Cheers! I'd contemplated that one - so it's getting hot but not all over, & when more air's blown over it, it cools down / can't heat it all...?

Worth trying to flush it, or am I looking at a swap? I presume that's as horrible a job as on most other cars?!

 

From what I have read, it might be worse. If it happens to me, I'll either stay home on cold days or drive my XK8. Not many really cold days here.

 

Definitely do a flush. Keep at it until you get it cleared. Our heater cores are more prone to plugging because of the design of the system. Unlike a traditional closed loop system with a heater valve opening and closing to restrict the flow of hot engine coolant through the heater core, ours uses an open loop system where gravity keeps the flow of coolant inside the engine compartment and relies on a pump to defeat gravity and pressure coolant up the "Spaghetti" hose and into the heater core. The problem is that after a bit of non use of heat, air is naturally reintroduced into the heater core circuit. Air-Moisture- & Metal = rusting/clogging.

The standard pump rates at about 180gph. I replaced mine with a cheap $35 240gph standard use marine pump so that I'd have increased flow & pressure to provide a better "flush" when in use. When the weather warms it's a good idea to turn on your heater once a week or so, for a few minutes, to keep your heater core flushed. Turn it on right after start up before the motor is producing heat, and you'll not even be bothered by it, but your heater core will thank you.

 

I have heard of flushing the heater core with a garden hose, would that be your recommendation?

 

That is what I would do. If it doesn't work you could try pouring some radiator flush in and let it stand in hot water before flushing.

 

Anyone ever try using Borax in the coolant system? Supposedly it flushes and cleans the whole system and is quite beneficial.

I, however would be reluctant to try it without a little discussion first.

FTR, I had similar heater problems with my 95 earlier in the fall. Temp gauge was only getting to just below N. Hardly any heat to speak of at all. Changed the thermostat and it was fixed in a flash.

 

I recall boric acid being used, but not borax, which is sodium borate. I don't like using chemicals in the cooling system of an aluminum engine. Flushing the heater core is one thing, but trying to flush the whole cooling system is not easy.
The use of acids was fairly common when tap water was mixed with antifreeze, the deposits due to hard water were dissolved that way, but I would hope that our Jags have distilled water in them.

 

Flushing the matrix out seems to have done the job

The water coming out looked pretty grim, but it flows well enough now and keeps me a lot warmer!

 

I'd be interested as to people's views on the various cooling system cleaning products you can get. I'll be using water from the dehumidifier (paying for it's just daft!) but from the state of it now, I suspect a previous owner preferred used dishwater...

 

I'm in Canada, so not sure if these products are available in the UK, but this is what I use:

Cooling System Treatments | Car Care | Products | Prestone® Canada

I use the Super Radiator Cleaner for each coolant flush as so far I've not bought a really neglected used vehicle. If so, I'd use the Super Radiator Flush.

Old style acid coolant flushes worked great but can degrade aluminum components much more easily so these aren't recommended for modern cars.

It is impressive when you drain the first bit after using a coolant flush: "was there really that much gunk in there?" BTW: water from the dehumidifier? Very ingenious! I'm on a well for potable water, so I have to buy deionized water for cooling systems and battery maintenance.

 

Finally got round to doing this - it was filthy, but it's nice and quiet now and surely must be working better too! Yet again one thing crossed off the to-do list to be replaced by another though. The duct/vent that should channel air onto my feet is missing! No wonder I've got cold toes...

 

Footwell Air Ducts - Parts for XJ Series from (V)720125 to (V)812255 (X300) | Jaguar Classic Parts UK

Looks like part 2 is what I'm missing? 3&5 look like they go towards the rear footwell? Typical that they've got the LH available but not the RH!

 

Vent fitted. Mmmmm warm feet. Perfectly happy with my heater now - eventually!