I have a problem with a 1992 Mercedes 300E. The car has done 43000 miles, and runs very smoothly, but will not achieve more than 4500RPM. Even with it in PARK, I can only manage to get just over 5000 RPM. When you bury the throttle peddle ( with the car in PARK), it quickly revs to about 2500RPM, then the revs take a little longer to 3500RPM, then really struggle to get up to the 5000RPM. When Driving, kicking it down often slows the car down, as you are calling for more revs than the engine will easily deliver. Also when you set off, with the throttle half pressed, it changes up with a jerk, obviously as the gearbox is expecting much more power at that throttle position.
I have tried and tested the following to try to correct it.
1. Checked the air filter, which looked clean, and tried revving the car with the air filter removed. No noticeable change
2. Replaced the fuel filter, with no noticeable change
3. Changed the spark plugs. Managed to get from 4500RPM to about 4750RPM
4. Checked the cylinder compression - all OK.
5. Checked the timing (dealers checked on computer, advances OK to about 40 degrees BTDC)
6. Checked timing chain to make sure it had not jumped a tooth or stretched - Chain installed correctly
7. Check fuel pump delivery flows and pressures with the engine OFF, and are as follows. Open ended flow (no restriction), gives 2400 litters per minute. 5 bar pressure gives a flow of 1860 liters per minute, 7 bar pressure gives a flow of 1620 liters per minute, 9 bar pressure gives a flow of 1140 liters per minute, 11 bar pressure gives a flow of 600 liters per minute. These according to MB dealer are all OK.
8. Tested the fuel supply pressure with the engine running to make sure the fuel regulator was working OK. The delivery pressure held at 5.5 bar regardless of REVS. Suggests it is OK.
9. Tested the air flow sensor, and found the resistance to be none linear. Around the middle of the range, it did not really go up, but jumped around, then at the bottom end of the range went linear again. Tried a resistance box onto the sensor plug, to emulate the sensor output resistance, this made no noticeable change to the engine revs. When the plug was removed from the resistance box, or from the sensor when the engine was running, the engine revs changes for a fraction of a second, then recovered to normal.
10. Lastly, the only thing I could then think of was the catalyst was blocked, cut this out, and welded new pipe in place, and still no change (well very little about an extra 200-400 RPM). Even tried revving with no exhaust at all, with no change.
I have now run out of Ideas, has any one got any clues????
This should be quite straight forward for the dealers to solve as it is NOT an intermittent problem it is like this all the time, but the dealers solution to all the problems is throw enough money at it and they will solve the problem

Well you are running out of something. My first thought is available voltage. That is my first thought because my real first thought was the catalyst. and now that I am thinking twice I still think its your cats. I'll wager you cut out the cat under the car which isn't what plugs. What plugs is the two precats right off the exhaust manifolds; not under the car.

But if you had the whole exhaust off all the way to the manifolds then we go back to my first thought: that you are running out of available voltage. To cause your symptoms I would expect the coil or dist rotor finger. This can be checked on a secondary ignition scope, if you know what your doing. Pulling a plug lead should raise the necessaary voltage to 25-30kv. If the spark line doesn't rise above necessary idle voltage or a little more then either the coil available voltage isn't there or it has found a low voltage path; say through the rotor to ground.

Take Steve's advise. Check the pre-cats. I have seen many clog up.

Also, I have replaced quite a few ignition coils for this condition with good results.

I had precisely the same trouble with my '86 300E, and it WAS the Catalytic converter. (I didn't have to change the pre-cats at all.)

Good luck!

J

I had th whole exhaust off to the manifold, and tried reving the car. it made no difference the same 4500 ish RPM. Very loud though!!!

I will have the ignition checked. Are there any DIY tools available to check this, or do I have to take it to a garage, and does it have to be a merc garage??

I take it from this that you dont suspect the oxygen sensor, or air flow sensor??.
What about rev limiter, could that be causing the problem??
If it helps, the car is sometime hard to start when it is cold, needs cranking twice, and reving when it fires to get it going. The car is also quite economical, about 26 -28 mpg on a good run.

Thanks

Ian

If you have eliminated the exhaust system I would suspect that the timing has jumped. From what is described it sounds like the timing chain is off by one, possibly two teeth. Pull the cam cover and verify that the components are in time. I have seen the engines run like this with as many and three teeth out or time.

Mark

Could it be the throttle valve sticking in the throttle body?? Has this car ever backfired through the intake system?? I don't think this has a rev limiter in the strict sense of the words. (such as BMW etc.)

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Jeff Lawrence
1987 300e
1989 300e
2000 Dodge Grand Caravan SE

[This message has been edited by jeffsr (edited 07-25-2000).]