156,k miles on it. Was driving home last night from work on the thruway, cruise at about 80, display lit up with amber and red lights and TRAC unavailable, ABS unavailable and a few cycles between those and it added Engine Failsafe to the list. Cruise stopped working, car kept going but was decelerating. I made it off the next exit and stopped at a hotel parking lot, shut it down and let it sit for a minute, then restarted. High idle, stumbling almost like a misfire, but no flashing MIL, MIL was lit by this time. Opted to call my wife for a lift and let the hotel know I'd be classing up their lot for the night and I'd retrieve it with a trailer the next day after work.

On my way in with the pickup I stopped to visit it and pulled the codes to get a head start on diagnostics. Showed a P1121 code, which I believe indicates a bad output from the pedal position sensor, correct? Best permanent solution is get the throttle body rebuilt or replace it with a rebuilt one? Is there a good interim solution that gets me back on the road? Little uncertain if the TPPS is a relatively easily replaced part, but I get the sense it might not be from another thread I found.

Throttle body is relatively straightforward to remove I think? Are those big hoses on it coolant or is everything vacuum?

I have a decent code reader that lets me read live data, so I can check to see if throttle percentage is all over the place later as a confirmation. I had a few run ins with the throttle body wigging out on me about a year ago, rebooting the car addressed it each time, it happened a handful of times and then vanished, but now I guess its back with a vengeance, which honestly isn't too shocking I suppose.

One last bit of info, I've just become aware that I have a very small coolant leak, seems to be coming from the bottom of the overflow tank, to me it looks like maybe the seal on what I think is the coolant level sensor is leaking a bit...since its right next to the throttle cable, maybe this is related, maybe not. I became aware of it when I opened the hood after a drive and saw a tiny puff of steam rising and a small bit of liquid pooled in the frame rail adjacent to the rear of the left side cam cover...I assume a few drops of coolant are getting on the exhaust. Coolant level is good and I did not overheat. Related or not, obviously I have to fix this too, is the a new tank with sensor or can I just get away with maybe replacing the sensor? Probably have to drain the tank and pull the sensor to determine if its just a bad gasket or o-ring or if I have a cracked tank I guess?

 

few things. First I would try disconnecting and cleaning the throttle position sensor and pedal position sensor (if the 00+ cars use them). You can replace the TPS, but may make sense to clean it first. The throttle bodies, while overengineered, are not really that complicated once apart. There are several coolant lines to the throttle body, which is what makes it a bit of a pain to remove and reinstall. There is one line on the drivers side, and another on the bottom of the passenger side. Other than that, its a few electrical connectors, and 4 bolts.

As to the coolant sensor. Its a magnetic setup so there is no seal between the external sensor and coolant, its separated by the tank plastic itself. The tanks have a tendency to leak at the seams when hot and under pressure, but it could also be any of the numerous hoses in the area. Probably best to pressure test it and look for leaks.

 

Talking Throttle Bodies, if you do replace it then it doesn't matter if the new one is off a supercharged car or not. The main part of it is the same, you just have swap a small (2" square ish) plate on the back & the throttle cable bracket. I know as I put an NA one on my XJR but doing just that.

 

If you determine it is probably being caused by worn tracks on the Pedal Position Sensor (erratic readings over range, fault codes), then this part is replaceable with a refurbished part (no oem available as far as I recall) or you can get your throttle body rebuilt with refurb PPS amongst other items.
- e.g. autoecu.com or similar (sometimes their ebay listings are cheaper than through their website)
There is a procedure in IDS to reprogram the idle/resting position for the PPS - if the resistance falls below a threshold then the engine goes into failsafe. It is unlikely in my opinion that this is your issue, but just mentioning it.
- If the idle portion of the track(s) is(are) worn then this could be a short term workaround along with cable adjustment, but only if you have a solid/stable IDS system that you can reprogram with. Or you can 'adjust' the output of the bad track with an arduino for those 'versed in the art'
- Similar form factor PPSs are used on Toyota, etc. and much much cheaper, but they rotate in the opposite direction and have different resistance tracks - so after some investigation I concluded time and money on trying to adapt these is likely not worth it

 

Update.

Hard reset and throttle relearn and I'm back in action. At some point in the near future I'll send it out for a throttle body rebuild, just needed to get past the holiday season. I'll probably replace the pedal cable while its all out too.

Thanks all!

 

Got past the holidays and have been living with the occasional car reboot when the throttle crosses its eyes, too little time and too much money going in other directions to address it at the moment.

Bit of bright news however, used a Christmas gift card for Amazon to puck up a replacement coolant overflow tank, swapped it out over the past weekend. Took me a bit as I am utterly paranoid of the hard lines coming off that tank and don't want to deal with having to replace any right now. Have had it out for a few spirited runs and I once again have no coolant leakage, hooray!