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P38A GEMS idle problem

Discussion in 'Range Rover' started by JamesBB, Jul 17, 2018.

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  1. JamesBB

    JamesBB Active Member

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    Hi All

    I have had an issue on my 1996 4.6 GEMs engine running on petrol (not LPG).

    All is ok from cold, but starting from warm/hot causes the revs to drop so low it stalls. Touching the accelerator would stop the stall, but it would often miss a few beats until it settled down after a few seconds.

    I decided to check the usual as follows:
    • Spark plugs (which are NGK BPR6ES fitted about 10K ago). They were in good condition albeit a little sooty, so I cleaned them up with a wire brush. I was guessing the sooting was down to the occasional missing as mentioned above.
    • Cleaned up leads which are LR OEM leads, maybe even originals. They look ok, no signs of wear, arching, oil or water contamination.
    • Cleaned up throttle linkage. I am not sure the springs are correct on mine, but the throttle does swing shut when released, no kinks in the cables and moves freely.
    • Cleaned up throttle body, and removed IACV, both a little oily but nothing much at all.
    • Cleaned TPS.
    • Cleaned MAF with carb cleaner and cotton bud (carefully).

    Since doing all that the engine fires up fine, no problems, however some new problems have occurred which were not there before.

    1. The revs gradually run up to about 2000 then drop off to about 1600-1800 rpm.

    2. Touching the accelerator (or manually operating the throttle at the plenum by hand) causes it to suddenly/instantaneously drop in revs to the point it wants to stall, release the throttle and it climbs back to 1800.

    No errors on the ECU. So all in all other than the very high idle it runs fine, until you touch the gas. No misfires.

    I tried to pin it down to a sensor by plugging them. Here is what happens:
    • If I unplug the MAF while running nothing happens, but the ECU does log an error as expected.
    • If I unplug the TPS while running nothing happens, but the ECU does log an error as expected.
    • If I unplug the IACV while running nothing happens, but no error logged.
    • If I unplug the input air temp sensor while running nothing happens, but the
    • ECU does log an error as expected.
    All in all, I am lost, please can any or the gurus advise, it would be most welcome.
    Thanks in advance.
    James
     
  2. Greg Sharman

    Greg Sharman Member

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  3. JamesBB

    JamesBB Active Member

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    Thanks for the reply, but the issue is not being able to drive now. As soon as you touch the throttle it dies. This is a new problem.
    I know there are 2 levels of adaptive values, the long term need to be reset with diags, but surely it will not stop it being driven?

    Thoughts?
     
  4. Grrrrrr

    Grrrrrr Jedwood Spannerhands

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    OK cold? Not hot? Only guessing here but I am thinking the O2 sensors will be ignored cold and are only used when hot. Would affect petrol fuelling but guess not LPG?
     
  5. JamesBB

    JamesBB Active Member

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    The initial problem of low idle/near stall was when hot only. It was fine starting from cold.

    Since I cleaned stuff up it seems to start hot or cold but the revs are high. I know this may be adaptive values, but what is causing the revs to die (almost entirely) when I open the throttle?

    Its a weird one. Is it a TPS problem? Its all I can think of.

    Cheers
     
  6. Grrrrrr

    Grrrrrr Jedwood Spannerhands

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    Not played with the GEMS much. Someone like @Saint.V8 or @martyuk would be a better bet.

    Should be idling around 700rpm as far as I know. What does it say in RAVE? Presumably it has some sort of choke for cold starts. Could be that is on for some reason and when you open the throttle it floods? Not got RAVE handy to have a look but that's what I would be reading next.
     
  7. Saint.V8

    Saint.V8 Dyed-in-the-wool 100% RR Junkie

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    I would say you need to look at the IACV (Idle Air Control Valve) settings in the ECU, I seem to remember it needs to be set at 30-50 or something...I'll see if I can find out where I read it.

    EDIT: File attached
     

    Attached Files:

  8. johnnyb70

    johnnyb70 Well-Known Member

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    Sound like an idle valve (stepper motor)problem. The throttle cable should be slack when closed. This then brings the idle valve into play due to the tps registering shut. When you apply the accelerator the tps senses this and brings into play the main air inlet. At which point yours dies.
    I think you'll need nanocom because when it's warm most of the sensors are working at that moment Ie when the accelerator is pressed.
    It won't be the intake air temp sensor. It could be the maf.
     
  9. JamesBB

    JamesBB Active Member

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    Progress update. But first thanks for your replies and help guys.

    I have reset all the adaptive values using GEMS Lite, an utterly excellent Android app.
    Now my idle speed is bang on 700 plus minus 50 or so, smoother than its been in ages.
    Not sure if the warm start problem is fixed, but I did find another issue.
    The TPS is damaged. The little grey guides on the pot where the throttle blade slots into, are badly damaged so I need a replacement.
    I can only guess when I reassembled the throttle linkage I put too much tension on the springs which wanted to slam it shut at rest. While that was ok the tension must have been too much for the plastic lugs when the throttle opened. Since I have slackened off the spring tension but the TPS must be shot now.
    So my suspicion is when the throttle opens the TPS thinks its still closed so does not fuel the injectors as it expects, hence it tries to stall.

    Anybody else think this may be the case?
    At least the idle is stable and smooth.

    Cheers
     
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  10. johnnyb70

    johnnyb70 Well-Known Member

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    The gems tps is as rare as a rare thing. It usually only interferes with auto box changes if it's got drop outs on the wiper. If it's static it will cause trouble. Good luck with your search I ended up getting a used one from the states. Someone did say new ones are around in America.( Henrikb?)
     
  11. Greg Sharman

    Greg Sharman Member

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    What symptoms have you got now?

    Greg
     
  12. JamesBB

    JamesBB Active Member

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    Same, idle then touch gas and it dies.

    Looking for a TPS to replace mine as mine is damaged for sure. Whether it is causing the problem is unknown.
    In my view the TPS should see the throttle open and tell the ECU to pulse more fuel in through the injectors. If that does not happen it will stall of fuel starvation.

    Since taking the TPS off again and messing about with it the idle speed has gone back to near 2k, so I will reset the adaptive values again when home and after putting on a replacement TPS.
    I notice he Freelander and some of the Rover K series engines use the same sensor, but with a different physical blade on the shaft (more of a D shape as opposed to simple blade). I wonder whether one can be "adapted" to fit the old GEMS?
     
  13. Saint.V8

    Saint.V8 Dyed-in-the-wool 100% RR Junkie

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    Does the document I attached further up not tell you the function of the TPS? yes it is based on the Rover V8 in a Morgan application - but still the same engine and GEMS control schema
     
  14. Greg Sharman

    Greg Sharman Member

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    I would put my bet on you've got an unmetered air leak then... don't reckon its got much to do with your TPS. (check this by before resetting the adaptions, see what the TPS value at closed throttle is - then reset the adaptions and see what the TPS value is... shouldn't have moved.)

    Go around all of your hoses, check theyre good end to end (inc the rocker cover hoses on both sides as before), if air is getting in and not going through the MAF your confusing the EMS, hence why you end up with a sky high idle.
     
    Last edited: Jul 21, 2018
  15. JamesBB

    JamesBB Active Member

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    Checked the plenum and the rocker breathers which are brand new, no visible leaks. The clamps either end of the large induction hose are tight.

    Some data prior to reset (engine not running):
    TPS Close Throttle Value =0.045
    Fuel Flow Rate =0.092
    Air Flow Rate =2.1166666
    Short Term Idle =0
    Long Term Idle =236

    Same data after reset (engine not running):
    TPS Close Throttle Value =0.855
    Fuel Flow Rate =0
    Air Flow Rate =0
    Short Term Idle =0
    Long Term Idle =236

    So with what the GEMSbyPoole file says is does reset the TPS voltage to 0.85v for closed throttle. Its has done that and the idle is fine.
    Touch the gas and it dies is the main problem, not the idle. How can an air leak or faulty IACV cause that? Surely they affect idle only?

    Cheers
     
  16. Greg Sharman

    Greg Sharman Member

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    Air leak can cause the EMS to trim in error potentially causing strange behaviour - IACV can be compensating that out at idle.

    With your tool can you get a live TPS value?

    Greg
     
  17. johnnyb70

    johnnyb70 Well-Known Member

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    For reliability I found a proper tps. Could one be adapted? Maybe but not a job I'd do. I found the aj16 gems one was identical but worked in reverse!. The maf on these can cause plenty issues. The maf will take over as soon as the throttle opens. Unplug whilst running no change dodgy. Slight change then normal fine. But these are more sophisticated and the reading is completely vital to proper running. Check by getting the values at different rev ranges. The info is around somewhere!
     
  18. JamesBB

    JamesBB Active Member

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    OK guys, sorry for the delay. I have been away and also hunting a TPS.

    So, its fixed.
    Second hand TPS fitted properly so that its not under excess tension from the springs.
    New gasket on the IACV.
    Adjusted throttle springs to make sure its closes the throttle fully, but not with any large amount of excess tension.
    Reset ECU errors caused by me testing and unplugging things while running.
    Reset adaptive values.
    Idles bang on the mark.
    Opening the throttle by hand (or via pedal) revs the engine as expected.

    Not taken for a test drive yet, but its behaving like normal on the driveway.

    Cheers for the pointers.
    TPS was definitely damaged, as soon as I replaced that the revs went up as the throtttle opened.
    Slight air leak on the IACV gasket, did not help.
    Spring tension on the throttle did not help.

    Thanks again.
     
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  19. johnnyb70

    johnnyb70 Well-Known Member

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    Great result.
     
  20. Grrrrrr

    Grrrrrr Jedwood Spannerhands

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    Thanks for sharing the result. It really helps us all.
     
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