Question about 1983 Mercedes-Benz 300-Class

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Idles very bad has compression no bad injectors

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  • jeronimos Sep 12, 2010

    This is a disel car no fire is envolved



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Ensure that there is no misfire. Check that the spark plugs are OK, ensure that there is fuel circulation, and check the air filter adn fuel cap (try removing it for testing).

Check the MAF and AIT sensors. This two sensors detect the air flow mass and the air flow temperature, and determine the amount of air to be mixed with fuel in the injection.
Both sensors are located in the air intake and are both common cause for idleing problem.

Check IAC (idle air control) valve. This opens and closes to let extra air in the mixture when the engine is idleing.

Check the vacuum system. The vacuum system is used to recirculate engine gases. If there is a vacuum leak you get a leaner mixture, resulting in engine running rough particularly at idle, when vacuum makes a bigger difference.

Other problems like a defective EGR valve for example, may affect idle, but will also create big problems when driving.

I think I have listed all problem. If possible pul OBD1 codes using a Mercedes OBD1 scanner (go to authorized garage). Standard OBD2 scanners do not work on this car.

Good luck.


Posted on Sep 12, 2010

  • Ginko
    Ginko Sep 12, 2010

    Well if this is the diesel engine, check the rest.

    The two sensors are still there, the vacuum system has the same function, and the rest is pretty much the same.

    Start grabbing a hand gauge and test vacuum and fuel pressure. PAy particular attention to vacuum, this is tested reading pressure with gauge while heating or spraying with lube the vacuum hose (I use heating, but for safety reasons it should be done by a mechanic). If there is a leak you will see pressure going up.

    Most common reason for idle problem is air intake and sensor. Check temporary removing filter. Check the fuel circulation removing fuel tank cap. Check the sensors by disconnecting wires and seeing if that makes any difference. If that makes no difference, the sensor is bad. Clean the MAF sensor screen. This engine should have both MAF and iat. check them both. Check idle control cable and valve.

    Here is a forum that may interest you. I know it is not the same engine, but I suggest reading it.



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