92 Prism int. heat

I just picked up a 92 Prism, it runs great the only problem is the heat, it only works when the engine is rev'ed up, otherwise it only blows cold air. I cant figure it out, the radiator is completely full and the heat is super warm when driving until you come to a stop and let the car idle such as when at a stop light, it then gets cold. Forget warming up the car in the morning, the temps are in the 20's here and it doesnt matter how long I let the car run, the engine gets warm but there is no heat at all until the engine is rev'ed up.

any ideas ????

Thanks for any input,

Mark

Check the level on the overflow\recovery reservior when cold. The recovery tank is to the left of the battery, and should be half full when warm. Are the radiator fans shutting off when cold. Most likely, it may be a partially open thermostat.

the coolant level in the radaiator and the overflow bottle are fine, the engine thermostat appears to functioning fine since the engine does warm up to the operating temp. (according to the guage) however, there is absolutely NO heat from the vents until the engines rpm's are increased.

I have seen something simliar to this when the coolant level is low that there is not enough preasure to push the low fluid into the heater core when ideling however, that is not the case from what I can tell.

Very strange.....

Well just because the guage says the engine is up to temp doesn't necesessarly mean the thermostat doesn't have a weak spring, or is operating properly.
Are both heater hoses warm at idle?

Try bringing the engine up to temp with the radiator cap off, or on the first click, to remove any trapped air within the system.

Is the belt tight on the water pump pully?

I'd replace the thermostat. Compare the old with the new by submerging them in hot water. Maybe the previous owner had a lower temp thermostat installed

The hoses are warm, the temp is where is should be but there is no warm air from the heater unless the engine is above an idle.

I just might try a cooling system flush, maybe there is a partial blockage that wont allow the warmed coolant into the heater core unless there is a higher preasure that is caused by the increased engine speed