01. XJR, travel8ng on the highway on cruise about 70-75. No issues prior, but the amber and red warning lights came on, abs/tcs unavailable and Engine Failsafe Mode atarted showing on the display, wouldnt go over about 25mph. Pulled off to a parking lot, parked and had a rough but fast idle, about 1250 rpm. Shut it down, waited about 30 sec or so and restarted, seemed fine. No check engine light.

Another maybe 45-60 min later it did it again, this time with a ceck engine light. Pulled over, restarted and again, seems to run fine, check engine still on.

This is about an hour or so of runtime. No temp spikes, premium fuel used. Nothing obvious to me.

I recently recalibrated my throttle cable, it read 98.2% wot max, dunno if maybe its tightemed up and threw it for a loop, but i havent floored it at all today. I did the right side valve cover and plug well gaskets, but haven't gotten to the left side yet. Right side did have oil in the plug wells, cleaned them out and assume the left is similar. Plugs are not new but were in good shape when i examined them.

I feel like this is likely an electronic or computer issue, not mechanical since it goes away with a reboot. Still new to me though, so what do i know?

Any thoughts? Still have to get home tonight, so i assume a couple reboots are in my future before i can get it in my garage and check the codes.

Thanks in advance!

 

read the codes

 

Entire trip home was uneventful, apart from the check engine light being on, you wouldn't know anything was wrong. After I dropped off my passengers I did notice that while idle was rock solid about 600rpm, when I pulled away from a stop sign at very light throttle it would fluctuate the power, surge, normal, surge, normal. Went into fault mode, restarted and it was ok, but stull fluttery at light throttle, so I just gave it more gas and kept it in the lower gears longer and it seemed to be more drivable and felt normal.

Got home, fine, plugged in the code reader, active code is P1121, which my reader says is TPS intermittent high voltage. I also have a pending code of P1111, which my reader says is an IAT sensor, but I beleive is just the "all clear" code, right? I think I can ignore that my reader is saying IAT high voltage?

AJ27_ECM_2001 obd codes document says P1121 is a Pedal Position Sensor, which I could see as a TPS, but I assume Jag would just call it a TPS. The description of the action seems to line up with what I experienced, failsafe mode, serverly restricted power, funky idle, no cruise control, TCS off.

I switched to live data and monitored TPS and a week ago when I adjusted the throttle cable, I had it set at 98.2% WOT when throttle floored and kickdown switch depressed, right now the max throttle is varying...its in the 90's still, but each time I floored the throttle I got a different percentage open, so something is wonky here, it should be very consistent I would thinkk.

Thoughts? I can back off the throttle adjustment a bit and see if it smooths out, but somehow I doubt it will or I wouldn't be getting the fluttery idle and that varying WOT reading, right?

So is it more likely a TPS or PPS in this case? I could see either being correct, assuming there is a sensor on the pedal itself, which I don't know if there is or is not, but I'm kinda thinking Jag wouldn't call it that if it didn't exist.

Thoughts? If I need to replace a part, are there brand recommendations? I don't see a TPS on rockauto, but I do see them new and reman on ebay, amazon, etc...if anyone has the correct part number I'd be grateful...assuming I need that of course. Or I suppose I can get it off the old one.

Thanks!

 

the whitebox $10 tps are just straight up junk as far as i know. idk if the later 32bit (x202 400 350 103 etc) tps works but these things are pretty reliable.

the APP sensor is cable operated like on some of the toyotas you might be able to salvage parts off one.

 

When I did the throttle cable adjustment I did a hard reset, but when i restarted the car I didn't get the fast idle and it didn't wander around like I've read it should have while doing a throttle relearn, so its entirely possible since the cable adjustment was to the PPS and the code I'm getting indicates the PPS, that I just didn't do my work properly. So my plan tonight is to back the throttle adjustment off maybe 1/4-1/2 turn, verify the throttle angle is always below 100% and then redo the hard reset and see if I can get it to calibrate. Drive it and see how it goes.

 

I cannot be all certain but let me share my experience. I had a 4 hour drive and car kept installing on Taconic parkway without much side shoulder, scary.

it was a bad TB and I sent to rebuild. problem solved.

 

Stalling? Like it would just die on you or go into a failsafe mode and keep running but rough and very low power like happened to me?

I'll keep the rebuild option in mind if I get to that point, thank you!

 

engine stalled/died. that is even a message on the dash. it kept dying evening driving on high speed.

 

Tonight I pulled the negative cable off and did the hard reset, puled the PPS and TPS plugs and sprayed them with electrical cleaner and let them dry out, cleaned the throttle body (it was fairly good, but a bit of buildup around the edges of the butterly which may not be a problem but it can't be helping the low throttle fluttering I felt yesterday. I also backed the throttle cable off a quarter turn on the off chance I bumped into a 100%+ reading here and there. Car pulls strong when I get on it, nice and smooth when I feather the gas, which is how it should be.

All said and done and reassembled I plugged the battery back in and started it up, got a high idle that wandered for a few seconds and then settled itself down. Let it idle for a bit and took it for a drive, seems ok, but too soon to tell I think. Check engine light is gone, but the 1121 code still shows on the computer as a stored code, but not pending. I'll drive it some and see if the code self-clears before I erase it. The throttle input on live data is still not rock solid so I suspect this isn't over yet, I'm getting a more narrow fluctuation in the WOT reading, varying by a couple tenths while I hold the throttle on the floor and kickdown switch depressed. I also cxhecked the switch and it seems secure and mounted properly as near as I can tell.

So now we wait. I drive a lot so it probably won't be long.

 

I posted my battle a looong time ago about P1121, let me explain….

each sensor has 2 pcb ‘tracks’ - tpps & tps.

Once a track is worn faults appear. Intermittent fault codes that become ever more present depending on the sensor and wear…

Back then there wasn’t an answer apart from sticking another used TB on there.
(Now there are rebuilders)
My issue was similar and obvious once I plugged an OBD in.

Slow, really slow depression of throttle pedal vs smooth percentage rise on OBD, then suddenly zero and back up to percentage readings meant the tracks had failed. A drop in signal even if momentary causes the ECU to go fail safe, and a hard reset ensues.

I swapped out sensors and built up a TB from lower miles parts to get it running reliably but this really isn’t recommended as live TPS sensor setting on a running engine is very dicey.

My suggestion would be to get a low miles TB and install it, and send the original for rebuild to Vincent’s ATE guys. Unfortunately they don’t ship internationally or I’d be using them myself.

 

Thanks Sean! Thats the direction I was thinking I'd probably end up going in if/when the issue returned.

Swapping out a TB with the sensors is just plug an play, then hard reset to calibrate? Different parts for AJ26 and AJ27 I bet?

 

2000, 2001, 2002, 2003 JAGUAR XJR THROTTLE BODY SUPERCHARGED | eBay

Not a bad price.