Question about 1995 Mercedes-Benz SL-Class

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MB 95 SL500 Fuel Prob, Running rough, etc.

I have a 95 MB SL500 80K miles that during the past month or so has "SOMETIMES" started running rough at lights, etc. but would recover and then seem to run fine until the next incident on another trip or similar. I just changed the fuel filter (it is installed correctly) and now everything is worse. After it sits for a while and is cold, it runs and accelerates fine and exhibits no fuel starvation problems, but after it gets to operating temp, it idles rough and I have to shift into neutral at lights to keep it running. Also it will not take the gas when accelerating but after I get it to say 30-50 mph it seems OK. But it still is fuel starved on acceleration and hesitates when doing that at any speed. Can you help me fix this please. Thanks

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  • rocketguy2 Aug 12, 2008

    How does the throttle positioning sensor or fuel pump relate to this problem?

  • rocketguy2 Aug 12, 2008

    What I don't understand is why changing the fuel filter would cause this problem to become so much worse. Any ideas on that?

  • rocketguy2 Aug 12, 2008

    Please answer my comments to the original post. Thanks

  • rocketguy2 Aug 12, 2008

    Does anyone else think that the fuel pump could be the problem?

  • rocketguy2 Aug 13, 2008

    Does it make sense that a new fuel filter would require more fuel pump pressure for gas to flow than the old one? If so how? Because I know from the engines behavior before and afterwards that this problem is somehow related to changing the fuel filter.

  • rocketguy2 Aug 13, 2008

    What should the fuel pump pressure value be?

  • rocketguy2 Aug 13, 2008

    What should the fuel pump pressure be?

  • rocketguy2 Aug 14, 2008

    I have 5 different answers someone please try again!

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9 Answers

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I have a 1993 SL500 mine would die and then after sitting would start and run, tried fuel and all kinds of stuff. Turned out it was the distributor caps and rotors. Easy to take off and easy to see if they have dirt and carbon tracking on them. I had to replace them once and cleaned them with WD40 recently. If my car runs rough its the first thing I check.

Posted on Jan 24, 2015

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Testing Fuel System Pressure Most vehicles utilize a fuel injection system to deliver fuel to the engine. Correct fuel system pressure is essential for proper engine operation, in fact many engine run-ability problems can be related to weak fuel system pressure. A fuel injection system contains a fuel pump to supply fuel from the tank into the fuel system. The fuel filter cleans fuel before fuel enters the fuel injectors. The fuel pressure regulator adjusts fuel system pressure depending on the load of the engine.

Posted on Aug 13, 2008

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You must check also fuel pump pressure and fuel pressure regulator if operate normal and replace if necessary.

Posted on Aug 12, 2008

  • Kleanthis Mavropoulos
    Kleanthis Mavropoulos Aug 13, 2008

    Fuel pump pressure must be in the range 40-45 PSI

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If you know how to reset your map sensor and your maf sensor this also helps

Posted on Aug 12, 2008

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Im almost pos its your intake system, just from the way it sounds i had a old dodge truck that did the same thing and it was in the intake system, had to put a new one on... it wasn't breathing right, change your air filter and get your some intake cleaner see if that helps if not you will have to change the whole thing out... possible down to the carb...

Posted on Aug 12, 2008

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Throttle position sensor help automatic trans control module to understand how to use throttle, and affects automatic gear shifting.
If it is a gear problem that is the only case when problem may be related, among other things to TPS.
A broken or faulty fuel pump can be related to this problem only assuming that your old fuel filter was doing far less resistance to fuel pressure than new one.
To troubleshoot the problem it will be useful to understand if it is a fuel problem or a transmission problem.
If it got worse after replacing the fuel filter it is probably a fuel problem.
Check also that you do not have oxide bits coming out from fuel tank and getting stuck somewhere in fuel pipes or in filter.

Posted on Aug 12, 2008

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You need to check you airflow meter... The airflow meter tells the computer how much air goes into the engine and then the ecu sends signal to injectors to inject gas.... But if the MAF don't give the right measuring then you have less so then you have no acceleration going but then when you engine revs up it sucks more air and then the fuel has to be there in order to make your car fly

Posted on Aug 12, 2008

  • gopinath pasupathy Aug 19, 2008

    pressure value should be around 35-40......

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That doesn't sound like fuel starvation, that sounds more like excessive fuel at idle. So at low speed when you do hit the accelerator, you basically flood the motor. Running it up to higher revs will allow the motor to burn off any excessive fuel.

Has your fuel mileage gone down at all since this problem started? That would be a good indication of the rich setting.

That's where Pyrhc is correct, it could be the TPS, or you have an O2 or another sensor going out.

Have you run any code read out at all?
That would be my first place to interrogate the car.

Posted on Aug 12, 2008

  • Coss Aug 12, 2008

    Having this happen right after you changed the filter could just be a matter of circumstance.

    Not necessarily the cause for this problem.



    There was one comment about the resistance in the old filter, but that is pretty unlikely.

  • Coss Aug 13, 2008

    There is one way to test that....

    Do you still have the old filter?

    If so, put it back on and see if it makes a difference.

    I don't think it's the fuel pump.



    I will still go with my one post.



    Have you plugged it in to get any read outs?

    The onboard computer will tell you a lot about the car.

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This sounds like the throttle positioning sensor (TPS) is going out. Could also be a bad O2 sensor or partially blocked EGR valve. Start with the TPS, then got to the O2 and EGR.

Posted on Aug 12, 2008

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  • Ryan Rogers
    Ryan Rogers Aug 12, 2008

    The throttle positioning sensor (TPS) works in conjunction with the ECM (control module or computer) to ensure that the proper amount of gas is getting to the engine at a certain rpm with the timing being adjusted accordingly. If the TPS is malfunctioning it can cause a multitude of things to happen, such as, not getting enough gas to the engine (starving) at certain rpm's, usually in two different stages, either low rpm or at higher rpm's, getting to much gas to the engine, low rpm will flood the engine and high rpm will stutter, causing the ECM to adjust the timing incorrectly, resulting in rough idle at low rpm, and falling flat or having a flat spot at higher rpm's. A weak fuel pump can cause this by not getting the proper pressure to the injectors, in turn the injectors aren't getting a full injection of gas into the cylinders. Also keep in mind the O2 and EGR. A bad O2 sensor can adjust the air intake too high and create the illusion of not enough gas, where in fact there s simply too much air, and the EGR, if partially blocked, can create a back pressure into the cylinders which doesn't allow the exhaust to flow out properly, causing an effect as if the car were not getting enough gas. Any of these will cause the problem, and, again, start with the TPS.

  • Ryan Rogers
    Ryan Rogers Aug 12, 2008

    I have to agree with Coss71, the fuel filter is most likely circumstantial, but yes the pump could be the problem. I'm still leaning towards the TPS or O2 sensor.

  • Ryan Rogers
    Ryan Rogers Aug 13, 2008

    I was reading info on the SL500, and found that some of the 94-96 models have a two fuel pump system. If yours is a model that has a two pump system then it could be that one of the pumps is failing. Unfortunately I couldn't find the pressure rating, but the service dept at your local dealership will be able to tell you the rating, and if you have the two pump system. You will probably need the VIN in order for them to find out if you have the two pump system.

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