Cold heater at tickover speads

[GrahamS]

The heater on my Polo goes cold when the engine is at tick over. It's warm-hot when the engine gets up to about 2000 revs, but goes cold at slower speads. Both radiator hoses are hot & the two heater hoses feeding the heater matrix feel hot. I've checked the water level and the system for leaks, it all looks ok.
The car is 1300 N reg, any ideas please?

[Marc97]

Mmmmmmm, tricky one.
Try giving the cooling system a flush then replace water with a 50/50 antifreeze/water mix.
Perfect timing for winter.

[GrahamS]

Yeh just in time for winter, I'll give the system a good flush through. Do you know if there's a drain plug on the radiator?
Graham

[Marc97]

Not a clue mate, sorry.

[GrahamS]

It was the water pump. The original pump has a plastic impeler which is a force fit on a metal shaft. This wears and the impeler starts to slip on the shaft, so just enough circulation to keep the engine cool, but not enough force to push the water through the heater. New pump cured it.