300SDL misfire between 3000 and 4000 RPM - PeachParts Mercedes-Benz Forum

AJEYA · #1 11-14-2003, 01:44 AM

My 300SDL '86 runs fine till about 3000 RPM in any gear, but as soon as I step on the gas, specially going uphill or trying to pass on the freeway, the engine seems to hesitate and lurch continuously, till the RPM goes over 4000 or the load on the engine is reduced by easing off on the gas pedal. It will run fine if I accelerate gradually through the 3000-4000 RPM range.
I have changed the filters at the engine, with no difference in performance.
Any suggestions will be helpful.
Thanks.

BoostnBenz · #2 11-14-2003, 07:33 AM

Does it normally get any louder while doing this? If so it is knocking. Does the turbo seem to make a pretty good difference in performance at lower rpms?

Was the prefilter very dirty? If it was perhaps the screen inside the fuel tank could be getting clogged and starving they system of fuel.

AJEYA · #3 11-16-2003, 12:50 AM

Quote:

Originally posted by BoostnBenz
Does it normally get any louder while doing this? If so it is knocking. Does the turbo seem to make a pretty good difference in performance at lower rpms?

Was the prefilter very dirty? If it was perhaps the screen inside the fuel tank could be getting clogged and starving they system of fuel.

No, there is no difference in the noise level. I'm not sure if the turbo is making a difference or not at lower rpms. I have noticed that of late the power seems to have lessened.
I did not notice if the pre-filter was dirty.
Is there any way to check the filter screen in the fuel tank without taking out the tank?

BoostnBenz · #4 11-16-2003, 01:22 AM

It is normal that the power falls off at those speeds, after ~3500 it seems as if it is decreasing. Well if you had a vacuum gauge hooked up to your fuel line and the vacuum was quite high while driving you'd know it is having to pull to hard. I've heard before that on some vehicles you can reverse the incoming and return and it'll blow the crud off of the fuel tank screen.

AJEYA · #5 11-16-2003, 01:34 AM

Quote:

Originally posted by BoostnBenz
It is normal that the power falls off at those speeds, after ~3500 it seems as if it is decreasing. Well if you had a vacuum gauge hooked up to your fuel line and the vacuum was quite high while driving you'd know it is having to pull to hard. I've heard before that on some vehicles you can reverse the incoming and return and it'll blow the crud off of the fuel tank screen.

I will try this tomorrow.

Thanks

BoostnBenz · #6 11-16-2003, 01:38 AM

Make sure when you do this that you are watching the fuel in the prefilter, if it doesn't work it'll obviously drain out the prefilter before running the IP out of fuel. Good luck.

mercedesmatt · #7 11-16-2003, 11:20 PM

also check the rubber hose on the turbo, runs from the compressor side to the waste gate, this actuates the waste gate. i had a split in one on one of my 300sd's and it ran just like your describing. if in doubt, check it out!!!!
good luck

AJEYA · #8 11-17-2003, 12:02 AM

Quote:

Originally posted by BoostnBenz
Make sure when you do this that you are watching the fuel in the prefilter, if it doesn't work it'll obviously drain out the prefilter before running the IP out of fuel. Good luck.

How can I take out the screen and clean it? If I reverse the flow and blow the crud off, it will just be a matter of time before this happens again.

BoostnBenz · #9 11-17-2003, 01:01 AM

This is true, but it is also assuming that is the problem which we aren't sure of yet. If you take out the first aid box you'll see the huge nut there on top of the tank. You'd need the socket to take off the ~32mm nut that holds it in place, a lot of towels, and also a new oring for the screen.

AJEYA · #10 11-17-2003, 01:09 AM

Quote:

Originally posted by BoostnBenz
This is true, but it is also assuming that is the problem which we aren't sure of yet. If you take out the first aid box you'll see the huge nut there on top of the tank. You'd need the socket to take off the ~32mm nut that holds it in place, a lot of towels, and also a new oring for the screen.

Thanks Jeff

gsxr · #11 11-17-2003, 02:35 PM

Before you get too wound up with all that fuel testing, I'd plug in a pressure gauge at the line to the ALDA and see what your boost pressure is. That "lurching" under heavy throttle sounds a LOT like the overboost sensor kicking in. It will turn off when load is reduced and boost pressure drops. You could just yank the wire off the sender (round thingy on the intake with hex flats, about 1.25 inch diameter) and go for a drive and see if that cures it. If so, the cause is either a faulty sender (assuming boost is in spec), OR your turbo wastegate is stuck closed and boost pressure is too high.

BTW - lack of fuel doesn't usually make the car lurch like that. It's usually just a smooth lack of power - just gutless. Regardless, you did mention that power has dropped recently, so you need to test boost pressure (install a "T" at the ALDA & connect the test gauge to that), and also replace the fuel filters if they're not pretty recent.

HTH,

AJEYA · #12 11-18-2003, 12:16 AM

Quote:

Originally posted by gsxr
Before you get too wound up with all that fuel testing, I'd plug in a pressure gauge at the line to the ALDA and see what your boost pressure is. That "lurching" under heavy throttle sounds a LOT like the overboost sensor kicking in. It will turn off when load is reduced and boost pressure drops. You could just yank the wire off the sender (round thingy on the intake with hex flats, about 1.25 inch diameter) and go for a drive and see if that cures it. If so, the cause is either a faulty sender (assuming boost is in spec), OR your turbo wastegate is stuck closed and boost pressure is too high.

BTW - lack of fuel doesn't usually make the car lurch like that. It's usually just a smooth lack of power - just gutless. Regardless, you did mention that power has dropped recently, so you need to test boost pressure (install a "T" at the ALDA & connect the test gauge to that), and also replace the fuel filters if they're not pretty recent.

HTH,

What should the normal boost pressure be, and at what RPM?

BoostnBenz · #13 11-18-2003, 12:26 AM

It should be 12psi, most of the motor's range.

AJEYA · #14 11-18-2003, 12:31 AM

Quote:

Originally posted by BoostnBenz
It should be 12psi, most of the motor's range.

Thanks for all your help, will keep you informed on progress.

gsxr · #15 11-18-2003, 10:27 AM

Official spec is 0.85-0.95 bar pressure (about 12-14psi), at 4000rpm, at full load in 3rd gear. That means you need to be driving the car with your foot on the floor to get a proper reading. A steep hill makes it easier if there's one handy. Most cars seem to be in the 12.0-12.5psi range. You should get close to full boost from about 2000rpm to redline at full load.

Thread Tools
Show Printable Version Email this Page
Display Modes
Linear Mode Switch to Hybrid Mode Switch to Threaded Mode