[xj40] High Idle Problem

There is nothing in the intake tract. Goes from the air filter box, through a flexible bellows, past the air temp sensor, takes a 90 degree and into the throttle body.

Sorry guys. I am getting confused with this car. I can find no MAF sensors. A MAP sensor can be mounted anywhere, with a vacuum hose and electrical connection. This car appears to not have any MAF sensors. Which begs the question, how does it know how much air is going to the engine to calculate the amount of fuel to add? I still have not found the MAP sensor.

Figured it out. The MAP sensor is located inside the PCM and is not serviceable. So my replacement PCM must have a bad MAP sensor and the original one is ok. So now back to my original problem. The car idles at 1200rpm and I can not get it down to 800.

I know this ground has been plowed before, but if at first you don’t succeed…re the idle air control valve: I know you replaced the galled-up-and-wouldn’t-move valve with a new OEM unit. However…I found this YouTube video where the valve was the source of a high idle like you are experiencing;

I did once try a third party IACV on my car, specified for a Renault Megane but alleged to be compatible with the XJ6 4.0L I6, and while it was a perfect fit it was never right on my car. The base idle was a bit high and on coast-down from speed the revs would start bouncing between about 800 and 1200 IIRC with the unintended “cruise control” effect. I finally replaced it with an OEM spec unit and upon comparing the two I realized the cone on Renault unit was beveled at a slightly different angle ftom the OEM unit. The moral of the story is the angle must be correct (and of course the mating surfaces must be smooth) for the valve to seal and function correctly.

I have been wondering about that valve myself. I am going to remove it again tomorrow. Test it in cold and hot water and see what it does. I found a gentlemen on this site that showed how to disassemble the valve without tearing it up. Auxiliary Air Valve (AAV) disassembly and rebuild guide He also shows how to use a radiator thermostat heat bulb to replace the stock one. I do not really want to do that but may be forced to do so. Can’t afford another.

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OK I am either missing something here or we are talking apples and oranges. The idle air control valve is electrically operated; I don’t understand the hot/cold water test.

I would think a way to test the IACV would be to remove the bit to which the IACV attaches (with IACV attached, of course), use the following procedure to fully close the valve, and use a liquid to test for leakdown like one would test for valve seating in a cylinder head. While this procedure and specs are for the AJ6 4.0 L engine I would think the process for closing the valve would be applicable.

Provided by AJ6 Engineering, 60, Henshall Road, Bollington, Macclesfield, Cheshire SK10 5DN, England
Email aj6engineering@ntlworld.com, Tel/Fax:- 0044 (0)1625 573556

4.0 Litre

  1. Run engine until fully warm.
  2. Switch off ignition.
  3. After 5 seconds switch on ignition - wait 5 seconds - remove connector from stepper motor.
  4. Switch off ignition.
  5. Wait 15 seconds then replace connector on stepper motor.
  6. Repeat operation sequence 3 - 5 twice - on the last occasion do not reconnect stepper motor.
  7. Start engine and set base idle to 550 - 600 r.p.m. via air bypass adjuster.
  8. Switch ignition off and reconnect idle stepper motor.
  9. Restart engine - idle speed should now stabilise around speed shown below.
  10. Check idle CO as for 3.6 (if non-cat).
    Specified Idle Speed.
    Cold in neutral - 800 (Auto)
    Hot in neutral - 700 (Auto)
    Cold in drive - 650 (Auto)
    Hot in drive - 580 (Auto) (3.2L = 650)
    Hot in Neutral - 800 (Manual)

Note: Cold idle speeds will be slightly higher at very low temperatures.

If the above procedures do not result in stable idling check that the throttle potentiometer is correctly set. Whilst the ECU will automatically compensate for minor errors within the range of the adaptive memory, a badly adjusted throttle potentiometer can cause spurious drivability and idling problems.

Um, yeah, my mistake. The OP was referring to the auxiliary air valve - that my car does not have, and about which I know nada - whilst I was referring to the idle air control valve. I am going to lapse back into my coma now…

As it has been extremely hot and humid here for the past week so I have not worked on the car. Today is better so have been working on it. I pinched off one of the air lines to the AAV so it should not be in the circuit (so to speak). The idle did not change, still 1200rpm. So I see no sense in removing it at this time to check it. I am at a complete and total loss on this car. I feel fairly confident there is not an air leak as I have been all over the car with carb spray (while running) with no change in idle. If I disconnect the MAP at the computer the car will idle at 800rpm for about 10 seconds then the check engine light comes on and 1200rpm there after. Which leads me to believe the problem is electronic. But I do not know what it could be or what to do next.

Does this car have some other kind of timing control besides the computer? I have two computers, I have changed them out with no change. Is there some kind of timing retard that may not be working properly? Grasping at straws here I know but have absolutely no idea what’s wrong with this car.

I feel your pain (and frustration) with the high idle situation but not owning a V12 I am unable to offer any practical advice. I know you have visited the V12 Engine category here on J-L, and it seems like that is going to offer the best hope of finding a solution to your problem. You might try posting over in that V12 Engine category. I did find this LENGTHY thread (500+ posts) there - I apologize if you have already been down this rabbit hole. While I did not read all 500 posts I read enough of them to realize there is a ton of advice about possible problem areas:

I wish I could be of more help with your problem.

It must have an air leak, otherwise the engine would stall when you pinch off the idle air valve.

As you have found, that Ecu uses a Map sensor, so disconnect the intake pipe and completely block the throttle housing and idle control valve and it should stall.

Keep plugging vacuum pipes until either you find the one leaking air in, or are down to a bare intake manifold.
Use a smoke tester to find leaks. I’ve never had success using carb spray.

I suspect the evaporative system has a leak.

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