2004 Civic Sedan 118,000 miles.

Noticed on the way home that my heater seems to go from hot to cold then hot again. It just cycles intermittently without my touching anything, so I dont believe it to be the **** itself. It seems to get worse when I slow down or stop, and stays more consistently hot when traveling at a steady speed.

The thermostat I believe to be functioning correctly since the temperature of the engine is exactly like it always has been since it were new. My most likely guess would be a clog in the heater core? According to the maint schedule, the coolant should be replaced every 120,000 miles, which was when I planned to flush, so I don't know if it should be a clog.

 

When the engine is cool, check whether the coolant level is low in the radiator as well as the reservoir. Top off if necessary. If the coolant level seems fine, then you may want to try bleeding air from the cooling system:

 

I think it was the coolant. The reserve was dry and I added some to the radiator as well. I had the water pump replaced with the timing belt service about 7000 miles ago, so there is a chance that the tech may not have filled it properly or maybe something is slowly leaking. I'll keep my eye out for that. I dont suspect anything any more serious than that since everything else seems fine. I took it down the road for 10 minutes and the level looked the same, so no major leaks.

It blows crazy hot now. The intermittent part was what was stumping me. Seemed like it wouldnt do that for low coolant.

 

Well you need coolant for heat. Some people even have low temp on the temp gauge when they have low coolant in their system because the water doesn't touch the thermocouple. The intermittent heat happens when your heater core has air bubbles which doesn't transfer much heat to the coils.

 

In simpler terms, the heater heats the air from hot coolant. Less hot coolant = less heated air. The intermittent is the thermostat opening and closing, letting the coolant flow through the engine. If there's only enough to fill the coolant passages in the engine coolant will be drawn away from the heater core, leaving you with frosty nipples.