My '99 XJ8 with 60k has had a rather odd issue.

About once every 3 or 4 days, it will go into "Fail-safe Performance Mode", with all the fun speed restrictions that entails.

Strange facts about this:
It happens only once every few days.
Happens ONLY when the car is idleing. It does NOT occur at any other time.
Issue is resolved by simply waiting about 3-4 minutes with the engine off.
Does not cause a check engine light, or any OBDII codes at all for that matter.

Any ideas on what this could possibly be? My best guess is a glitch in the Body control module is receiving some false positive and dropping into failsafe mode when it doesn't need to.

 

If you have an OBDII reader plug it in and drive the car. When the FailSafe & Restricted Performance comes up see if the reader captured the code. This process worked for me when I had a bad Knock Sensor.

 

^^^ Me too. I am afraid that the knock sensor on the other bank is bad now. The first one went bad at 135k, and I just rolled over 170k, so I wouldnt be surprised if mine is bad. You will know what you are up against as soon as you get the code number though.

 

I would follow advice above if that does not solve your issue check/clean the throttle body connections, there are posts about how to do this

my 02 XJR was randomly failsafing for several years, the throttle body ended up being the issue

 

This is a link to cleaning the TB plugs Throttle Body Plugs But I would try getting the codes first.

 

Throttle body connections might be the culprit.

Code pulled up was P0121. The scanner's text mentioned it as a throttle sensor issue.

Also, something that occured to me. I have had a few rather minor sensor issues recently, which I have mostly ignored as they are relatively harmless.

First, the least harmless, is a code P0455. A "gross evaporative leak" code. There is no leak. Its fine, the sensor is most likely bad.

Second, a "engine coolant low" message on the dash. Coolant isn't low, I have quadruple checked it, the coolant is fine. Again, most likely a bad sensor.

Then I thought....... wait a tic, I had the relay for my headlights get fried not that long ago. Maybe my body control module is randomly killing stuff?

Any possibility that my theory is accurate? Also, I'll work on cleaning up that throttle body tomorrow when I wake up.

 

I think the BCM theory is off - check the front lamps for bad connections including the fogs. There's a number of earthing posts with dome headed nuts at the front of the engine compartment behind each headlamp pod, check these and the wiring.

Hope this helps.

 

Looked over the service bulletins, it seems 303-58 matches my issue and my VIN. Is this repair covered gratis at a dealership? or should I order the leads myself?

 

Order them, the service action ran out in 08.

 

Really? they put a time limit on a service action done "on customer complaint only"? Know of a good place to order them?

 

I had my TB rebuilt recently and it was a heck of a lot cheaper than replacement with a warranty. This is a link to my page that has all the info JagRepair.com - XK8ThrottleBodyRR

 

I don't think I need a new throttle body. My VIN doesn't fall under the throttle body service bulletin. I'll double check the date code on it to be sure. As far as I am aware the issue is with the tin plated leads.

EDIT: I must say though Gus. You have been awesome. That website of yours has provided me with more information then I thought possible. I'm going to try the repair listed under the 303-58 TSB, and hopefully that does the trick. Found the leads for $15 a pop through a link on your site. Although at $60, I wonder if the whole cable might be cheaper.....

 

My TB along with others did not fall under the TSB so do not be disillusioned, they do fail. Clean and/or replace your plugs and also clean the TB and see what takes place.

 

Hmmmmm, I'll take note of that. Trying the leads first as I already ordered them, and when the same symptoms point to different problems, my rule is to try the fastest and cheapest solution first. Occam's razor and what not.

Also of note while the discussion is still live, I noticed a that TSB 303-63 seems to fit the symptoms of another issue (yes, she needs some fixin, but she's worth it). Now that TSB states that its only for XK8, but I'm getting the same exact fuel smell and DTC P0455. I'm thinking I might want to look into that now.... after the P0121 is fixed of course.

 

I got the new leads. And I can't get the old ones out of the connector. The service bulliten does not match with the connector I have. I got the stupid yellow thing out of the way, but I can't get the old leads out. I'm on the verge of a breakdown. I can't get to work tomorrow without this. Anyone know how the hell to remove those leads?

 

Look on the bodies of the terminals. You will see some type of catch that needs to be released to slide the lead out of the connector body. Most of the time you will need some type of mini ice pick/awl or very small flat screwdriver to release the catch. I have never re-pinned a connector on a Jag, but all the others I have messed with have been this way. I have only screwed up one If you are seriously in a pinch, you can always crimp the leads on and plug them into the proper places so you can drive to work. You will be good as long as nothing touches and shorts out the power or signal circuits.

 

Well, this one was kinda weird. But i got it to work. Percussive maintenance was involved.

 

Congrats! Did it take care of your "restricted performance" error?

 

For the time being. It would appear so. It made it to the local in-and-out for some celebratory food, and it has made it in to work today without issue. A vast improvement considering that last night it decided to finally die and no amount of letting the car "sit" would allow it to work.

On the other hand, there was a whole month between the first and second time I got the issue. So I'm not going to believe it to be 100% gone til a full month goes by. If not, I get to cry a little bit and hunt down a new throttle body.

For now though, that's one thing off the checklist, next task, drain the oil, pop off those valve covers, and hop to god that the previous owner already changed the tensioners.

 

yesterday I experienced almost the same issue. After a 200+ miles long trip a check engine came on, a few minutes a "restricted performance" at idle at the intersection which went off within some 300 yards of driving, than "coolant low level" warning. So I checked the coolant - it is full. The OBDII gave me P0455 and P0172 codes. I cannot find the EVAP in the motor bay of 1999 XK8. I located a canister behind the splash shield in front of LR wheel. No broken or corroded hoses. I have this problem for a year. The car otherwise performs well, a gas milege while driving at speed limits +10% came to 25.2 mpg...