman850]

Thank you both for your advices.

The Pattern was rev and cut, then rev and cut continuously, this only happened when I was at a speed of about 120-130 kmph, the rpm was about 5000, maybe a little higher, I tried kickdown at night, so I didn't see clearly what the rpm was, but definitely it was very close to the redline.

I have also tried kickdown at low speed, at 30-40 kmph, I could slip rear tires and didn't see any problems.

I really hope this is caused by the rev limiter, but why does the limiter kick in so violently, and doesn't allow the transmission to upshift and maintain acceleration?

[speace]

My 92 400E does the same thing. I can back off on the throttle a little and the oscillation will stop. I can then continue to accellerate once I settle it out. Yes, it is near redline, yes the tranny is kicked down...

My shop doesn't see the problem because they refuse to drive the car that hard when testing. I can understand their policy.

I had always thought it might be a fuel delivery problem. That the pump(s) just can't deliver enough.
My 500SL can sometimes do it too... The fuel injection systems on them are completely different...
...and that's the funny thing... they don't always do it...

I do hope that someone on this forum knows exactly what it is.

[JoeR]

Really sounds like a fuel filter problem from your description. Under the condition you describe, that is the highest demand for fuel volume (high load & rpm).

[psfred]

The rev limiter in the fuel computer shuts the fuel off until the rpm drops, then turns it back on. This will shake the car pretty hard.

Lift your foot and let the tranny upshift -- if the problem goes away, it's the rev limiter.

You might want to change the fuel filters, though -- lack of fuel will cause a missfire, and that will cause the computer to shut the fuel off to the offending cylinders for a predetermined time, then attempt to deliver again. This will also cause surging.

You could also drive slower -- I doubt it's legal in north America to go as fast as you would be in third at 5000 rpm.....

Peter

[speace]

hmm... Okay, Okay!!! I guess it downshifted to second...
Doing the math at 5,000 in 3rd boggles the mind!

It goes away once it upshifts. I think you are right that the fuel shuts down. I guess it is a "feature" to protect the car from ME!