Heat only works when engine NOT running

Hi All,

I recently ran very low on coolant with my 2005 Prius and had the overheat light coming on. Immediately before the overheat light came on, I realized that the blower was blowing cold air even though the engine was running. When the light came on, I immediately stopped and refilled coolant at the overflow reservoir and the radiator and let it run through a few times. I have re-topped off the coolant levels and it seems to be holding steady. Hopefully I have not done long-term damage to the engine, but that remains to be seen, and is, of course, very possible given that I let it get so low.

A symptom I have right now is that when I turn the car/engine on, hot air does not come out of the blower. If I remain in park, when the engine turns off, hot air begins to come out. When the engine kicks back in, the hot air stops. Any idea what this may be a symptom of?

 

Thanks everyone! I was hoping that it was just an air lock or a weird thing with the engine's pump...will spend some time this weekend and week testing things out. Not having heat in Vermont in the winter is inconvenient, but I can live with it for a week.

I noticed about 15k miles ago that after long downhills (i.e. 3-5 minutes with the engine totally off), when it kicked back in, I had some white smoke that would clear up. I assume that meant that I was somehow burning coolant, and I was very good at keeping on top of it for awhile. Then, I left the country for 4 months and forgot about it...thus I didn't refill it recently...thus, no coolant.

To be honest, with 195k very hard miles on this car (think 4000 feet of elevation gain and loss on back roads every day), and a hybrid battery that has, at times, been on the rocks, I wasn't too interested in looking into a head gasket issue and preferred just to refill the coolant...until I forgot...

 

Hi again,

Just as a final update (I hope), I ended up taking the car to the dealership after being strongly encouraged by my mechanic to do so (he suspected it might be a head gasket issue from my description over the phone).

The dealership said that the belt from my engine water pump was missing, so they replaced it and did a pressure test--the head gasket seems to be fine, and the fix seems to have worked.