Heater blows cold under 1500 RPM

[canoecraft]

On my 2009 Saturn Vue 4-cylinder, the heater works great, but only when engine RPM is over 1500. At highway speeds, heater works normally blowing hot air, but when idling under 1500 RPM, the heater just blows cold air. In cold weather, driving at highway speed, heater blows hot; if I come to a red light, within about 10 seconds of stopping, the heater blows cold while idling. Resuming highway speed returns the heater to blowing hot air. On the highway, the coolant temp runs from 195-210 degrees.

We've drained the coolant and done a complete system flush including the heater core. No difference. Any suggestions on possible causes or solutions?

[fdryer]

1-Does your '09 Vue use a cabin filter?

2-Felt the heater hoses after a full warm up when both should be hot?

If there's a loss of circulation when above idle but normal heat at speed, the hoses, engine fittings, coolant return line may be constricted, strangling coolant flow. You might have to stand back and visualize the two heater hoses for the heater, how coolant flows into and out of the heater core and if there's a restriction somewhere, hidden or visible. If sealer or crud built up anywhere, it may still be restricting coolant flow (engine block, fittings, bent hoses, collapsed hose if suction collapses a hose. Older hoses soften and are usually seen as bulging around clamps. Collapsing hoses may not be seen unless the engine is revved above 1500 rpm and seen.

[canoecraft]

The Vue has a cabin air filter. How might the condition of the filter cause loss of heat at low RPM?

The temperature of the heater inlet hose and outlet hose are nearly the same.

[fdryer]

It's easy to play armchair quarterback with guesses why a clogged cabin filter chokes off airflow at low speed and then allows good airflow at speed. Why ask when it's easier to examine the filter and determine if its clogged or not?