My 2004 Jaguar XJR heats up spiratically. One day it is fine the next it will heat up idleing or in slower traffic. Sometimes it will be days in between. What could be the possible problems and what should I check first?
First suspect would be cooling fan controller. Big silver unit close to main fans. To check correct fan operation, leave it idling for a while, (best to do this test after it has had a run) and fully warm, then check both fans run up - they should go really hard after a while, the air flow will gradually increase to try to maintain a constant temperature. If they just run at a single speed, suspect controller, if only one runs, then the other fan or controller has failed (put multimeter across fan terminals to see which it is). Another possibility would be air lock in cooling system, these models are not prone to this, but it is a possibility, undo water pressure cap and make sure you fill until water is spilling down overflow tube, keep going for another pint or so, quickly put the cap on whilst water is still flowing down the tube. There is a second container at the front where excess coolant is dumped and returned when cold. If this is not primed, you can sometimes get a small vapour lock which can cause problems. Less likely is thermostat, these tend to fail in a predictable way, but thought I should mention it if all else fails.
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