What happens if cooling system isn't bled properly?

I did some work on a 1990’s Toyota Corolla recently, and I had to take off the hose coming from the heater and back to the engine. About 1 cup of coolant leaked out. I just re-installed the hose after the repair, and didn’t bother to bleed the cooling system. My plan was to just top the cooling system off after a week or two when everything equalized.

Actually, I’ve never bleedthe cooling system in any car I’ve worked on, but usually working on cooling systems involves draining the whole system, then refilling it by filling the radiator and leaving the cap off and topping it off until the thermostat opens, which I think bleeds the system. This time I didn’t do anything except just to replace the hose. Then I wondered if that could have created and air pocket with some undesireable effects. What should look out for?

The air should work its way through the system and bubble up in the overflow tank, then as the car cools coolant from the tank will be pulled into the system. Keep an eye on the coolant level in the coolant overflow tank for a few days and adjust as needed.

I was worried an air pocket lodged somewhere & it could cause the engine to overheat. But I’ve watched the guage and it has always remained at normal levels with no indication of overheating. I did hear a sort of knocking sound sync’d to the rpm, soon after the fix, but it gradually went away and has never returned.

Because the heater core is the highest point in the cooling system, and if enough air is trapped in the cooling system, the air would collect in the heater core and there would be no heat. If the heater works there’s nothing to worry about.


what if the heater isn’t working, does the heater core need to be replaced or is it fixable?

Varies vehicle to vehicle. Low slung sports cars tend to have more problems with air pockets forming which block coolant flow than typical econoboxes. I recently completely drained and then re-filled the cooling system on my Corolla and all I did to bleed the air out of the system was idle the engine for 15 minutes with the heater turned on. At first I could see air bubbles appearing in the coolant overflow tank. This lasted about 5 minutes, then all the air was bled out apparently, and no more air bubbles appeared in the overflow tank.

In any event no heat from the heater doesn’t necessarily imply the heater core has failed.