My heater works well when it's behaving. But not infrequently it starts imitating a pneumaticly activated accordion-playing monkey.

Flaps open and close randomly. Wheezing and whirring sounds try to accomplish some task that they simply aren't capable of. Air shoots out the defrost then the dash then both then FLAWAP! Just the dash. Then the feet, kind of.

I know the XJ6 jags use a complex mechanical computer style control. I'm pretty sure there's one under the XJS dash somewhere too. I assume that I have a vacuum leak under there and the system just doesn't work correctly when manifold vacuum drops. I am planning on buying some silicone vacuum line and fixing it good. I have a couple questions.

1. Where is the dash release lever?

2. How many meters of what size line will I need?

 

Dash release lever, HAHAHAHA, about 15 hours to remove it all on a good day.

I luv a Jag owner with a sense of humour.

I would first and foremost check the one way valve in the vac supply hose under the bonnet. They are plastic and fail, and when the vac drops, as you have said, the innards do strange things.

Your car is the Delaniar 2 version, and yes, there are vac and servo controls all crammed under the dash.

I would also be checking the 2 TINY setscews that secure the RH mode switch to the metal frame of the dash, they come loose, its an age thing, just like us humans gets loose as we age. Those 2 screws being loose allow mis-alignment of teh micro switches behind that panel, and can also reek havoc with the system. That little exercise is about 2 hours of fun, and has fixed all but 1 for me over many years.

Hose sizes, no idea.

How many metres, no idea.

The supply hose inside the engine bay would be the first place I look at. It is the larger of the 2, with the smaller hose being the supply TO the heater tap from the internal system.

 

If the checks that Grant mentions do not fix it, you might look under the blanking plate where the pedal box fixings would be if your car was RHD, under where you probably have the washer reservoir. Under this plate (four bolts one at each corner)you will find the vac reservoir bolted up to it. This might have failed (it should audibly hiss when you pull off the vac supply pipe) or the vac line to and from it may have perished or got holed.
Greg