This is a 98 XJ8.
My daughter has been driving this car so this issue hasn't happen to me.
While driving at a moderate speed and quickly lifting off the throttle, as if pulling off the
freeway or lifting for traffic, the throttle when push down will not accelerate. The engine is still idling but won't accelerate. The way to get the accelerator to function again is to turn the engine off and then restart. Then it will operate normally again. It happens intermittently a couple of times a week on average. The are no codes or check engine light.
I leaning towards throttle position sensor but would like your thoughts.

Thanks, Bob

 

Engine did not rev up either ?

 

No. Just keeps idling. As if the throttle cable was broke.

 

I’d start by checking the TB relay. You could swap it out with a fog lamp relay if the relay contacts looks iffy. If you haven’t done it already, now might be a good time to open up all the brown 5 pin relays to check their condition. Plan on the A/C compressor and the starter relays to be toast. Since you have multiple AJ26 X100/X308’s, you could swap a TB out of there? It also couldn’t hurt to do a hard reset and unplug the ECM and plug it back in for a possible micro-corrosion. Within the last 6 months, my daughters 98’ XJR was throwing P0172/175 B1 & B2 rich codes. Fuel pressure was good, MAF was good and air filter was good. I had my Snap-On scanner with live data for STFT/LTFT. While the engine was running I literally lifted up the ECM and the fuel trims went back to normal. Albeit the engine wasn’t running bad persay but, it ran noticeably better after I lifted it up and sat it back down. I’m sure you already know that these AJ26/S ECM’s can and will fail due to capacitors(??) inside. Oh, one last thing... when the engine is **cold**, remove the air intake tube from the TB. Have her go inside the car and turn the key to position 2 but *do not* start the engine. While looking at the TB butterfly, have her push the accelerator all the way to the floor. The butterfly should open completely vertical. If it doesn’t, either the throttle cable needs adjusted or you have several of the roller bearings that are seized inside the throttle shaft of the TB. If the bearings are seized, it’s possible that the amount of amperage to force open the butterfly may be too high and cause the restricted performance light to come on. This stuck butterfly will be more pronounced when it’s cold vs fully warmed up. Yes, it’s still possible that the TPS could still be bad. Years ago I did a detailed post of rebuilding the AJ26 TB.

*Edit* I thought I had pix within this post below...

https://www.jaguarforums.com/forum/xj-xj8-xjr-x308-27/1998-xjr-throttle-body-concerns-159443/