No heat in my 99 XJ8

You don't say... can we assume the there is cold air coming out all the vents inside the car?

I'm not familiar with your model, but there should be a valve controlling flow of hot engine coolant to the heater core. Hopefully the dealer checked it. Usually it is a squat metal cannister valve with a tube that is spliced into one of the two heater hoses that go from the engine through the firewall. It has a small vacuum hose attached to it.

Look around the engine compartment for the pair of heater hoses and find the valve. Check to see that BOTH hoses feel hot at the firewall when the car is warmed up and the heater is on. If they aren't, then check to see if there's vacuum at the small hose on the valve... if you have one cold hose and there is vacuum at the valve, replace the valve (as a temporary measure, you could replace the valve with a short length of copper tubing). If no vacuum signal, then the vacuum hose has to be traced to find out where/why it lost vacuum Beyond that you are into checking vents and switches (eg, switch, disconnected hose, deteriorated hose, etc). and I can't help you much there.

Hi John,

It might sound a silly question, but are your front fog lights working OK? If not, you probably have the same problem I had....

I lost all my heat although everything was OK. Engine temp OK etc.

It turned out to be the fuse had blown that also protects the front fog lights and alarm security sounder.

The sounder itself had packed in due to water ingress and as soon as I disconnected it, put in a new fuse, I had heat again.

If yours is the same problem, you will find the sounder tucked down the passenger side of the engine radiator.

A new one fiutted by Jag Main Dealer cost just short of £100.

Adrian .. your spot-on.

This is a common fault. The wiring diagram (at least on mine) shows no connection between the Heater Pump and the fog lights and security sounder.... but there is, and as you have written.... change the fuse and the heater pump will run agian giving heat to the interior.... until the first time the sounder 'sounds' or you use the fog lights and it lifts the fuse again.

Ian

Adrian Higd> Hi John,

Hi John,

Pleased that it is solved for you, we all need some heat in winter.

Regards Adrian

This is a common fault. The wiring diagram (at least on mine) shows no connection between the Heater Pump and the fog lights and security sounder.... but there is, and as you have written.... change the fuse and the heater pump will run agian giving heat to the interior.... until the first time the sounder 'sounds' or you use the fog lights and it lifts the fuse again.

Ian

Adrian Higd> Hi John,