I picked up a 1987 V12 XJ-S last week, and now that I’ve been driving it for a bit I’ve noticed there’s nowhere near as much power as I’d expect - 0-60 is taking 13+ seconds! It’s like you get increases in power upto about 50% throttle, then further increases make a lot more intake noise (its had the air filter inlets modified to be noisier) but doesn’t appear to produce any more power.
It also seems to miss slightly at idle when it’s warm. Idle seems to be either 1000rpm or more like 300rpm depending on what mood the car is in - I suspect the AAV is defective!
The previous owner barely drove it and didn’t service it in years, which may be a factor.
I decided to service it, and when I drained the coolant I noticed some gunk under the filler caps and the coolant was rather cloudly - oil was pretty horrible but no sign of coolant contamination. Bought a test kit for combustion gases in the coolant and it reads consistently negative. I did a compression test and most cylinders showed around 200psi except one which was more like 175psi, but I don’t think one cylinder at a lower compression would cause such a significant power issue.
So far I’ve replaced the fuel filter, checked the TPS (it was slightly out, 0.25v at idle, so adjusted), checked the throttles are set correctly (they seem to be), checked the centrifugal advance wasn’t stuck (it wasn’t) andchecked the coolant temperature sensor.
From reading the forums I’m wondering if this a fuel pressure issue, or an ignition issue (it’s still on the 2-coil system and the spark plugs look quite old). I have noticed a fairly loud hiss from the LH bank at idle but not been able to locate an obvious vacuum leak.
Aside from the slightly rough idle when warm and lack of power, the engine seems to behave itself, starts up fine etc. Idle when cold is fine, and it passed the MOT emissions test last week with flying colours.
Any thoughts would be greatly appreciated!