Hello, T

his is my first post here.

A fairly butchered 1992 XJS V12 is currently in my shop. The previous shop it was at swapped a complete engine in the car and attempted to replace the wiring harness. They couldn't get the car to run and gave up on the project. When the car came in it did not idle or run in any capacity.

I have since rebuilt the fuel system and replaced the fuel regulator, sorted out the wiring issues with the harness, tracked down and fixed a series of vacuum leaks, replaced the sparkplug wires, and checked the quality of the cap and rotor. I have also re-synced the butterflys.

The car now runs but suffers from a low idle (cold idle of about 700) which drops after about a minute of running and then the car stalls. When trying to increase RPM the throttle response is weak/stutters and is followed by backfiring. I can't seem to track down the cause of this. Fuel pressure is strong while the car is running.

Any ideas?

Thanks for any and all help.

 

Almost sounds like it could be running on only 6 cylinders. Check distributor and rotor, ignition coils, and then ignition amplifiers on top of the radiator. A simple first check would be to put a timing light on plug wires on each bank to check for firing.