So recently picked up myself a 1992 Jaguar XJS with the 5.3L V12 engine. The one and only issue that prohibits me from even driving it is that on startup, wether warm or cold, it decides to rev to full throttle and just sit there. I've done a bit of research and not finding much. This most likely is an easy fix, just don't know what it is.

 

Maybe an iacv issue but it sounds like you just have a big honkin vacuum leak somewhere

 

Assuming you've checked for a mechanical problem (something physically holding the throttles open) I agree that you have a very big air leak.

Do you say 'full throttle' literally? Like, the engine revs to 6000-7000 rpm?

Are the air filter housings is place? if they are removed, and the bolt holes left open, you'd get a very high idle

Is the 1" diameter balance tube at the rear of the engine in place?

Cheers
DD

 

. . . and, really basic, check your throttle return springs. Is the accelerator pedal "flat to the floor"?

Cheers,

Ken