Just replaced the ECU after the mechanic and a rebuilder company said it was no good. Anyway I put in a used ECU with the same matching numbers and engine specifications. When I received the car the idle was 1,500 but I just figured it needed to relearn idle. It also tries to stay at 1,500 while in drive. Needless to say the car doesn't coast. Should the ECU be replaced again or possibly reprogrammed. The mechanic just plugged it in and said it's normal for it to idle high when the battery and ECU were disconnected. Any help is appreciated. -N

 

I'm not the expert, and others here are.
But the TPS (throttle position sensor) may not be properly 'zeroed' or returning to it's idle position. Also, air flow meter may need checking and the two oxygen sensors on the exhaust downpipes may need to be checked (lambda orientation test, I think from memory). Although oxygen sensor disorientation would probably make for rough running rather than high idle.
Can you find a local mechanic with experience with X300 Jaguars? Not always easy, I know..

 

The ECM needs throttle adaptation with a WDS. You should also perform an Oxygen Orientation as AL NZ stated. There are threads on this procedure if you do a forum search.

There is talk that you can cut some slots or drill out the mount holes and move the throttle clockwise/anticlockwise and try to get the idle correct (like the AJ6 throttle). I have never tried it with AJ16 because I have a WDS so there is no need.

bob