Fixed! TPS Problem Resolved!

This project took far too long, but I'm ecstatic to say that I've finally fixed my '99 XJ8 and her throttle body / throttle position sensor issues.  Everything is back to normal and I'm happy to be able to drive my car again.

It's been just over 3 weeks now, about 23 days, since everything fell apart.  I had just come back from Regina and upon trying to restart my car I instantly got a failsafe warning - code P0121 relating to the throttle position sensor (TPS).  For those of you unfamiliar with Jaguars, failsafe is like an extreme check engine light that actually limits your car to about 20km/h.  The idea being that you can just get it off the road but can't actually drive it around without fixing the problem.

*I should post a disclaimer here that while my repair ended up working for me, I can't be held responsible for any of these things that you might try yourself.  I'm simply sharing my experience in the hopes that others can better identify their problem and take the necessary steps to fix it - whatever route you think that is for your vehicle.

I already knew I was having issues with my throttle body before this as my scanner that I plug into my car was telling me that the problem was the sensor on the throttle body - a very common failure on '99 XJ8s.  The real issue is and was that Jaguar, in their infinite wisdom, doesn't sell the sensor separately from the throttle body (unlike pretty much every other manufacturer).  This means that you're forced to buy the entire throttle body, which costs several thousand dollars plus the cost of installation.  It's ridiculous, and I wasn't going for that.

Soon after the failure I went on eBay and found a salvaged throttle body that was a perfect match for my car.  My strategy was that I was going to transplant the TPS from the salvaged throttle body and install it on the throttle body that was already in my car.  This process seemed much easier and affordable than replacing an entire assembly when just one little sensor wasn't working.  The only concern was that I'd be playing Dr. Frankenstein by removing parts that were never meant to be removed and switching them around, but what did I have to lose? 

The delay began after it took a week just to find a suitable salvaged part, and then just over two weeks for it to actually get to me.  I have to say that over the course of 23 days you use up all of your favors pretty quickly.  I was largely borrowing vehicles from my parents, but as the process dragged on they became as aggravated as I was.  When the part finally arrived I felt excited and incredibly nervous because this was going to be the moment of truth.  Either it would work or I'd be shelling out a few thousand dollars.

I did my research online about how this should go exactly, but found no one actually suggesting what I was willing to attempt.  People said buy a used throttle body and get the shop to install it or bite the bullet and buy a new one, but no one actually suggested buying the used TB and just exchanging the sensors.  And now I see why.

The plug simply snaps off and two screws hold the TPS in place.

Two screws stand between you and this sensor.  These screws are Jaguar specific and require a Jaguar tool to remove them.  The shape looks incredibly common, but after trying every tool in my toolbox, running around Home Hardware and consulting with the mechanics at Canadian Tire, it's clear that no one has this tool but Jaguar.  Time for plan B.

I used my Mastercraft Grabit bits for stripped screws and was able to remove them quite easily.  I then transplanted the sensors and reinstalled the screws by hand tightening them and then using pliers to lock it in place.  This actually worked quite well.  The screws won't look pretty when you're done, but they'll hold. 

The next part of this challenge was making sure the TPS was calibrated properly - start by disconnecting the battery.  Because this part isn't meant to come off there isn't an exact science to this, but essentially you want to get it as close to how the other one was sitting as possible.  You can rotate the TPS slightly (before you've screwed it down) to calibrate how the TPS will respond to the throttle.  The TPS is essentially just a spring with a sealed sensor, and the part it hooks to on the throttle body is a just a pin that will push that spring.  Rotate it too much one way and the car will floor it on start up and too much the other way and your throttle will feel completely limp.

As an added fix for the TPS it's a good idea to clean the connectors with electrical contact spray.  Simply spray the plug and the pins of the TPS or lightly scrub them with a toothbrush and this will insure a better connection.  You can get the same P0121 code for the TPS if the connection is bad, so clean the TPS before thinking you need to do a full replacement.

You can click here for more detail about the throttle body cleaning and electrical connections.

Anyway, once you think you've got the TPS in a good position, a hard reset of the cars computer can help to readjust how it's reading the throttle.  A hard reset is done by touching one of the disconnected battery cables to the other one that is still attached to the battery for about 30 seconds.  This is like doing a full restart and your car will be forced to learn again - you can also temporarily remove stored problem codes this way.  It is normal after disconnecting the battery that you're RPMs will be high when restarting your car and your acceleration will be very brisk.  This is part of the learning process and on your next start up it should be at normal levels. 

This project was a bit finicky, but I finished the repair in a single evening and my Jaguar has now returned to normal.  No codes, no failsafe, and she runs as smooth as ever.  In total I spent around $500 to do this for what could have ended up costing over $3000 through the dealer.  She's still slowly leaking coolant . . . but that's another story.  For now I'm just happy I can drive my own car again and that I didn't have to sell a kidney to make it happen.