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  • Trouble Starting

    Hi Everyone,

    I've got an issue with the car that I've been trying to track down. When I go to start the car I turn the key to the start position, the engine begins to crank and then it feels like I'm getting ignition. But, then when it feels like there's ignition I let go of the key so it goes to the run position and the engine just dies. I had tried replacing the fuel injectors, the fuel pressure regulator and the fuel pump relay, and it's still the same, the engine won't start.

    I continued diagnosing the problem and I found that if I disconnect the coolant temperature sensor before starting the engine, it will start just fine. It starts on the first attempt every time. Without that sensor plugged in the engine feels like it's running a little rough and the idle speed will slowly climb from about 880 where it should be to around 2,500 - 3,000 rpm. The odd thing is, when I plug the sensor back in the engine stays running and the idle speed returns to about 880 rpm. It's that way even if the engine has only been started and run for 20 seconds before I plug the sensor back in, but it won't start with the sensor plugged in.

    I've taken readings from the sensor, and on a day when the air temperature was 18 Celsius I got 2.3k ohms resistance. At the same time I checked the air temperature sensor in the air flow meter and I got 2.2k ohms. 2.3k ohms is about right for 18 Celsius, but the day had been warmer and I would have thought the coolant in the system would have been slightly warmer than the air temperature from it being warmer earlier in day, so 2.3k ohms might be a little higher than it should be.

    Does anyone know if the ECU's default is to go full rich or full lean when there's no coolant temperature sensor reading? What do you folks think? Are the facts indicating a faulty coolant temperature sensor even though it's testing roughly within spec, or is unplugging the sensor just causing the ECU to run a certain way that compensates for what's actually causing the car to not start?

    Thanks.

  • #2
    A little off topic, but have you changed the cap and rotar? Had a start issue like yours and it was the cap and rotar...
    1989 E30 M3 Cabrio - Nogaro Sliver / Black - 1 of 786
    1990 E30 M3 - Sterling Silver / Black
    1991 E30 318is - Sterling / Black (Mtech II project)

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    • #3
      You cold try just plugging the harness into a resistor of the appropriate value for the given temperature. Another possibility is that the connector is loose or intermittent, and reseating the connector fixes the problem temporarily.

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      • #4
        James, the cap and rotor were changed a while ago, but they only have a little over 3k miles on them. I would have thought they should still have been good, but I'll check them out to be sure.

        Colin, thanks for the suggestion. I'll try wiggling the connector a bit to make sure it's seated correctly and see if it starts any better.

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        • #5
          Thanks everyone for the suggestions. I finally found the problem. In my case it ended up being a bad barometric pressure sensor. Most of the time when it fails, it's supposed to make the engine run lean, but in my case it was reading as if my altitude was about 800' lower than I actually am so the engine was running too rich. When the engine was hot, I could still get the car started but the car felt sluggish when accelerating, and when the engine was cold it wouldn't start at all. I guess unplugging the coolant temperature sensor caused the ECU to just lean out the mixture enough to get the car started and running.

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