I just bought a 1998 Jaguar XK8 and the heater was good and warm but seemed to be coming mostly from the lower vents when I had chosen both and when changing the selector only wisped out of the dash vents. Then on the drive it would cycle from warm to not so warm air and then near the end of an hour drive it was blowing only cold air. Any suggestions or clues would be very welcomed.

 

You probably need to firm up the diagnostics. There is an air box that moves air around the heater core. The air box is fed by 2 separate blowers. Each blower has its own re-circulation flap. You need to drive more, especially testing with the recirc on or off (press and hold to keep recirc on). Also, test with the air in the face. There is a diagnostics procedure built-in. Check the end of this doc:

climate control diagnostics PDF

Separately, the more common problem is the separate water pump that brings hot water to the core. They are known to fail (brushes wear out). Unfortunately, they are a pain to replace, so firm up the diagnostic before jumping in.

Over the years, Jaguar has changed the coolant spec, and the different types are incompatible (they form a gel). If that happened, there may be some clogging going on. Double check the coolant level, too, to be sure.

Last, thermostats fail, maybe yours is stuck open. Should be worst at speed. Get a cheap ELM327 and read the engine coolant temp to validate.

Best of luck, keep us posted.

 

Welcome to the forum elantoys,

You'll be in a very small and very lucky minority if you DON'T have engine cooling and heater system work to do on your 1998 XK8. Check the FAQ's for information on these related systems and recommendations.

Please follow this link New Member Area - Intro a MUST - Jaguar Forums - Jaguar Enthusiasts Forum to the New Member Area - Intro a MUST forum and post some information about yourself and your vehicle for all members to see. In return you'll get a proper welcome and some useful advice about posting to the forum.

Graham