I have a 2001 Olsmobile Alero 3.4 V6. The heater only blows hot air when under acceleration or cruising at highway speed.

I have a 2001 Olsmobile Alero 3.4 V6. The heater only blows hot air when under acceleration or cruising at highway speed.

I have a 2001 Olsmobile Alero 3.4 V6. The heater only blows hot air when under acceleration or cruising at highway speed. Coolant is full and circulating, and there are no problems with engine temperature. The thermostat was recently replaced because the previous owner took it out for some reason. Now the engine is at the correct operating temperature but the heat is sporadic. Suggestions? Could this be a symptom of a heater valve failing? Possible clogged (or clogging) heater core? Why would it only work sometimes?

If the T-stat was changed, was the system bled to release trapped air? It sounds as though you may have that issue.

—Jerry Ives
Assistant Professor, Parts Technology
Alfred State College, Wellsville Campus

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