Shortly after buying my '92 Formula a bit over a year ago, the heat stopped working. The previous owner had replaced the heater core right before I bought the car. This fall, I replaced the thermostat, which I thought might be sticking. Still, no heat, and air only blows from the defrost vents, not the heater vents.

Any ideas? With a new heater core and t-stat, I'm at a loss to explain the issue.

 

Possible air pocket in the heater core....take off the radiator cap and run the car and look down into the radiator to see if there are bubbles coming up..basically your purging the air from the system....of course this suggestion is based on the assumption that you have the correct fluid levels in the motor...something easy to try

 

Fluid levels were correct last time I checked, will double-check today. Will also try looking in radiator with cap off and engine running. So, if there is air in the core, does removing the cap and letting the engine run purge all of the air out, or is there something else I have to do?

 

Air in the core will all get blasted out within 10 seconds of running, after filling up the radiator. Most likely that's not your problem.

Sounds to me like a heat control valve problem.The problem being, the vacuum control system is farkled, and no vacuum is getting to it to open it and let water flow to the core.

Find the disconnected vacuum line; verify that vacuum is reaching the control head in the dash; and make sure that the vacuum switch thing on it is working right. Hint: if you hear a hissing sound coming from somewhere underneath your radio, it's probably bad.

See my signature for a hint on the mental discipline of troubleshooting. Consider that a vacuum issue accounts for BOTH of your symptoms. Direct your attention accordingly.

 

Thank you, sofakingdom, I'll use the above as a guide. Troubleshooting tends not to be one of my strong suits...sort of like Latin. But I do like the quote.

 

If your thermo ever sticks you will know. Never gonna be like oh I just don't have heat hmmmmmm. When them suckers stick they will let you know loud and clear that something is wrong.


If it stopped for no reason its probably the vacuum valve for the heat slider.


If say you only didn't have heat when its was really cold. Sometimes its just to cold out. Use to have to stick cardboard in front of most of the rad on my 80 V8 camaro to get good heat in the winter.