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Thread: high idle when driving

  1. #1
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    high idle when driving

    For some months I have been trying to sort out the high idle (1500 2000rpm) when driving with the throttle closed.
    When stationary idle returns to normal.
    As I live in a hilly area this is really a problem when descending grades.

    This is where I am at so far.
    Checked operation of IACV valve ok. When vehicle is stationary idle comes back to normal. Responds to AC operation ok. Speed sensor ok
    Cleaned throttle body and checked for air leaks etc.
    Swapped out throttle position sensor.
    Distributor advance checked ok
    Set base idle at 525rpm
    Purchased Rover Gauge cable / program. No faults detected with ECU. All values normal.
    Swapped out ECU no change in high idle condition.

    When starting and vehicle stationary idle returns to normal prior to driving.
    When driving with the throttle closed Rover Gauge indicates that the IACV is opening allowing extra air to enter manifold and excessively increasing idle.

    It would be reasonable to suggest that the ECU is receiving an input from a sensor / input that instructs the IACV to open even with the throttle closed.

    My question is which sensor / input do I need to check that can be influencing the ECU / IACV.

    Are MAF and coolant temperature sensors worth checking ??

    Again this is only an issue when driving with the throttle closed and when stopping the idle is excessively high until the speed sensor tells the ECU to activate the IACV to achieve normal idle at 750rpm. This usually takes only a couple of seconds.

    I will sort it out eventually but need some help.

  2. #2
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    The Vehicle Speed Sensor VSS instructs the IACV to open when the vehicle is moving and the throttle is closed (less than .49 volts)

    The VSS also causes injector shutoff at 1500RPM or lower with throttle closed and VSS active.

    It is entirely normal for the idle speed to increase to approx 1200RPM under these conditions.

    The reason for this is to ease the change to 1st gear in the auto while coming to a stop.

    Funny I had a Falcon that did this years ago and the dealer even didn't know of this feature. It is common in most older injection/auto setups. It probably still is.

    Regards Philip A

  3. #3
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    Find the VSS wire to the ECU and chomp it. Fit a switch if you want high idle at times. My reading of the wiring diags suggests the wire is yellow/pink stripe, pin 29. Use a LED test light or a logic probe to double check you've got the VSS output before hacking into it.

  4. #4
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    Find the VSS wire to the ECU and chomp it. Fit a switch if you want high idle at times.
    Er it also runs the speedometer.
    Regards Philip A

  5. #5
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    Quote Originally Posted by PhilipA View Post
    Er it also runs the speedometer.
    Regards Philip A
    Correct, that's why you cut it near the ECU not near the VSS. The ECU is not used to generate a speed signal as the 300TDi speedo works perfectly well without an ECU.

    Another spot might be at the multi plugs above your left foot, the engine/ECU harness connects to the body harness here. Haven't done this myself but have done a 300TDi conversion as well as retrofitting a 14CUX to a Defender.

  6. #6
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    High Idling Disco 1 3.9

    Quote Originally Posted by bee utey View Post
    Correct, that's why you cut it near the ECU not near the VSS. The ECU is not used to generate a speed signal as the 300TDi speedo works perfectly well without an ECU.Another spot might be at the multi plugs above your left foot, the engine/ECU harness connects to the body harness here. Haven't done this myself but have done a 300TDi conversion as well as retrofitting a 14CUX to a Defender.
    Thanks for the info I am still looking for the source of the fault. At last resort I can isolate the VSS to the ECU as suggested at the ECU end of the loom.I am going to check the resistance on the coolant temp sensor to ECU. Rover Gauge indicates a reading and it looks ok, but I would like to eliminate the sensor as a potential cause.Whist leaning over the engine I noticed that there is a vacuum line running to the fuel vapour canister and from the canister to the fuel tank.I also have considered an air leak at the brake power booster but figure that this leak would be constant across idle and driving conditions.The manual advises that the purge of the canister is controlled by a solenoid which pulses when the throttle is open and the engine is under load.Is it possible that the purge solenoid is faulty and allowing air to be drawn in to the plenum under load conditions.Also if the fuel tank filler gasket is not sealing properly could it then be combined with the purge solenoid issue to introduce an uncontrolled source of extra air entering the plenum when driving as distinct from being stationary. Its all part of owning a landrover. Just need to sort it out.Any further discussion is greatly appreciated.

  7. #7
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    It's highly unlikely there are any mechanical faults, seeing as your engine is working basically as designed. Tho other things you can do is to

    1. Unscrew the stepper motor, remove the fibre washer from the body, and reinstall it. This limits the amount it can open.

    2. Play with the base idle screw on the top of the throttle body (3/16" hex key needed) and lower the idle a bit as required.

    Once you get the engine to the point where it only revs high enough for a smooth cold start, you don't need any more.

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