Heaters only blow hot air when car is revved

[Pezperry12]

As the title says really only time the heaters blow hot is when car is going at some speed and otherwise it blows cold got plenty of coolant in car so no leaks can't hear and whirring noise from water pump and wouldn't of thought it would be thermostat as it would either be open or closed? Anybody experienced this before car always heats up never drops or goes over the temp gauge

[jonathanfenton]

Poor circulation to the heater matrix. Caused by failing water pumps or failed tandom pumps in my experience. If your gauge is staying at 90°c I wouldn’t think it would be a thermostat but for the price of em and whilst changing water pump in is just change them both for price of them.

[Pezperry12]

How can you check the water pump and how much is that going to set me back?

[Pezperry12]

And would it not overheat if it was waterpump

[jonathanfenton]

I said failing not failed, heater matrix is highest and furthest away. Take a hose off matrix at firewall and whilst idling and see if any water comes out.