So what would be the correct steps to diagnose the problem?

The water is getting hot in the engine and the levels are OK, I turn the heat up to max and run it with A/C on and off, no hot air. If I boot it up to about 90Mph it starts to come through hot until I slow down again.

Not sure where to start looking, so would love some advice.

 

Probably a clogged heater pump.
The pump is located in the engine compartement, left side.

Here are a few pics:
http://www.gusglikas.com/AutoRepXJ6heaterpump.htm

HTH,

David

 

The heater pump or valve could be out or the heater matrix may be clogged.
The heater motor brushes can be replaced, the valve cleaned out and the matrix flushed or replaced.
Check the boards.

 

Thank you chaps. I will look into these things.

 

I have located the pump and it doesn't appear to be vibrating when I would expect it to be running. I.E. ignition on, temp set to max and fan running.

Am I able to remove it and still drive the car? I would like to take it to a local auto electrician to check the brushes and winding but only have the one car.

 

Get yourself a piece of tube that matches the hose inner diameter, remove the pump, plug the tube in between the hoses fix it with hose clamps and you can still use your car.

 

Found a re-con unit for £59. Should be here tomorrow.

 

Where did you find it Kev? I don't know how much a new one is, but that's reasonable, do they do recon wiper motors too, they're known to fail on the early cars.

 

I am having exactly the same problem as BigKev with my 1999 XJ8, including the improved performance with increased engine speed. BigKeg, please keep posting with your progress. Thanks.

 

This is where I got it from:

http://www.jaguarspares-northeast.co...%26aid%3D43%26

They do them for X300 and X308. Different bracket apparently. I had to pay £15 deposit on the exchange unit.

Not sure about wiper motor, check out the website.

 

These heaters are dying like flies, check the other boards.

I wonder how many went in the summer when we never noticed?

Regular heater maintenance could become advised for Jags of a certain age?

 

Mine went last winter and I barely survived the cold. I forgot about it during the summer and now that winter is here again, it struck me. I pulled the hoses off the valve and flushed the heater matrix by pumping water through the lines both directions. I connected them directly to one another, bypassing the valve, and I have sweltering heat, but I can't modulate the temp (it is either hot or off). I had the valve bench tested and it failed. I am picking up a new valve this weekend and will replace it.

 

I can confirm that replacing the pump has fixed the heating issue.

 

Is there a write up on how the hvac systems work in these cars? Does this pump take the place of the blend door in other cars? Is heater matrix just a fancy word for heater core?

 

I think the glib answers to the last questions are no, sort of and yes. On most (at least, American) cars, hot coolant goes to the heater core all the time, and a door blends heated and unheated air to deliver to the inside of the car. In jaguars, for some reason, the regular coolant flow is insufficient to the heater core, and must be boosted. In addition, there is a mixing valve near the pump. You might say that instead of a blend-air system, there is a blend-coolant one. I don't know that it's better, but it certainly is more complicated.

 

Sounds like it my man. Thanks for the quick clarification.

 

I have a 1998 xjr v8, there is a fuse for the pump that you are on about ,i have had to replace mine before, its in the fuse box under the bonnet.