96 XJS stuck throttle - high idle

Took my 96 XJS 4.0 out for a drive yesterday and the throttle seemed to be sticking. I could take my foot off the accelerator pedal at 40MPH and the engine would maintain the rpm and speed. I then shifted into neutral and the rpm increased confirming the throttle was stuck. However, when I applied the brakes and slowly came to a stop, the engine rpms came back to normal idle. The throttle seemed to stay at the highest rpm after letting off the accelerator pedal.

Today, I removed the throttle body and cleaned it with throttle body cleaner, checked for vacuum leaks, and inspected the idle control valve (it was clean but upon inspection, it appeared to have been replaced at some point because the exposed part of the gasket looked new) Also checked the throttle cable and cam mechanism, all good. After buttoning it all back together, I then disconnected the battery as I read this might reset the ECU. Started it up and it was idling at 1300. I then took it for a short drive and experienced the same issues as the day before.

Where do I start from a diagnosing standpoint?

Thanks - Kevin

Hi Kev,

The magic of “poor man’s cruise control” - should start from checking your fuel pressure. It is much easier with inline fuel pump, lower pressure will affect fuel regulator (i think there is one in 96 version - still vacuum operated).

In terms of vacuum leaks - remove the cover from fuel rail and check the bolts/nuts securing the manifold to the cylinder head - I bet those will be quite loose…

To check fuel pressure variation just start up ypur Jag, and switch to.Drive on level road. As soon as the speed goes above 5-7mph - your idle control valve will be switched off, if RPMs are will jump slightly in that point of time with increased drag - probably your fuel pressure is lower then it should be.

You can also try to slightly squeeze the return hose in one place (wrapping up wirh insulating tape until hose will be squeezed slightly), then have a ride -but that’s difficult to control…

The last thing to exclude - vacuum operated system. Disconnect the vacuum lines from the inlet manifold and blank the ports (all apart from fuel regulator of course…). Have a ride and see if something has changed…

Disconnecting the battery does not reset the ECU.

You need a TPS reset. Common issue.

If you find someone with a diagnostic machine that is loaded with Jag software, they can do it for you in minutes.

There is a manual way to perform this. I may write up a thorough explanation and ask that it be made a sticky.

Thanks, but curious. What would cause the need for the TPS to be reset? I have an appointment next week with a retired Jag mechanic. I’ll double check to make sure he has the needed software.

How difficult is the manual reset?


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I’m not exactly sure what causes is. My speculation is that as the years go by, and the engine blowback oil that eventually gums up a throttle body continues to restrict the butterfly, the ECU adapts and learns as the incremental gumming occurs slowly, over a long period of time.

Then one day, all of that gunk is removed, and whatever the ECU learned to do, to adjust for the butterfly is no longer needed.

The service manuals do say that the ECU will relearn over a period of time and get back to where it should be, but I’ve never been patient enough to find out, nor has anyone else I know.

With the diagnostic equipment, it should take someone 5 minutes to perform the reset. It’s a matter of plugging into the OBD2 port, letting the computer read the car, working your way through the menus and once you get there, the computer takes about 5 seconds. Then you can disconnect and be on your way!

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Interesting… A couple of months ago (~150 miles ago), I was having some rough idle issues. I installed new spark plugs and carefully cleaned the throttle body butterfly and inlet side of the TB using TB cleaner on a rag. Wondering if I dislodged some crud behind the butterfly. The TB did not seem too dirty when I removed it for cleaning three days ago.

My 95 has the same problem. The car had a ton of miles on it (probably 150k then, I parked it at 198k). The TPS itself was worn to the point of being notchy. A new TPS fixed everything. I didn’t use a PDU to calibrate it, I think I just went for .3v at idle with a 5v input. I don’t remember exactly and I’d just use the PDU now. The Coventry Foundation in Columbia SC is probably the nearest PDU to you.

The TPS is at the underside of the throttle body. Gunk and stuff collects not only behind and in front of the butterfly, but in the bottom of the butterfly spindle, and then into the TPS as well. Bad design.

Some people damage their TPS by cleaning the butterfly in place and the harsh cleaning chemicals sprayed around the butterfly all drain down to the spindle and into the TPS. Bad design.

I have since installed an oil catch can that hopefully eliminates any oil blowback from reaching the butterfly.

Just removed the throttle body again to inspect and clean the TPS. The first thing I noticed was the Ford insignia on TPS. I cleaned the oil off of the unit and reassembled. Cranked the engine with no change in idle rpm’s ~1400.

Did you remove the butterfly and spindle to clean these components and the TB thoroughly? Veekay makes a good point that effective cleaning requires dismantling. I used a very light rubbing with a scotch pad on mine to cure a persistent high idle. Don’t forget to clean the spindle bearings. When re-assembling carefully re-locate the butterfly so it clears the TB barrel when closed (a little adjustment is possible in the clearance of the screw holes) and make sure the butterfly screws are locked.


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Thanks for the advice. I may tackle the complete disassembly tomorrow. I am experiencing the opposite symptoms of a TPS going bad. Is there anyway to test it? No check engine light illuminated.

TPS values should be 0.58V fully closed and 4.8V fully open.


TPS is not going bad over day and the voltage has nothing to do with correct functioning as the butterfly gap/possible jam must be checked first, then TPS. TPS is not adjustable on aj16 as I remember.
Your ECU will show an error when TPS and MAF values are workig out of pre-defined ranges…

ALSO - if it’s idling high but getting back to normal while driving (over 5mph) it seems to be idle control valve rather than TPS…

TPS isn’t supposed to be adjustable, but can be made to be. Just make the holes for the screws larger and boom!, you have an adjustable TPS.

TPS Values are only good if you know for a fact that they have never been reset by a diagnostic machine.

TPS Sensor is notorious for causing mysterious issues, while bench testing perfectly fine! Unfortunately with your high idle issue, there wouldn’t be anything to bench test. It’s all within what the ECU has learned over the years.

You need at TPS reset. Either manually, or through the diagnostic machine at the shop.

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Ford inisgnia is the OEM part.

…boom! You’ve just trashed your butterfly.
Butterfly has machined flats. Just close it, tighten both screws. Check with feeler gauge (just mentioning…)

It’s been done many times by forum members. Not sure how opening up the holes on the TPS can trash the butterfly. Can you explain your concerns Janusz?

In any case, if you have access to a mechanic with a diagnostic machine with Jaguar specific software, then you don’t have to concern yourself with this manual adjustment.

Well, ehm, err… What can go wrong. Nothing. No loose screws going down through the cylinder head…

Just saying that butterfly in AJ16 has oval holes allowing adjustment without drilling anything…

You guys are talking about two different things, aren’t you?

Yes Kirbert.

Janusz, you’re talking about the screws on the butterfly spindle. I’ve never adjusted those screws.

I’m talking about the two screws that hold the Throttle Potentiometer Sensor (TPS) in place…under the throttle body itself. If the screws fall out of that thing, they would fall to the ground. Look up part number JLM12074 to understand what I am talking about.

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