I have an XJ-S 1988, V12 HE. I’ve recently done quite a bit of work on the car. When I got it, it was overheating in traffic.
I’ve replaced the radiator, thermostats, fan clutch and fan blades, and the car will now idle all day without overheating.
However, when driving, the gauge moved up off the N towards the H if I’m pushing it hard or going up a hill. As soon as I slow down or the hill levels off, it very quickly (within about 20-30 seconds) settles back down to the middle of the N.
I also notice the car still feels pretty slow - I was wondering before if this was a timing issue but I’ve had the timing checked and the garage said it was now ok.
I’ve previously checked the vacuum and centrifugal advance systems and they seemed to be working too.
One thing i’ve not worked out how to check is fuel pressure, I did wonder if pressure is dropping when the engine is under load and causing a lean burn?
I did wonder about the head gasket but there’s no sign of contamination to the coolant or oil, and I bought a gas ‘sniffer’ thing which is showing no evidence of combustion products in the coolant.
At no point has it got hot enough to throw coolant or steam out or anything like that.
Any tips on the next thing to check? I’m running out of things to replace!