It sound like the throttle is not returning to fully closed when the cable is connected - perhaps a build up of deposits inside the throttle body or weak return springs. Have you cleaned the throttle body (inside)? With the engine at high idle can you lower the idle by “snapping” the throttle shut or turning the butterfly spindle by hand?
Frankie
Thank you very much. I will try spraying some break cleaner around the spindle tomorrow. I keep thinking there must be something really obvious that I am missing. I guess I’ll find out soon enough.
I have tried pushing the throttle shut and snapping it shut, but it is restricted by the throttle stop screw, which is absolutely seized solid. This makes me think that it might not be the position of the linkage that is the problem so much, as it looks as if it has been seized for years and I don’t imagine it has been idling that high for that long as the previous owner was a Jaguar specialist. I am also fairly sure that the problem is not due to a build up of deposits, as my cousin (who I found the car through) dismantled and cleaned the inlet system a few weeks ago, and he hasn’t had any trouble with idle. Who knows. Thanks for the help though
It could be the throttle potentiometer or the idle air control valve/stepper motor. If you are sure that cable disconnection allows idle to return to normal but cable connected but slack raises idle, logic suggests that something in that operation causes the issue to occur. Trial and error may be able to narrow it down.
Frankie
I will try as best I can to test everything. Thanks very much
I’m going to recommend that you remove the throttle body and clean it thoroughly, on both sides, being careful not to let the carb cleaners collect and damage the TPS on the bottom of the butterfly.
If that doesn’t help, then you will need to reset the TPS. Its easy to do with a WDS, PDU, Mongoose, or any other diagnostic device with Jaguar software, or it can be done by hand. It would require you to loosen the TPS just enough that it can be adjusted by hand with some effort. Once you get the throttle body back on the car, get the engine started, and hot. Then by hand, adjust the TPS until the throttle is just right, then tighten it.
Those last two parts happen when the parts are all on the car, so you’ll essentially be doing them by feel, “in the blind”.
Thanks for the help, I have cleaned my throttle body and it is all back together, however I cannot see how to adjust the throttle position sensor. I don’t know if you could give me any advice on how to do this. Thank you.
The picture shows the idle air control valve. The Throttle Potentiometer Sensor (TPS) is underneath the throttle.
As mentioned before when you remove the throttle body for cleaning, leave the two torx screws just loose enough that the TPS can be rotated, with a little bit of force. Then reinstall the throttle body and adjust the TPS (without being able to see it) until you get the idle right, then tighten the torx screws (again, by feel, since you won’t see it) a little more to lock it in place.
Finally! The problem is fixed. I eventually got an X300 specialist to come over and have a look. He scanned the ECU (and cleared some past faults) and found the fault. The ECU wasn’t getting any signal from the TPS. After checking, it turned out that there was no continuity from one end of the wire to the other, and after taking the injection cover trim price off, he found a frayed and broken wire. He fixed this and, hey presto, 700rpm idle! He also said he hasn’t seen that problem arise in that exact manner before, so first time for everything.
Thanks for all your help on the forums and I hope someone out there finds this useful.
Cheers
Thank you for coming back to close up this thread. So many people start asking for help and never come back to tell everyone how the problem was resolved.
Joshua you could go to the heading and bracket (resolved) to show people that there is an answer.
