Heater tdi 90

Hi, My landy 90 heater is still cold... we have flushed the heater matrix, put in a new thermostat and made sure there is no air lock. The fan is working and it goes warm for a while then blows cold again. The engine is at the correct temp., could this still be some kind of air lock in the heater? Can any of you out there please help, had the same problem and found some obscure fault. Many thanks.

air lock in the heater?

i recently taken my heater box out to replace the heater motor, and didnt even bother about any air locks, just connected the hoses back to the matrix and that's it...

and it blows hot, hot air!

Pipes the right way around?

Heater matrix knackered.
Even when you flush the matrix and water comes out clear it can still be blocked.
Fit a new matrix.
I know from experience.

Correct thermostat?

I have the same problem. But never gets hot. Flushed the whole system. When I put an airline in a pipe and blew it through I got hot air but soon went cold

try vacuum filling

Am I correct in believing that the later matrix (with straight pipes) is a direct replacement for the earlier ones (pipes bent down)?

Check the 2 pipes that go into the heater matrix and see if they stay how.
If they stay hot then it's likely the matrix is scrwed.
If the pipes suddently go cool then it's an air bubble trapped somewhere, being moved by the water flow into the matrix.
Also from recent experience, check for air leaks where the heater box meets the bulkhead. If the foam has perished (very likely) then the air being blown out of the matrix is leaking out of this joint and into the engine bay instead of into the cabin.