No power above 4000 rpm? - PeachParts Mercedes-Benz Forum

Legoinsaniac · #1 03-06-2010, 12:43 PM

My '92 400E w/ 220,000 miles has developed a problem over the last couple months. Normal driving is unchanged. But if I get to about 4000 rpm, acceleration diminishes and the car just won't go any faster. Once the transmission shifts to the next gear, and gets below that 4000 rpm, the car will accelerate again. It's like the car starts going up an imaginary steep hill once it hits those higher rpms. Any thoughts? I've noticed a reduction in gas mileage from about 22 combined to about 18 combined over the same time period. I would imagine it's related.

The engine isn't missing and there are no unusual sounds.

Thanks in advance!

mpolli · #2 03-06-2010, 01:59 PM

You might try searching. I think I have seen that symptom before. Also investigate if you can make a code reader for that car. I don't know. There are probably a few things that can make that symptom. I wouldn't want to guess since I would probably guess wrong.

400Eric · #3 03-07-2010, 06:13 AM

I'm a big believer in being sure a part needs to be replaced before I replace it. So check before you replace. Having said that, this smells (pun intended) like a bad cat or two to me.
Regards, Eric

Legoinsaniac · #4 03-07-2010, 08:18 AM

Thanks for the input guys. I made a code reader last week, and will finally have a chance to use it today. I'll post the results.

It'd be nice if this was just a clogged cat. The car does seem remarkably quiet for having just straight pipe aft of those two cats...

mpolli · #5 03-07-2010, 01:55 PM

Well, now that people are making blind guesses, I will guess it could be the MAF sensor, AKA AMM. But that is just a guess.

Legoinsaniac · #6 03-07-2010, 04:55 PM

I read the codes, and found the following:

Pin 17
#8- Transmission overload protection switch does not close

Pin 8
#9- LH-SFI control module voltage supply, open circuit

Pin 7
#5- stop lamp switch
#6- starter lockout / back-up lamp switch
#14- Closed throttle position switch
#3- this one references numerous things, including:

-reference pot.
-actual value pot.
-safety contact switch
-closed throttle pos. switch
-actuator motor
-magnetic clutch

I erased all the codes and will read them again in a few days. I imagine some of these may be leftover from before I replaced the wiring harness, years ago.

I'll be looking more into what specifically these codes could be pointing to. Any information regarding these codes or similar experiences would be greatly appreciated!

el sea · #7 03-07-2010, 09:45 PM

I am betting on bad fuel!

El Sea,

Suckin Sludge & Havin a Gas

400Eric · #8 03-08-2010, 03:30 AM

Remember, bad cats won't show up on the code reader. But you already knew that. I'm just saying it for the benefit of the readers of this thread who don't.

Straight pipes after the cats? So I'm not the only one. Where did you get the idea to do that?
Regards, Eric

ps2cho · #9 03-08-2010, 12:10 PM

Clogged fuel filter?

Legoinsaniac · #10 03-09-2010, 05:35 PM

Today I read the codes again, after having put more than 60 miles on the car since the last time, and now: No codes at all! So that narrows down what the potential problems could be. Thanks everyone for the suggestions.

400Eric's suggestion of a clogged cat is high on my list right now. They're original, and the car does seem to be exceptionally quiet. It would explain the lack of power as exhaust flow increases (or tries to increase).

This issue has been ongoing over several tanks of gas and several months, so I doubt it's a bad gas issue.

A clogged fuel filter is possible, as ps2cho suggested, although I replaced this (as well as cleaned out the filter in the tank) about 20k miles ago. I think the interval is 60k? Since it's cheap and easy I might just do it anyways.

I'll also clean the MAF sensor with some electrical contact stuff.

I'm going to take a look at a couple spark plugs as well. But I replaced these not so long ago.

ps2cho · #11 03-09-2010, 09:01 PM

Fuel filter should be replaced every 30k. Start with that and go from there...as you said its cheap enough.

400Eric · #12 03-10-2010, 05:40 AM

Straight pipes after the cats? So I'm not the only one who has done that. Where did you get the idea to do that?
Regards, Eric

Legoinsaniac · #13 03-10-2010, 04:35 PM

400Eric,

Such a nice engine deserves to sound good as well, right? I guess that was my thinking. I was half planning to put a muffler on it further down the road, but I just love the way it sounds.

If my cats are clogged, I'll probably hack those off and have a shop weld up a nice exhaust and a magnaflow or something. I luckily live in a state without emissions testing. I don't think I'd be mad if I went this route and it wasn't my problem!

400Eric · #14 03-11-2010, 01:45 AM

Yes, Mine sounds good as well but it gets kinda "droney" below 2500 RPM. Above that it sounds great. Approaching 3,000 RPM and above, it sounds downright beautiful. It just purrs and hums. It's also fun to listen to how the automatic retarding of the intake cams as the r's increase affects the sound as the overlap decreases.
Regards, Eric

garymand · #15 04-01-2010, 08:32 PM

My cat broke loose inside at 180K 560SEL, and ended up rotated and mostly blocked the housing. Low end was fine but I couldn't accelerate up a hill and almost got back ended by a semi as it got pretty bad. All you can do is drop the exhaust and look. Mine was close to the intake and visible.

Gary