If it was the water pump the engine would be overheating. Thermostat seems
much more likely. Air locks and a sludged heater matrix are also possible
but if the coolant temp is low then your thermostat is the prime suspect.
Oops - I should have said, - have changed the thermostat.
Yes I wondered why the engine temp is on the low side too. Can't
really explain it but am certain it is the case. Steady Rev the engine
for a few second and warm air comes out the heater. Back to idle, back
to cold air.
I put the van up on racks, open the heater to full open and ran engine
for 10 mins - still the same situation.
Looks like a sludged-up heater matrix then - which is a damn awful
thing to get at in this VW.
When I changed the 'stat, the coolant that came out looked very clean!
I would look for an airlock first. you could find if the heater is
sludged up by running a domestic hose pipe to it and seeing how easily
the water flows through if it runs through at a dribble it is blocked,
otherwise it is OK
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