Hi everyone. I thought I'd tell you the latest tech tale with my old kitty. The car has been running great this summer and, apart from a cracked headlight lens resulting from a truck kicking up a rock, its been shaping up to be a pretty uneventful year. Just as the car was lulling me to sleep with its reliability, I start the car while running about town and it starts idling around 2000 RPM and drove itself home. The problem quickly got worse, idling around 3000 RPM, sometimes higher, but settling down to a still high 1500 RPM. The car would also surge up and down the rev range while driving itself, and just before i was going to take her off the road to do more research, it finally threw a P0507 and temporarily went into the glorious failsafe mode. The car still actually drove ok, and was much better than my previous experiences with failsafe mode, but I started looking here in the forums to start my troubleshooting.

This thread had some interesting information and really gave me some ideas to check: https://www.jaguarforums.com/forum/x...igh-rpm-84948/

My first stop was the intake tube. Mine was really in pretty good shape, but I did notice that there was a small pinhole in one of the accordion folds. While I doubted such a small hole could produce such a drastic result, I decided to take action, as any hole was certainly not a good thing. I elected to forgo the roughly $75+ a new one runs, and spent around $10 on some "JDM sport" boy racer silicone hose (3.25" diameter, cut down to 6.5" length) and, with some hose clamps, ended up looking like this:


I also checked the partial load breather, and it looked perfect. Testing this out and....no change.
I really was not shocked that this was not the solution, but it eliminated the easy stuff from the list. The throttle body looked clean, but I cleaned it up a little more and started to clean contacts on the throttle body. After pulling the intake completely, I started to look at the throttle position sensor. I had a rebuilt TPS already from a previous P0121 issue (see: https://www.jaguarforums.com/forum/x...d-96436/page2/) and thought perhaps that a minor adjustment may be the culprit.

I hadn't noticed it when I was working on the intake tube, but when I started to focus on the TPS more, it was easy to spot a problem:



The epoxy holding together my rebuilt TPS had failed. I'm pretty meticulous with my paperwork, but somehow I had misplaced the warranty information, naturally, and I didn't want to bother trying to fight the warranty process when aftermarket TPS are now under $40 online. I decided to order a new one and epoxy my old one back together to see if I could make that work. I carefully cleaned up the TPS, epoxied it together, and reinstalled it at the same orientation that it originally was. I got really excited to fire the car back up, but I did take the time to do a hard reset first....and now it was idling at 4000 RPMs. My hopes were dashed. I thought maybe being exposed to the heat after it was open had caused more issues, and I really didn't have the heart to mess with it more, especially with a new TPS coming in the mail.

Finally, the new, generic TPS arrived. Once again I was excited, as I hate the car being down, especially with something that required a bit more advanced diagnosis and not a more straightforward fix. I eagerly installed the new TPS at the same position as my old one was and fired up the Jag. I have a rather loud, some may say occasionally obnoxious exhaust, and I started the car in the garage as normal. It quickly revved to 4500 RPMs and my ears were practically bleeding. This was worse than before. I shut it down and adjusted the TPS counterclockwise (backward) and tried again. Yikes, 5000 RPM idle this time. Well, the other way should be the treat, right? More adjustment, and 2000 RPM idle. Continuing to rotate clockwise, and the car almost seemed good, but then stalled with too low of an idle. Starting the car again immediately resulted in a engine failsafe, no abs, no traction control, and an old nemisis, P0121. Feeling somewhat closer, but more frustrated, I adjusted a little more counterclockwise to raise the idle just a bit, did a hard reset, and back to the 1500 RPM idle with surging, and now both P0121 and P0507. I wondered if my new TPS was now the problem since it wasn't OEM and I was just about at my ropes end. I dropped it clockwise again to the same, or close to the same spot as when it stalled before, and tried one last time. I figured it was at least better to have that failsafe with idle too low, rather than the higher idle failsafe, and I had planned to call it quits for more research and a fresh mind on another day. I fired the car up one last time as during one of my attempts I backed out the car so I didn't have to deal with exhaust fumes or noise, and the car again seemed to be really close. As the idle dropped, I quickly hopped in and feathered the throttle to keep it from dying. The car seemed close enough I decided to take it for a drive just to see what it was like. I held the revs close to 1000 RPM and slipped it in gear. The car seemed to be driving really well and was no longer surging. I thought that this current state could at least leave the car driveable as I looked for more answers, and as I pulled up to a stop light while headed back home the car didn't seem to drop RPMs as far. I threw it in neutral to see what idle was now like, and the car seems to have learned as it was now purring away at around 700 RPMs.

The car seemed to be fixed at last! I continue to test things, but this certainly seems to be the solution. The funny thing is the new TPS needs to be clocked almost all the way clockwise to work right, while the old one was almost all the way counterclockwise for proper adjustment. To summarize, turning the TPS counterclockwise, raises the idle (very quickly), while clockwise lowers the idle. The car seems to not learn properly if it is allowed to stall after adjusting the TPS, but if the car gets a little help to keep from stalling, it does learning the proper range.

I have to say, this was some of the most frustrating, but somewhat rewarding, issues I've dealt with on this car. I had heard horror stories on the TPS not being something that Jaguar wants to deal with separately from the throttle body and requiring a dealer if the adjustment is wrong. When I replaced the TPS the first time, it worked flawlessly without any adjustment issues. This replacement brought all those worries right back, but, with some persistence, I was able to overcome and now the car is running beautifully again.

I hope this admittedly very long thread helps someone, or is, at the very least, an entertaining read.

 

That’s a good story and just shows that even when you’ve done the job before, it’ll always come back eventually.

This is a car that just keeps giving... but that’s one of the reasons you love her ....