Poor performance - PeachParts Mercedes-Benz Forum

wbrian63 · #1 07-02-2007, 11:27 AM

'92 300SE

Just replaced the fuel filter. New plugs, cap, rotor (obscene amout of $ for the previous two parts), wires.

Car runs like it's short on fuel at higher revs, like when attempting to pass at speed. Starts to "run out of steam" at about 4,500-5,000rpm. I would think the motor would pull strongly to at least 6,000 given it's redline of 6,500+.

Seems like a fuel issue - no misfire as far as I can tell, and when you back out of the throttle and let it shift, you feel the acceleration increase.

Could this be fuel-pump related? I know there are two, maybe one is on its last legs?

Any ideas?

david s poole · #2 07-02-2007, 11:41 AM

are you sure of model? 92 300se has 104 motor with 3 coils and some wires under a cover on top of engine.

wbrian63 · #3 07-02-2007, 11:51 AM

positive - 1992 300SE (badged as S320) - M104 motor, one coil, distributor driven off passenger side camshaft with 6 individual plug wires eminating from there.

vin wdbga32e0na023154 (working from memory here)

saumil · #4 07-02-2007, 12:51 PM

Can you check vaccum on intake manifold somewhere, it should be between 16 and 22 inch Hg at idle. If it is too low at idle, there is a problem in the exahust system, most likely a clogged catalytic converter.

Fuel pump possible but is less likely given that you can get the car to get to say 4000 rpm and keep it there without a problem.

mbfanatic · #5 07-12-2007, 01:32 AM

have you done some tune up, lately? yeah, try to check for some leaks, as what other have suggested.. it would be best to check all your parts to see what's really the problem, a clogged mercedes catalytic converter, exhaust leak, bad 02 sensor might be causing this problem.. anyways, keep us posted of what other things did you notice.. good luck

mbdoc · #6 07-12-2007, 08:07 AM

1992 300SE has a M104.990 engine with LH fuel injection & EZL ignition(cap & rotor).

On that car fuel volume & air mass are the 2 keys for proper acceleration.
The fuel system should have 3.8-4.0bar of pressure UNDER acceleration. Also the pump should deliver 1 liter of fuel in under 25 seconds.
If that checks out then a exhaust pressure test (less than 1 psi at idle) measured at the O2 sensor hole would be the next test.
If that checks out..then either a replacement air mass sensor OR inspection of the timing chain condition.

wbrian63 · #7 07-12-2007, 08:10 AM

Haven't had the time to check the manifold vaccuum.

O2 sensor is less than 1 year old. Car passed inspection in Nov with flying colors. I would thinkg that clogged or restricted catalytic converter would have adverse effects on the emissions.

New plugs, wires, cap & rotor. Coil is original. No exhaust leaks.

Additionally, I believe a clogged converter's restriction on exhaust flow will not differ based on car speed, but on engine RPM. I think I noted in my post that the car pulls like gangbusters through 1st and 2nd. It's at freeway speeds in 3rd that I see the fall-off in acceleration.

If the converter were plugging up, I'd see the same "nosedive" regardless of the gear.

I'm more inclined to think this is a fuel issue. In the lower gears there's less load on the motor because of the mechanical advantage provided by the lower gear ratios. Additionally, the car spends far less time in each of those gears - especially 1st.

In the higher gears, there's a lot working against the motor - wind resistance (50+ mph), and the lower mechanical advantage of the higher gear ratio means the motor works harder for longer at higher revs.

If this problem is fuel-related, then there would ordinarily be indications because of lean-mixture induced detonation. The knock sensors will likely prevent me from ever noticing the problem - the computer will prevent pinging by retarding the timing - which will further exaggerate the acceleration fall-off.

The car is completely driveable and gets good fuel mileage at 75 (around 22.5-23.5). It's just when I'm flogging the snot out of it to pass on a 2-lane road, or trying to move around some dimwit driving 55 in the fast lane when traffic is flowing 70+ that I notice the problem.

Far as I know, the fuel pumps are original and the car has 121K miles.

I need to get a fuel pressure gauge so I can check for pressure fall-off when the car is misbehaving.

But I'll still check the manifold vaccuum. Couldn't hurt.

mbdoc · #8 07-12-2007, 08:25 AM

A restricted cat won't cause emission problems..a melted down cat will.

IN 1st or 2nd gear the engine doesn't require the volume of air OR fuel needed in the higher gears.

IF there is a cam timing issue that would show up on a vacuum guage & most likely have in-gear idle problems with the a/c on.

Checking the fuell pressure/ volume is the logical starting point. Good luck.

david s poole · #9 07-12-2007, 09:50 AM

perhaps the fuel filter is partially blocked and won't deliver enough for full throttle in higher gears.a fuel volume test as M.B.Doc suggests would be a good starting point.

Arthur Dalton · #10 07-12-2007, 09:51 AM

Quote:

Originally Posted by david s poole

are you sure of model? 92 300se has 104 motor with 3 coils and some wires under a cover on top of engine.

104 w/DIS , coil-over did not come out until 104.992 HFM/SFI Engine Management.
Early104 [ in this case 104.990], were LH with standard EZL ignition

wbrian63 · #11 07-12-2007, 09:54 AM

I just replaced the fuel filter, so I think we can safely rule that out.

I would think that if I can find a good gauge to do a pressure test, if the pressure falls off at higher rpm/loads then that indicates the pump(s) are my problem.

Am I correct in assuming that I'm not likely to have insufficient volume and sufficient pressure - they go hand-in-hand, right?

Can someone suggest a source for a good fuel pressure gauge? I'm guessing it's something I can attach mechanically to the fuel rail with a hose long enough for me to do a road test with the gauge attached.

Arthur Dalton · #12 07-12-2007, 10:20 AM

< >>
When all conditions are normal, they are closely related..BUT, if you have a restriction [ like a partially blocked filter] , the pumps can have the pressure capacity and the sytem can still have low volume b/c volume flow is a TIME measure.
A partial restriction will still show good pressure at a fuel rail test port, but how long did it take to get to pressure ?? that is where the flow volume comes into play.
So, a fuel pressure test checks both FP Regulated, FP Unregulated, and fuel volume flow RATE. There is also a HOLD pressure time test after you test the FP..that is how well the lines hold pressure after the pumps are off..that test the check valves or accumulators , whichever system is used on that particular chassis.
Flow/Volume measures are particularly important at high demand/load conditions.

wbrian63 · #13 07-12-2007, 11:53 AM

Is there a document somewhere that details how to perform that type of test?

alabbasi · #14 07-15-2007, 11:18 PM

I had a simiilar problem on my E420, it turned out to be a bad cat. It did not throw and codes or fail an emission test.

LUVMBDiesels · #15 07-16-2007, 12:09 PM

I had similar trouble with the BMW (I know a totally different engine...) and when I unplugged the O2 sensor it cleared up. It could be that the new O2 sensor you put in is not working correctly and causing the engine to run too lean...