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  1. #1
    Junior Member
    Join Date
    Jun 2010

    High idle problem - XJS 4.0 R6, AJ16

    Hello All,
    I do have an high idle problem (1500rpm) with my AJ16 engine (in an XJS 1995 US spec).

    Prior the engine had ...
    - a high idle (1200rpm)
    - sometimes very lumpy idle and stalled
    - negative (5...15) short term fuel trim - which I guess the ECU added some extra fuel

    so I...
    - cleaned the IAC-Valve, EGR-Valve, throttle body (very grubby)
    - changed the spark plugs, air filter
    - welded some exhaust manifold cracks
    - checked for any air leaks (none)

    .... and now the engine idles smooth but to high at 1500 rpm. O2 sensor signals are ok, short term fuel trims are back to normal. IAC-Valve and EGR-Valve seems to work - I can hear the piston movement after ignition.

    - Do you have any clue what I have to check next?
    - Some forum posts deal with TPS adjustment / orientation and O2 Sensor adjustment / orientation. What do you think about that suggestion?
    - The X300 should have an "apaptive fuelling" which could be reset - do you have any experience with that.

    Any tips would be great, unfortunately the local dealer here in Germany aren´t that experienced with the JAGS.

    @admins / mods
    I placed this post also in the XJ6 group (same engine-trans) - if not - please delete this post here in the XJS section as there might me quite more X300 driver with the same problem or even driver with solved problems

  2. #2

    Re: High idle problem - XJS 4.0 R6, AJ16

    Before you going digging in the electronics check the basics like the throttle linkage and stop adjustment. IIRC a few thou clearance between butterfly and body when throttle fully closed. The exact figure is in the workshop manual.

  3. #3
    Junior Member
    Join Date
    Jun 2010

    Re: High idle problem - XJS 4.0 R6, AJ16

    Thanks a lot for your reply Steve.
    The throttle mechanics seems to be ok and closes as it should, I already checked it while cleaning it.

    I had a telelephone call with an independet jag specialist garage in the UK - (nice service to advise experienced garages by the Jaguar Specialist Association btw).
    They suggested to perform a engine "base" setup via the Jaguar scanner (PDU or WDS system) and especially the throttle position orientation and O2 sensor orientation.
    Hopefully my local Jag dealer can perform the procedure.

    Also I dismantled the IAC-valve, plugged the cable on and turned the ignition on. The piston & spring stepped forewards and jumped out. After re-assembling of the piston and re-assembling of the IAC-valve the engine idled perfect at 750rpm. But unfortunately after starting the engine once again the idle raised again up to 1500 rpm.
    Does it mean that my IAC-valve could be worn?

    Any tipps are welcome !!!!
    Kind regards, Jens

  4. #4

    Re: High idle problem - XJS 4.0 R6, AJ16

    Do you have the workshop manual? There is a test procedure in there for the idle control.


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