[xj40] High Idle Problem


Does anyone know what could cause the Idle to jump to 2000 rpm
the moment I start the car? Its a 1991 Xj40 4.0 L.

Any help would be greatly appreciated. I hope its not
something expensive…

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In reply to a message from G@briel sent Thu 5 Mar 2009:

Try cleaning your intake system, including your MAF. If the sensor
wire on the MAF is sludged up, it will cause your car to do that.
Also make sure you clean the butterfly in your throttle plate. If
there is gak built up in there, the butterfly will not close all
the way, causing a high idle.
Gunner 94 VDP 176K+ miles Kingfisher Blue
Hendersonville/Tn, United States
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I had to have both my knees replaced so have been out of action for a year. Now doing well and back to my car. I have the same problem. My idle is 1200 rpm in neural no matter what I do. I have followed the manuals instructions for setting the idle. No joy. Suspecting the idle air control valve I removed it and sure enough the piston was galled and would not move. Taking it apart ruined it of course but it was defective anyway. Ordered new one. No joy. I have sprayed carb cleaner around all the intake manifold ports and any vacuum hoses. I am sure I do not have a vacuum leak. I cleaned the butterflys but they were not really dirty. The MAF looks good but perhaps it needs a shot of brake clean also. I do not use carb spray on MAF’s as it has some oil in it. At least most do. I have worked on cars for 50 years and this one stumps me so far. I know the butterflys are closing completely. This problem indicates to me a vacuum leak but cannot find one if there is. Can the crank position sensor cause this? Thank you all for your knowledge and help. I know it is hard won.


P.S. Forgot to mention the car is a 1993 6 liter V12.


Disconnect the butterfly’s from the linkage and see if the situation changes, also check the bushes that the linkages run in.

The last thing I did was disconnect the linkages and there was no change. Today going carefully double check the butterfly’s for centering and clean the MAF’s. If that doesn’t work don’t know what to do.

Firstly, you have an XJ81/6.0 L V12, not an XJ40/4.0 L I6 (which I have) so I cannot speak to the issues with the V12 engine. I am willing to bet there is a lot more collective experience with, and knowledge of, the V12s on the XJS forum. I will, though, volunteer that the butterfly on the 4.0 engine should not ‘close completely’ but rather have a tiny gap (.002 inch IIRC) when ‘closed’.

Have you tested the throttle position sensor for proper adjustment and operation (voltage change)? My understanding is an out-of-spec TPS can cause issues with the idle.

Good call Mike.

Thanks, still learning about these cars. Yes I checked the idle switch. It is functioning. Not only with a VOM but with the rods disconnected so the butterfly’s do not move, I can move the throttle quadrant off of the idle switch, the idle increases, then relax the throttle quadrant and the idle returns to 1200rpm. It acts like an air leak but so far I can not find one. I have sprayed carb cleaner all around the intake manifold and vacuum lines with no change. This is the method I have use to find air leaks in the past. The idle does not vary either. Very steady at 1200rpm.

Have you set the butterfly gap with a feeler gauge like Mike mentioned? How about the throttle position sensor voltages?

The idle switch works properly. And yes I measured the gap on the butterfly’s. Yesterday I double checked the cleanliness of the butterfly’s, their being centered in the throttle body and then relaxed the adjustment on the butterfly’s to make them close all the way. I wanted to eliminate them as any source of leakage. Then started the car. Still 1200 rpm. Somewhere the car is getting air and somehow the computer is seeing it as it is adding fuel because the car runs well. It does not surge or shudder at all as if the mixture is way lean. I am totally mystified at this point. The idle adjustment is wound all the way in also. I think at this point I am going to remove the idle air valve again and test it in hot water. I thought about that when I receive it but thought, its new so it works. Perhaps a mistake. So will see.

Referring to this diagram, which is for the 6.0 L V12 as used in the 1994 XJS models:

Is this the same as your 1993 XJ12 (note the diagram is for right hand drive vehicles)? My question in post #7 of this thread was about the throttle position sensor (TPS) that is mounted beneath the turntable, not the idle switch. Have you tested the TPS voltages at closed and wide open throttle positions?

No I have not. Didn’t realize that was a TPS. Do you know what the voltages should be? Sorry about the confusion. Thanks for the picture.

As I stated in a previous post, I believe you will find more information about the V12 engine in the XJ-S forum. I did a bit of searching (using the search tool [magnifying glass] at the top of the Forums page I entered “XJ-S tps voltage v12” - without the quote marks and note the first characters should be a pound sign followed by xj-s) and found numerous posts. Here are links to a couple of them:

Yes the TPS is a little flaky. I am going to take it off, take it apart, and clean it with some brake cleaner and see if that will fix it. I found on another forum that the TPS at idle should be .6v and wide open 4.2v. I thought also I would look for another just in case. I also found cross reference

P.S. Got finger happy pushed wrong button on computer. Any way cross reference to Lucas SEB205, Standard Motor TH416, TRW 73383, and Intermotor 19974. No luck on any. Now on to another question. I did find a used DBC2626. The part number was legible and was Lucas 73383A which indicates TRW. I also saw a Lucas 73379 or DBC2669. The only visible difference looked to be the connector. Instead of square black it is round blue. It is a 6 wire and the wire colors look the same color. These are much more available and cheaper. They were use on XJ6 and XJS 4 liters. If the connector is changed to proper square black would this one work?

P.P.S. Did it again. The 73379 is DBC2889 not DBC2669. Messing up all around. Sorry guys.

I don’t know about the TPS for the 6.0 V12, but IIRC the TPS for the 4.0 I6 is not really designed to be disassembled. It has been a long while ago that I cleaned mine but the generally accepted method of doing so is to drill a small hole in the bottom side (facing down when installed) right near the edge so as to avoid the drill bit damaging the bits inside the unit. The hole should be just large enough to accommodate the plastic ‘straw’ of the aerosol can of cleaner. I used electrical contact cleaner, but WD40 or similar should also work. Just bear in mind the plastic TPS housing and the electrical bits inside may not play well with just anything you might spray in there. :scream:

Read a post on another site where a gentleman carefully removed the bottom side to inspect and clean his (with pictures). I did assume he had to cut it open with something. I really would like to inspect the unit. Years ago I worked for a Nissan dealer. At that time the air flow meters for the fuel injection looked like the inside of our TPS (the electrical portion). A flapper valve moved needles across a potentiometer like our TPS uses the throttle quadrant to move the needles. They would get dirty and act up. The fix would be to take it apart, clean the needles and contacts with brake clean, lube with some lubriplate 105 and re-assemble. Today I would use either brake clean or a product I have been using recently called Deoxit D5. Made specifically to clean and deoxidize electrical contacts. I have been using this product on all of the electrical contacts inside the electrical switches in my car and it works wonders. I still would like to purchase a back up if possible. Do you know if the one for the xj6/xjs would work if one changed the connector?

I also think (after my experience with flapper valve MAF’s) that the potentiometer needs to be lubed to keep/slow down wear due to physical movement. So taking it apart would be the only option. When I clean any electrical switch I use lubriplate 105 to lube it before re-assembly.