I'm having an odd heating problem in my 2004 XJR. I had a stuck open thermostat, which I was blaming for the engine not reaching operating temperature and no heat in the passenger compartment. This was replaced in October and since then the engine temperature has been right where it should be. However, there's still not much heat. The first two or three days after replacing the thermostat the heat was good, nice and warm, but it faded away after that. Thank god for heated seats. A couple of weeks ago I tried to see if I could find anything wrong and noticed that the coolant was a little low in the expansion tank. I topped it up and then there was heat! For another two days. On about day four I checked the coolant again, and there was no change in the level.
The heat is currently set at 80, and the air coming out of the vents is in the 50s, if I had to guess, certainly no hotter than low 60s. It's currently 3 degrees out, and I'd really like the heat to work.
Has anyone experienced similar problems? Thanks!

BTW, the coolant is perfectly clear and the engine is very strong.

 

A couple of things to check as you may still have air in your system and my assumption is you don't have a leak as you don't mention it.

Put the front end on ramps, close the vents and put the heat on high until you reach normal operating temp. Shut engine off and let cool. Burp the air from the system by gently loosening (SLIGHTLY) the bleed screw on the reservoir. Alternatively you can loosen the SC fitting. Do this a few times as long as you have air.

If no air is coming out, turn on ignition to setting 2 (DO NOT START THE ENGINE) Set the heat on high and turn on the cabin fan. In the engine bay, reach down and feel the auxiliary heat pump. You will feel it running. It's a common failure and there are many threads on replacing or rebuilding

 

Sorry, I don't understand.

Which reservoir?

What's the failure and what needs to be rebuilt? Do you mean the auxiliary heat pump has failed if you don't feel it running, or the main heat pump has failed if you do feel it running, or what?

 

The 'reservoir' is the coolant header tank above the radiator, where the pressure cap is located and where you would re-fill or top-up the coolant.

The auxiliary heat pump is an electric-powered pump that enhances the flow of hot coolant through the heater. The main pump is, of course, the engine-driven coolant pump at the front of the engine. At low revs or tick-over, the main pump doesnt generate enough flow to keep the heater hot, especially for places where it gets really cold. Funnily enough, the V6-engined cars don't have this auxiliary pump. They weren't sold in the US.

If all the checks mentioned don't work it could be a clogged heater matrix, and whilst some people have had success with citric acid solutions to clear it, often only a new heater matrix works. Replacement is not a hugely costly job, but a nuisance as the coolant has to be drained.