No heat at lower speeds-Help! It's Cold!

[Jetskier]

We have a 98 Sable which is having a heating problem. It has 109,000 miles on it. After initial warm-up, the temp gauge stays steady in the operating range. However, the heater puts out cool air until the vehicle reaches about 2100 rpm. Then we instantly have heat. Drop under 2000 rpm and it goes cold. We can duplicate this by allowing the car to idle and pinching off the by-pass hose to force water through the heater core. The heater core hose on one side is always hot but the other side is cold until we pinch the bypass hose. Since the problem started, we have replaced the heater core, radiator & thermostat and it still is doing it. Is it possible the system doesn't have enough flow (water pump) to force water through the heater core? Please help-It's getting cold and my wife won't give me my pick-up back!

[KimMG]

When you replace the heater core and radiator did you have the cooling system flushed? The symptoms you describe are indicative of a partially clogged heater core. I have seen water pumps with worn vanes that did not circulate water very well. Although these pumps were over 15 years old.
To flush the heater core you will need these things:
two feet of 5/8" inner diameter hose
hose clamp
female end for a garden hose
clamp one end of the hose to the outlet of the heater core and attach the female end to the other end of the hose and attach to a garden hose and turn the faucet full on.

[Jetskier]

Originally Posted by KimMG

When you replace the heater core and radiator did you have the cooling system flushed? The symptoms you describe are indicative of a partially clogged heater core. I have seen water pumps with worn vanes that did not circulate water very well. Although these pumps were over 15 years old.
To flush the heater core you will need these things:
two feet of 5/8" inner diameter hose
hose clamp
female end for a garden hose
clamp one end of the hose to the outlet of the heater core and attach the female end to the other end of the hose and attach to a garden hose and turn the faucet full on.

We flushed it when the radiator & heater core were replaced and also this past weekend. It seems to flow fine.

[bussman240sx]

Had that problem with my Nissan, just needed a new thermostat

[vicv]

I'd check the heater control valve and the water pump. Remove the valve and clamp in a piece of hose, run the car up to temp and see if you get heat. If yes, the water pump is probably okay and the valve is bad. If no heat then maybe the water pump is bad. By the way, the valve operates on a vac line, might check that too.

[Wes026]

Its either gonna be a partially clogged Heater Core, or a work out Water Pump...... When you increase the RPM's water pressure is enough so that water is FORCED through the constriction of the heater core, or the extra speed of the water pump is allowing water to circulate through the heater core.... Its one or the other...

[bruzer321]

Sounds to me like a vaccum problem.