Heater blows hot air. Sometimes.

[Gert3116]

My heater wasn't working for awhile and after I got a new switch for and the blower started to work but now the only time it blows out hot air, is when I'm accelerating. Did the older cars just have crappy heaters or is there something else I need to check or replace. Plz reply quick. It's cold outside.

[J-(Chicago)]

Could be a bad heater control valve. It's on the back top of the motor by the firewall.

[joe_padavano]

Originally Posted by Gert3116

My heater wasn't working for awhile and after I got a new switch for and the blower started to work but now the only time it blows out hot air, is when I'm accelerating. Did the older cars just have crappy heaters or is there something else I need to check or replace. Plz reply quick. It's cold outside.

What year/model is it and do you have A/C? The A/C cars use vacuum actuated doors inside the HVAC box under the dash. If vacuum is lost, most of the GM systems like this default to actuator positions that allow airflow out the heater and defroster vents. This loss of vacuum can be caused by many sources - bad check valve in the vac lines, leaking lines, leaking control valve on the heater panel, etc. Under acceleration the engine manifold vacuum drops. The factory system uses the black plastic vacuum tanks with check valves to normally provide vacuum to the system under these conditions. You need to carefully figure out exactly what's going on.

First, determine what's happening to the airflow when you're not accelerating. Is the fan still running? If so, where is the air going? Check the defroster vents, dash vents, etc. Next, get a chassis service manual and be sure your vac hoses are routed correctly. Finally, listen for hissing from the heater control that would indicate a bad selector valve.

[Gert3116]

Well right now I don't have any vents in my car. I just got done painting it and the guy that I bought it from had them out when I got it and I haven't put them in yet so it just blows onto the floor.

[Olds64]

Since you don't have any vents I would suggest reinstalling the air plenum and re plumbing the vents and vacuum lines under your dash.