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Thread: '78 XJ V-12 Engine Control Module -- Likely Bad

  1. #1
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    '78 XJ V-12 Engine Control Module -- Likely Bad

    Howdy all!

    I've got a non-runner (mostly.) It's a Canadian-market V-12 with a likely bad ECU. (Fuel Injection) It will occasionally start but otherwise fail to keep running, especially the second it gets warmed up. A local british shop did their best with it and nailed it down to the ECU. I've done all other diagnostics -- sensors, fuel pump inertia switch, temperature senders, etc. All other is within spec.

    I understand these things are very difficult to find, and even if a used one is sourced, there is no way to ensure it's usefulness.

    Can anyone point me in the right direction here? Any advice on ECU replacement would be greatly appreciated. Any old threads on the subject you can link to? And, anyone ever taken one apart for a rebuild or refurbish?

    Thanks much.

    Brian in San Diego.
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  2. #2
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    hello Brian,
    I hope you have solved your problem?
    Due to my experience in Ser.3 fuel injection I would first check everything before reaching the ECU. ECU is not a common problem; the chassis grounding points of fuel injectors; the fuel relay; inertia switch; fuel pump itself.
    2- with a 12Volt low watt testing bulb (or LED 12V tester) I would check on every injector, the bulb should be flickering in starting mode (at least 3 injectors in each bank to be checked)
    3- fuel pressure (if the pump is running good) maybe when hot the pump is becoming lazy and loses pressure
    4- Sparks at several occasions (a bad coil or resistor will give weak spark when hot)

    keep us posted,
    cheers!

  3. #3
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    Quote Originally Posted by Rigajag View Post
    hello Brian,
    I hope you have solved your problem?
    Due to my experience in Ser.3 fuel injection I would first check everything before reaching the ECU. ECU is not a common problem; the chassis grounding points of fuel injectors; the fuel relay; inertia switch; fuel pump itself.
    2- with a 12Volt low watt testing bulb (or LED 12V tester) I would check on every injector, the bulb should be flickering in starting mode (at least 3 injectors in each bank to be checked)
    3- fuel pressure (if the pump is running good) maybe when hot the pump is becoming lazy and loses pressure
    4- Sparks at several occasions (a bad coil or resistor will give weak spark when hot)

    keep us posted,
    cheers!
    Thank you so much for the kind and thorough reply! I ended up selling the vehicle "as is" to a more motivated enthusiast. I just decided that the V-12 didn't match my skill level. As a longtime "above average" mechanic, this is the first vehicle that I never ended up solving the "mystery." Ahh.....it went to a good home to someone who will be delighted to find the solution, perhaps it's as easy as one of the above.

    Thanks again, perhaps this thread will serve another person who has a similar vehicle with similar ills.

    cheers!

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