Heater only heats when driving - '99 Jeep Cherokee



I recently had my heater core flushed and the flow reversed by my friendly neighborhood mechanic. For the last 3 weeks the heater has worked fine, until this morning. I let the car idle this morning for about 15 mins. before I left for work. I expected to find plenty of warmth when I sat in the car, unfortunately it was nearly as cold as when I started it. The engine temp. gauge was at the right spot, just no heat. When I started driving though the heater began to blow warm air. What do you think is going on?


You might want to read the posts for my heater problem.
Also, online research showed me a heater core can be damaged by excessive pressure used in flushing. Maybe that happened to you?
My sympathy for your shivering, whatever the cause.


Sounds like an air bubble that worked its way to your heater core, then eventually got pushed out, hopefully to the radiator. It’s hard to refill the coolant in some cars without ending up with a bubble.


Air bubble or leak. You haven’t confirmed that coolant level is currently correct. 3 weeks is enough time to have a slow leak at a clamp creating it’s own air bubble.


What do you man by “flow reversed?”


My mechanic said that after flushing the heater core he reversed the flow through the core. I’m assuming he swapped the “goes into and goes out of lines”. He said that this helps to prevent another plugged up situation in which the heater core is not getting coolant.


If the air bubble was pushed out wouldn’t that solve the problem then? I have plenty of coolant in the system according to the overfill mark. So far I can’t detect any leaks. When I started the car at lunch time today I had the same symptoms.