96 XJS stuck throttle - high idle

Ohh… I see :wink: yes we do…

Apologise, It’s not easy to live on alcohol addiction benefits for UK taxpayer’s money…

Still betting fuel pressure low…

Anyone else want to place a wager? :smirk: I hope to have an answer on Monday when I have a retired Jag mechanic take a look at it. I’ll keep you all post on the results.

Janusz you are on!

I got one beer on a TPS reset against any kind of Janusz’ fuel pressure problem. I’ll give you a wide definition here.

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I am familiar with the IACV manual re-set but not the TPS manual re-set. I would be interested to hear how its done, Veekay.

Frankie

If you take your TPS off, use a drill bit to enlarge the screw holes, just a little bit, you can then snug the bolts up just enough to keep it in place against the TPS back on to the throttle body, but not super tight so that it can’t be rotated with some effort.

With the car on, you should be able to slightly rotate the TPS, until your idle (hot) is in the 580rpm area. Then tighten down the bolts, locking the TPS in place! You’ll be doing that by feel, since the TPS is upside down in a difficult to see area.

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Interesting but that sounds like more of a modification that a re-set. My concern would be that the issue causing the high idle has been brushed aside rather than identified and addressed, but your solution may be OK in some situations.

Frankie

Frankie,

Less a modification than an adjustment.

To do a reset, you need a WDS, PDU, etc to reset the expectation the ECU has for the TPS voltage at idle. The ECU meets the new TPS at its current voltage at idle and is reset to accept that voltage value as the new idle voltage, and obviously the corresponding voltages for the butterfly positions through wide open throttle.

The manual process adjusts the TPS voltage at idle to match the ECU’s expected voltage setting at idle, without resetting anything.

Either way works.

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If you have a good idle at the minute and you aren’t sure the PO had the TPS adjusted by a PDU in the past - would it be a good idea to take some measurements of the base voltage for your TPS for future reference? Would it be that simple that you then know the base value for your own car for the future?

Just asking for friend!

Since the ECU learns over time, this seems like only a useful idea, if not absolutely critical without a device to do a proper reset, if you plan on replacing the TPS. This way you know what the base voltage was at idle for the one that is being replaced.

This assumes you’re replacing the TPS because it has a problem other than high/low idle.

If you take the reading now in hopes of it helping down the road, the ECU may adjust its expectations as time goes by and the TPS continues to be incrementally adjusted by gunk collecting at the throttle body. That reading today won’t be useful two years later.

Sure - but that wasn’t my point. The symptoms presented by the OP are an inconsistent idle - sometimes sticking high and then reducing to normal. That suggests a transient fault causing a variable output. If you re-set the TPS how will that make a variable voltage output stable?

Frankie

Have a look how the vacuum operated fuel pressure regulator works. Either the fuel pressure is incorrect or yuu have vacuum drop during closed throttle. You don’t need to do anything else apart from attaching vacuum gauge to the manifold - to diagnose variations. Poor man’s cruise control is an effect of overfuelling somewhere away from the sensors… Like mechanical fuel pressure regulator f.eg…

Well, my retired Jag mechanic friend came over to diagnose the high idle issue this morning. The culprit was a faulty intermittent working Idle Control Valve. After testing for some basic vacuum leaks, we pulled the unit to inspect. As I noted in a previous post, the unit looked new as the edges of the gasket was very clean in addition to the top of the devise.

With the unit removed from the throttle body, I turned the ignition key until my dash lights came on. No change in the valve. We did this a couple of times while my friend lightly tapped on the valve plug. After about the forth power cycle, the valve started operating correctly. We cycled power a few more times and the valve operated as it should. We then mounted the valve back in the throttle body, cranked the engine and it immediately idled at 750 RPM. Problem solved, although I will order a replacement unit as a spare.

What was interesting to me was that no check engine light ever came on when the Idle Control Valve had failed.

A big thanks to all of you for your input on this one. Hopefully this serves as a good starting point for others experiencing the same high idle issues.

Kevin

Thanks for coming back to let us know how it ended. I’m glad it was resolved. Some of these issues linger for awhile.

The AJ16 engine is extremely frugal with its codes. It seems that it does not like sharing information with others…

So I just went to source a replacement Idle Air Control Valve. Prices are all over the map, ranging from $9.50 on ebay to $500 at SNG and everywhere in between. Any recommendations from those who have ordered replacements would be appreciated. Thanks - Kevin

https://www.ebay.com/sch/i.html?_from=R40&_trksid=p2380057.m570.l1313&_nkw=jaguar+xjs+idle+control&_sacat=0

I’d either take a flyer on one that is being sold for $10, because why not?

or

Purchase one from someplace that has an easy return policy, preferably one who is a sponsor of this forum. Short of that FCPEuro is one place to consider. They do have a generous return and warranty policy.

So the replacement Idle Control Valve FINALLY arrived. I connected the plug prior to mounting the unit to the throttle body. I then turn the key to accessory and watched the plug actuate. I then turned the ignition off and mounted the ICV into the throttle body. Cranked the engine up and it would only idle up to 550rpm. I drove for a mile down the street, came back and turned off ignition. I then restarted it several times without it moving above 550rpm’s. I thought the ECU would self calibrate to the proper idle speed.

Do I need a computer reset or something else?

Thanks - Kevin

With limited knowledge of this system, that was to be my post.

Butterfly, The air valve to the engine Nice shiny brass in the picture. pretty much responsible for RPM’s in any ICE!!!

TPS throttle position sensor. The device that informs the ECU of the position of the butterfly…

Agreed point. Each needs to be crud free…

Carl

Thanks. I removed the throttle body earlier in the trouble shooting process and thoroughly cleaned with throttle body cleaner and a rag. I also made sure not to let the dippings reach the TPS on the bottom of the throttle body.

I replaced a faulty-intermittent Idle Control Valve.

You solved it. great!!!

Feels good, hun!!!

Carl

I think 550rpm is the right idle, for a warm engine. It’s 720rpm cold.