I'm wondering if anyone here can provide some insight into where I should be looking for the issue on my car.

89 XJS just got back from the shop, new dist spark plugs etc coolant hoses AAV oil change filters etc. The car ran flawlessly in the rain yesterday, and earlier this morning however upon driving around town it seems to just stall - usually at a red light or lower speeds in fairly warm weather.

After towing back to the house I noticed it seems to idle much longer (30+mins) with the hood open and the hot air blasting - I changed out the oil for 20w50 as well which seems to keep the pressure much higher at idle.

The temp gauge moves to the middle/top of the N when I hold the pedal down for awhile, then drop back to the low side of N, the bulk of the ignition system was just replaced at a british repair shop, it has new coolant (although I'm sure it needs a flush with chemicals)

What causes a V12 to run perfectly for 100 miles then die randomly then start up fine several minutes later?

 

I have to ask, coz the causes/answers will be different.

1989 is the change to Marelli Ignition system, as versus the Lucas of the earlier cars.

Which one is yours please??.

 

Marelli

 

Decent chance the amplifiers on the radiator cross panel are having hissy moments. Worth replacing, also worth replacing the two sensors (flywheel and front of engine by pulley) if they have not been. Almost bound to be electrical thingies getting hot and stopping. All of this sort of replacement is worth doing for general reliability, anyway.
Also check carefully the state of the loom between the amps and the coil.
Greg

 

Marelli is 2 separate I6 systems for ignition, unlikely (but not impossible) for 2 coils or 2 modules to fail simultaneously.

The Coolant Temp sensor is a likely suspect as are the front and rear sensors on the crank. Unplug the CTS and the engine will die. Check the wiring and connectors on both of these. As work was done on the top of the engine I would start with the CTS.