I have owned this 1991 XJS V12 for several years and have been fighting my way through all the things that I found wrong with it. This is my hobby, I have a 6000 sq ft air conditioned shop. more projects lined up than anyone can shake a stick at, and this was not purchased as a primary source of transport.
When I got it to where I was very comfortable with it's mechanical and material condition...I started taking it out for jaunts here and there, top down. I was out several weeks ago for a ride for approx. 40 miles each way. It has been very, very hot this summer here in Florida. The car performed well...except for getting back into it after a short spell.....it was difficult to start. It ran well within the temp range, not even rising due to the high ambient outside temps. The first thing I thought of was vapor-lock. It will start eventually, but it does NOT come back banging on all 12 cylinders....it creeps up gradually. It then drives fine.
I'd appreciate ALL theory's and repair approaches.
Thanks, Tony

 

May be vapor lock in the fuel lines. If your fuel temp sensor is working the car will increase fuel pressure to try to clear it.

Pull the vac line off the left side fuel regulator to force this.

Your ignition amp may be overheating and misbehaving electrically. You could cool it while in the failed condition and see if that helps. A can of dust off turned upside down will discharge compressed liquid which will rapidly cool anything it's sprayed onto. A bit harsh and rapid temp changes are not best for anything. You could also just relocate it to a cooler spot and see if it resolves the issue.

Jump out the coolant temp sensor harness plug to remove it from the equation. If it's thinking it's got a cold engine it can overfuel it.

 

JigJag - If the coolant temp sensor was bad, or going bad....wouldn't I see it act up on start up?? The revs are up where they should be, say 1500 and come down smoothly as it warms up.

 

I'm having this issue as well. In fact it doesn't even have to be hot outside. Mine will act this way after a full warm up and shutdown after it sits a few hours. I've checked the readings on my coolant temp sensor per the service manual. While they aren't spot on, they're close. I'll be following this thread for ideas. Looks like I already have two to try.