mk4 tdi heating issue

[phatpants]

Hi i new to this. I got mk4 golf tdi and have strange heater blower issues. When car at 90' the blower will blow nice warm air. (At standstill.) As soo  as i drive off blows cold??? I have recently changed heater matrix as was blocked and flushed everything else out to clear any blockage. Have had bew stat in and ran with that in and over heats so took out for bit to check collant getting round sysetem which if it is( not sure f enough). Any help as starting to lose my marbles

[itavaltalainen]

new thermostat fitted already?

sure that old one was dufft? maybe water pump impeller is spinning on shaft - not the first time thats happened on a mk4!

[phatpants]

Yeah had new stat. When tbermostat houseing is take  off cant move the impella and have cpllant returning back to header tank at all revs (glow increases higher revs )