This might be a bit long but hopefully will have enough detail for someone to point me in the right direction. Let me start by saying that I don't usually do my own repairs. I use a little local garage where the mechanics are superb but this particular problem has them a bit puzzled.

A couple of months ago, in the course of just doing some routine checks, I noticed that my coolant level was a little low (about 1cm below the max mark on the reservoir). I topped it up and made a mental note to keep an eye on it but less than a week later, the car's heating suddenly stopped working and when I looked under the bonnet, the coolant reservoir was completely empty. I topped it up and got it straight into the garage where they found a split in one of the hoses and fixed that. When I got the car back, the heating was working again - for about an hour! After that, no heat but the coolant level was fine and the engine temp was normal.

I got it into the garage again where they cleared an airlock. I got it back with the heating working but over a period of 3 or 4 days, the hot air became gradually cooler until there was no hot air at all. If I put the climate control on automatic and set the temperature right down to LO, the fans come on full blast and the air conditioning is definitely working. I get icy cold air. As I put the temperature up from there, the fans slow down, then speed up again and instead I get air at the outside temperature. Occasionally when I do this, I get lukewarm air until the engine is stopped and restarted, then it's usually back to cold air.

Throughout all of this, the engine temperature has seemed fine (gauge comes up to the half way point and never gets any hotter) and since the hose was repaired, the coolant level has remained on the max mark without needing topped up.

The guys in the garage haven't been able to put their finger on exactly what's wrong. I don't know what all they've checked but they've said they suspect it might be a control unit. They didn't want to go ahead with an expensive job that might not fix the problem. My last resort is probably to put it into my nearest Jaguar dealership to see if they can diagnose it but if I can avoid the expense of that, it would be ideal.

Would someone be able to guide me on how I can narrow down the source of the problem? Bear in mind I'm no expert on the inner workings of this car but if anyone can give me some guidance, I'd be most grateful.

 

There are a couple of possibilities for your problem. Below is links to other threads dealing with the same problem. It could be any of the problems and solutions found in the threads


https://www.jaguarforums.com/forum/x...2000-a-157376/

https://www.jaguarforums.com/forum/x...-vents-154169/

https://www.jaguarforums.com/forum/x...s-cold-127794/

 

The X type cooling system is a simple one, with no separate heater water pump or solenoid valves to worry about. Failure of the water pump impeller blades is not a common problem in these engines.
You may have a blend door problem in the cabin blower unit. The attached TSB will guide you through fixing this if it is your problem. Note that it applies to a LHD car, so in your case you won't have to remove the glove box - the blend motor is on the driver's side.

For all that it is possible that you still have an airlock that's being stubborn!

 

Thanks for the suggestions so far. I've had a busy weekend so I'll try to find time this week to read through those other threads properly and see what I can find. It decided to give me hot air today out of the blue, so it's a little hard to debug when it's randomly working.

 

OK, so I got back to the garage and asked them to follow the instructions posted by astomorg, but for the temperature valve in the blend valve. While it didn't fix the problem, it helped them pin down the fact that the actuator was faulty. A new one has been fitted and I now have heating which is both reliable and controllable again.

Thanks again for your help.