2001 p38 heater only hot when driving?

hi,

my 2001 p38 petrol/lpg 4.6, just started only getting warm/hot air when driving( or if reving the car above tickover revs ), when the is car stationary heater goes cool.

temperature gauge is normal, and no loss of coolent.

could it be a air lock in the heater matrix? or is it a thermostat problem?

any ideas?

thanks
karl

sounds like water isnt going to matrix. Run it with tickover and cap off to getrid of any air. Maybe need a new water pump. About 30 quid from island 4x4