Now the weather is getting a little cooler, I thought I’d try the heat in my XJS and whilst it blew warm for about 10 seconds, it soon tapered away until it was ambient temperature. I removed the fuses from the blowers and can hear the servos move when I select heat and have checked to see if the valve at the bulkhead moves, which it does, so assume I have a blockage in the matrix? I’ve had a look on the forums to see how to flush it out, but I can’t see where to attach a hose to do it. I don’t suppose anyone has a picture they could post? Many thanks

 

Blocked matrix is fairly common.

I disconnect the existing heater hoses* and attach lengths of new hose to each matrix nipple, one a supply hose and the a drain hose....the latter routed in such a way to keep your engine bay from getting splashed with the discharge. To the supply hose you'll attach a nipple to connect to your garden hose. Turn on the water and slowly increase pressure. You'll get quite a sploosh when the clog is pushed thru. Repeat a couple more times, reversing the flow.

Can't hurt to fill the matrix with radiator cleaner the night before. It'll soften the clog, hopefully.

If the matrix is rotten there's always that chance of bursting it if you use too much pressure .....but that's the chance you take.

Cheers
DD

*Be very careful in removing the hoses; excessive force/twisting may well break the elderly nipples on the matrix. I prefer to slice the hoses off...having ordered new replacements ahead of time.

 

I've had a blocked heater valve do exactly what you describe. Somehow, a piece of plastic got wedged into the valve, preventing proper water flow. A little would trickle through, allowing the core to get warm, but as soon as the blowers turned on, there wasn't enough water flow thru the core.

 

Assuming V12, you can disconnect the line at the rear of the right cylinder head water rail; this is the feed to the heater and is easy to access. The return line connects at the radiator on the left side at the bottom (where the lower rad hose connects); this is the return line and where you want to attach your garden hose to back flush. Flushing at these points covers the lines and heater valve, in addition to the matrix.
If you cannot get good flow, check the heater valve first (along firewall behind and below the crossover pipe)- they are known to seize.

 

Amazing, thanks all!

Just went outside to have a look under the bonnet (hood) - there’s not much room to get to the matrix inlet/outlet, but I can see where they are now at least. Will tackle it on my day off.

I just did some quick searches of the forum and it looks like, with my car having the mkIII air conditioning and being right hand drive, it won’t be a complete catastrophe if the matrix bursts as it’s easier to replace than early LHD cars…..

 

When you are ready to refill the system, be sure to click on the Sticky" at the top of forum regarding V12 Cooling System Maintaining and download the .pdf. Make to follow the refill and bleed procedure as described in the .pdf, it's very important to follow this to avoid having air trapped. Please don't skip this procedure.

Cheers

PS - love the color of your XJS

 

I flushed the heater matrix, but it didn’t seem blocked at all in the end. I followed the bleeding procedure for the coolant and tried the heater, but still no joy. Checked both these pipes going to and from the matrix and they were both almost too hot to touch, so assume coolant flowing ok with no blockages or air pockets:



I also removed the blower fuses again and can hear a definite whirring when I adjust the temp and can also see the middle of the red potentiometer rotating and also feel the lower blend flap move. Odd thing is I can feel a brief whisp of heat as it rotates, so wondering if it’s moving the drum too far or not enough? Couldn’t get to the upper blend flap. Bloody car!

 

Will try cleaning up all the connectors I can get to next, see if that helps.

Not sure if this makes a difference, but there’s no air coming from the centre vent at all.

 

Quote:

Not sure if this makes a difference, but there’s no air coming from the centre vent at all.

In this case it's a good sign. The center flap should open only when the system is in cooling mode. It should be closed in heating, blending, and defrost modes.

Cheers
DD

 

Sadly there’s never any air at that vent!

Anyway, I may have to try and bleed the coolant system again - just took the car for a drive, as opposed to sitting idling on my driveway, and was getting lots of gurgling sounds from behind the dash, so assume there’s still air trapped in there.

 

Thanks! It’s Brooklands Green, I think. It’s a beautiful car and fun with the manual gearbox. Just as well really as it’s severely testing my patience at the moment.

 

Yes, beautiful color. Ahhhhh... the joys of owning a classic Jaguar.

 

Righto - I’ve bled the coolant again and pretty confident there’s no more air locks.

I tried bypassing the heater valve with a straight hose joiner.

Cleaned all the connectors. One was really green and corroded, but the others were ok.

Removed the speedo so I could check if the upper blend flap is working. Seemed fine. Lower is definitely doing what it’s supposed to.

The only thing I can think of is that the coolant isn’t getting hot-enough or some issue with the matrix. The temperature gauge in the dash has never read much above the ‘C’ but was told by a mechanic that it runs at normal operating temperature. Maybe thermostats stuck open or something.

Stuck for ideas now.

 

I think you have identified your issue, the coolant is not at proper temperature. Perhaps block the air flow through the radiator with cardboard and get the temp gauge to mid scale and see if you have heat. From your initial post, it sounds like your car is a driver- are there any other cooling system symptoms? If running below normal, fuel economy is going to suck more than normal, and AAV will probably keep the idle high.

 

Great, thanks.

It is a driver, but I only manage a few hundred miles a year in it. In attempt to use it more, including during winter, I thought I’d try and rekindle the heater!

Idle is fine I think, though MPG is pretty bad considering it’s a manual gearbox.

 

I think a good starting point is to verify coolant temperature. There are 2 channel thermocouple and sensor sets on Amazon as cheap as $25, you could probably slip a probe in at a hose connection near the heater and another at one of the upper radiator hoses and see where you are at. May be hard to get a good seal but for temporary use should be ok.
I bought a K thermocouple that had a gland nut with 1/8NPT threads, it was close enough to fit the bleed plug on the left radiator tank; gave an accurate reading of what that thermostat was doing.