Had a situation where I got heat briefly after turning it on, (87 XJS with Delanair Mk2), but soon blew cool. After checking that all automatic and manual controls where working properly, vacuum operated flaps were flapping, bleeding the system numerous times, even using vacuum evacuation, nothing improved to situation. Finally I pulled off the heater control valve, a generic plastic replacement, I saw something in one of the ports. Got it out, and it was a piece of curled up plastic as shown in the picture. It was one piece, but I broke it trying to figure out it's composition. No idea how it got in there, as I have a filter inline before the valve. May have gotten in during manufacturing. I then back flushed the heater core because I was there, but got nothing of note out of it. Bolted everything back together, filled and bled the system. System fixed. Four days of trouble shooting for something so basic.

At least I learned a lot about how the Delanair system works, and not to overlook the simple things.

 

Should have said Delanair MK3 system.

 

I absolutely love to hate finding stuff like this...

Great find. Luckily, you didn't go any further. Like the deep deeeep dash dive - or sumpthin. Good stuff...

So was it blocking liquids, or the mechanical functioning of the valve - or both?

 

It must have been blocking flow since the normal position of the valve is open, except when a/c is on or near max. It didn't have vacuum going to it, so it had to be open. When I pulled it, it was open, as expected.
I had investigated a LOT of other possibilities. I thought at first is was a blend door actuator issue on the Delanair unit, but once I figured out that the air was starting off hot then cooling rapidly, I figured it had to be a coolant flow issue thru the core. I'd back flushed the core previously and have filters in the cooling system which were clean, along with all new hoses, so that left the valve. Frankly, before I pulled the valve and found the debris, I found it hard the believe it was the valve, because it's normally wide open. Even after finding the debris, I had a hard time believing that was the whole issue as it had to be in just the right place to block flow. But removing it cured the problem.