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Thread: Unstable RPM

  1. 27th October 2014, 07:28 PM #1
    pastr
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    Unstable RPM

    3.9 V8 auto
    Vehicle travels well.
    Most of the time, stable rpm at traffic lights.
    When I stop at home, rpm start to play up and down and the engine turn off.
    When I start the engine, it works like a clock, as if nothing had happened.
    What could be the problem?
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  2. 28th October 2014, 07:29 PM #2
    biglandy
    biglandy is offline Fossicker
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    Ah one of the troubles i had a while back . Not sure exact fault but i replaced fuel pressure reg, wires from dizzy to module a genuine bosch coil and a lucas module. The best it ever gone and no troubles since.
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  3. 28th October 2014, 08:56 PM #3
    pastr
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    I have checked the fuel pressure reg and it's look fin, new wires from dizzy to module, new coil and a bosch module like the Bee recommended.
    I'll put new pressure reg, or better put a fuel pressure gauge.
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  4. 29th October 2014, 01:31 PM #4
    Anthony
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    Check the Idle Air Control Valve (IAC) which sits on the rear of the air intake manifold and teh rubber pipes that connect it to the main air intake. The pipes can becom brittle and split and the IAC can not maintain the vacuum and control the air requirement at idle - this will cause the engine to hunt up and down as it tries to correct teh air requirement. Remove the IAC give it a clean and refit. Replacement pipes available from autobahn or supercheap for next to nothing.
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  5. 29th October 2014, 06:22 PM #5
    angus1
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    Don't overlook the coolant temperature sensor. I have had this problem and in the first instance it started with erratic idle speed which led me to clean the idle speed control. With no luck. As the fault developed it started to stall and sometimes run rough. Checking the coolant sensor revealed all. The resistance at 85C was 2kohms and cold was 28kohms. This should be 300 ohms approx. hot and 2kohms cold. So the ECU was delivering cold start fuel and revs when hot. The annoying thing is it's probably the easiest of the faults to check as the sensor is right at the front of the engine and only requires the plug to be pulled off and a minute with an ohmmeter, hot or cold will pin it down.
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