Winter is here and I'm feeling it bad! The heater in my 2002 Civic Ex isn't getting the job done. It heats up fairly quickly once I'm driving, but if I slow down for a stop light or to get off the highway, it will stop blowing hot and just blow medium temp air. Once I get back on the gas a few seconds later it'll get hot again.

It seems to me like the water pump isn't moving enough coolant through the heater core fast enough. I think the heater is sucking all the heat out of the coolant while its in the heater core. When I get back on the gas and build some RPM's the water pump then pushes enough hot water out of the engine and into the heater core to heat the air back up. At high rpm (2000+) it moves the water fast enough that the heater core always stays full of hot water. Under 2000 RPM the water stays in the heater core too long and looses its heat, causing the hair going through the heater core to NOT GET HOT.

What does everyone think? Anyone having this problem? Temps were in the teens today, not sub zero.

 

First and foremost, check the coolant level. With the car cold, take the radiator cap off and check in the radiator itself. It should be completely full. When the coolant gets low, the first symptom is the heater stops working.

Does the temperature gauge drop down when you slow down? If it does, thermostat may be stuck partway open. Also make sure the radiator fans are NOT running all the time. In extremely cold climates you can block the radiator partway with a piece of cardboard in front to reduce air flow and heat loss under the hood. This shouldn't be necessary if the thermostat is OK.

Make sure the water valve on the firewall is fully open with the dial on hot. It would be good to disconnect the heater hoses (easiest to disconnect at the engine end) and flush the heater core with water from a garden hose, in both directions.

Put the air control on recirculate to reduce demand on the heater, as much as you can. That can make it too humid inside the car and cause the windows to fog up though.

 

Temp gauge doesn't drop when I slow down. I'm guessing that temp sensor is on the engine somewhere, so the water IN the engine isn't cooling down, its the water in the heater core that's cooling down too fast.

I'll check the coolant level tonight. Changing the air control to recirculate has no effect on the problem I'm having. I already tried that.

 

buy a 195 degree thermostat and also flush your coolant system,

unhook the hoses going into the engine from the heater core and stick a water hose to one of the hoses and flush the core out, then put it on the other hose and flush it out,

 

I am having the exact same problem!

 

My daughters 2003 Civic is behaving in the same manner. When the car is at rest, and idling, with engine temperature at midpoint, the heater blows cold air. She says once she starts down the road, the temperature of the heater air warms up. I went out tonight (in the snow) and topped off the radiator fluid. I also had her move the heater thermostat from coldest to hottest while I watched the heater valve near the firewall operate back and forth. I am wondering if the valve needs replacing? Never heard of a core stopping up but I am not a mechanic.

 

If the radiator was not completely full, that's probably the problem. Did you test-drive after filling?

The core can get stopped up, especially if someone put "stop leak" in the system. Often it can be cleared by disconnecting the hoses from the engine and flushing with water from a garden hose.

 

My daughter called me on the way to work and said the heater was working fine. It must have been air locked. I did not drive it last night after filling the radiator. I will check the level again after she gets home. Thanks for your reply.

 

Yeah keep an eye on the level. If it goes down again look for leaks. If you find the radiator becomes empty even though the plastic tank on the side remains full, replace the radiator cap.

 

topped off the coolant and problem solved. Seems too easy to be true, but it worked!