I have a 2003 Jaguar XJR, and it runs well much of the time. Periodically, however, and especially in the summer months when it's warmer, when I am decelerating by letting my foot off the accelerator, the car will suddenly go into emergency mode, slow way down, and I have to pull over, shut it off for a few moments, then start it up again, and it runs just fine! I have had several shops look at it, and most of the time they say they can't find anything--looking at the vacuum hoses, running a "smoke test," etc. One guy replaced the computer,and it seemed to run ok for a year or two, then it started up again. Any ideas? Suggestions?

 

Sounds like the typical throttle position sensor, a component of the throttle body itself. My 2004 X350 started doing that, and after reading many of the threads addressing the "limp mode", I purchased a new unit on the Bay for $260, and had it installed for $150. No further issues. Good luck.

 

Other than the shutdown are there any other signs, IE codes or messages on the display?

Before going into replacing parts Id get the car car scanned with SDD/Mongoose and verify the battery is OK.

Check the volts when starting and make sure it does not drop below 12.2V

Also check the earth points. The locations are all in the JTIS

Cheers
34by151

 

Every Jag owner should have a spare throttle body in their garage. Failure of the TPS or PPS is soooo common, but the effect can be frightening. My XJ (4.0L V8 2002) went from 80mph to 15mph in a few yards on the motorway. Frightened me - nearly killed the guy behind me!

Set up a search on eBay and watch for a cheap one (UK prices approx £100), but make sure you get the right one. There are two types - one has a cruise control vacuum tank on it (early cars), the other is electronic (later cars).

I have 3 V8 4.0L Jags and two spare throttle bodies!

 

Wow, musta hit the brakes too! I was under the strong belief it wouldn't auto-brake due to a TPS problem....

 

It cuts out cylinders until it reaches 15mph. It then locks the gearbox in 3rd, sets the revs to 1100rpm, and disconnects the throttle. Scary. The real problem is that there is no warning - it just does it! By the time I realised that something was seriously wrong (and that I had no control over speed), the guy behind was humping my rear bumper.

 

It's in huge distress to go that far. It chooses not to risk things - like other car makers got wrong - such as uncontrolled acceleration.

In comparison the STR does not get to that state under any distress anyone has ever reported. In serious distress it does cut power and does reduce gear choice (a lot) plus makes slow gear changes, locks out cruise and so on, but not as you say yours does/did.

The guy behind was clearly a completely irresponsible idiot who ought to be banned from driving (here we lock them up as well).

 

The good side of the throttle body problems...

When mine first went out, Jag was the only one who sold them, and the price was $2,700. I delayed replacement until the limp mode became a weekly occurrence...for the first few years it would only fail every few months, and not nearly as drastically as Jim's experience. It would just lose power, but as soon as you cycled the ignition and pumped the pedal, it would work fine.

By the time I replaced my TB, it was down to $300. Now I guess it is pushing $200.