Rep Power: 0

Hi.

I just bought an 04 civic EX and noticed I wasn't getting heat on idle. When car is running heat comes back. I noticed there were not enough coolant in ther overflow reservoir so I added some and checked for air in the radiator (started the car, let it warm with the radiator cap off, etc...)

I don't have any overheating problem, the temp gauge stays right below the middle, but it's winter here and outside temp is around 0 since a few months.

Do you have any input to give me?

I thought

1) Thermostat stuck open. The car temp would not stay at operating temp when idling?

2) Bad head gasket? The car would overheat? But it's not. I let it ran idling for an hour or so...

3) Bad waterpump (not pumping enough coolant so it doesn't get to the heater core, but why is my heat coming back when the car is running?)

My upper rad hose is hot, the lower one is cold. (I think the upper hose is the hot coolant coming out the engine to the radiator while the bottom one shoots the cooled coolant to the engine block right?)

Thank you in advance... Having a hard time there during cold mornings and I don't want to start spending money on things that ain't causing this issue.

 
Rep Power: 211

assuming your coolant temp is in the "normal" range,

what you are describing sounds like an air lock in your heater core, either that or your heater core is clogged, and the coolant is passing through it too slow,

this could also be the very early signs of a failing head gasket, keep a close eye on your coolant level in both the rad and overflow, make sure they are both at proper levels,

if the coolant level in the overflow continues to go up, then you most likely have a bad head gasket

 
Rep Power: 0

Can you explain why an overflow issue would be caused by the head gasket...? My overflow was full today. I removed some. Rebleed everything...

 
Rep Power: 151

Your engine is an air pump. When the head gasket is breached the engine compression forces air out to any path it can release the pressure. The air escapes through the cooling system.
With air displacing coolant (normally a sealed system) the easiest path is out of the reservoir. It fills up and overflows. Soon you will be low on coolant and your heater core will be empty. You'll notice you don't have any heat inside the cabin. The engine will overheat because you'll be low on coolant, etc.
You add coolant and your engine pumps air back in and forces it out.

 
Rep Power: 211

when the head gasket fails, your combustion/compression gases get pushed through the head gasket and into the cooling system,

this air in the cooling system then pushes coolant out into the overflow bottle, eventually the overflow bottle will literally overflow with coolant

 
Rep Power: 0

Thanks for the info!

I changed my thermostat this weekend and did a complete flush with a heater core flush as well.

I'm getting heat again on idle and temperature is OK (it never overheated, but whatever)

I'll keep an eye on my overflow reservoir. If it fills up, that means I have a bad headgasket I guess.