Fellas, I'm pulling my hair out...

After driving by a 1999 XK8 with a "For Sale" sign in the window for 4-5 years, I picked it up and brought it home as a project.

The good: the car has been dead reliable. It has started every time I e asked it to, and has never left me stranded anywhere in the 18 months I've owned it. It has had some issues, and I've been working through them one by one. Thanks to our dry Nevada weather, she doesn't have a speck of rust anywhere. I watch guys on YouTube fighting with rusted fasteners, and begin to appreciate the desert more everyday.

Now to the bad. I have been fighting the "Failsafe Engine Mode" message since day one. Originally, it would come in in conjunction with the ABS light and antilock brake system not working message (I don't recall the exact verbiage). First, I scanned the codes with an Autel, and received an error message indicating two wheel speed sensors were bad, so I changed all four. Same issue. I scanned the codes again and received a message indicating the brake switch was not working so I changed it. I still have the scars from fitting my regular sized human hand through the tiny hole behind the pedal to prove it. Still the same issue. This time the scan indicated the ABS module was having issues. I tried the lid removal/resolder/glue the lid back procedure, and no change. I purchased a rebuilt ABS module, and I have not seen the ABS or traction control warnings since.

So what's the problem? I still get the Failsafe engine mode light. If I happen to set the cruise control before the light comes on, I can burn an entire tank of gas and the light never comes on. If I disengage the cruise, it comes on within a matter of minutes. If I do not set the cruise, it comes on within minutes. The vehicle never derates or restricts performance, but the option to engage the cruise control goes away. If I shut the car off and restart, the Failsafe message is gone, and, again, if I quickly engage the cruise it never comes on until I disengage it.

So I scanned the codes again. The error messages? Wheel speed sensor and brake switch. Back to square one, except without the ABS or TCS messages.

I find it hard to believe I purchased both a bad wheel speed sensor AND a bad brake switch. Also, if either or both were bad, why does the cruise control have any impact at all?

This one has me flummoxed. Any ideas?

 

The front wheel link harnesses are more likely to fail than the wheel speed sensors!

 

I think the only things that can cause an engine fail safe mode are issues with the pedal position sensor (PPS), and throttle position sensor (TPS) and associated control mechanisms (throttle motor, etc.), as well as the power/ground supplies feeding these sensors.
Assuming this is correct (!), and noting that you don't have issues when using cruise control, it could be an intermittent issue with the PPS (since you are not pressing it when cruise is on).....?
Although presumably the TPS outputs are fairly range bound as well when cruise is on, so also could be avoiding a problem area of the sensor....
- e.g. flaky connector or wire to PPS, bad section of PPS track(s), etc.
- although I would have expected some kind of error code to be logged....
- ( it's a bit strange to me that for the AJ27 DTC P1121 explicitly is triggered on a PPS track A versus track B mismatch according to the documentation, but the AJ26 docs do not mention the A vs B comparison as an error condition...)

Another, of many, causes of ABS issues can be bad rear wheel bearings, but that should not cause a fail safe condition to my understanding...

 

This happened to me with my first XK8 (also second-hand). It turned out there was almost no oil in the automatic transmission. I changed the filter and added new oil, and it never happened again.
Importantly, if it hasn't been done, I recommend replacing the oil lines that go to the heat exchanger (they can also be rebuilt at a hydraulics shop). I had a bad experience with this, and now I have another XK8 (I didn't see the warning) from the same era as the previous one (2004).

 

I have a 1999 XK8 with just over 40,000 miles. The Fail Safe and Traction Control warnings put me on a wrecker at least four times. I finally got a shop to catch it in the act, and it turned out to be a bad Throttle Body. They don't make those anymore, so you have to have it refurbished. Automotive Scientific in Tennessee is where I sent mine and it SEEMS to have fixed the problem.

 

Automotive Scientific is where I bought the rebuilt ABS controller. Works great so far.

 

Thank all for the suggestions.

I think I will pull the PPS and TPS off of the throttle body and give them a good cleaning. If that doesn't work, I'll go the Automotive Scientific route.