Low Coolant
While not having enough coolant may make your engine run hotter, it can also cause your Civic to not be able to create enough heat. Is there isn’t enough line pressure, due to low coolant, then the heater core will not be able to get any coolant itself.
You can verify this by checking the coolant level by taking a peek at the overflow reservoir. The overflow reservoir will be connected to the radiator by a tube coming from the top (usually right by the radiator cap). It should have separate lines to indicate “cool hot” and “cool cold”. Some vehicles only have full hot. Never touch the radiator cap unless the vehicle is ICE COLD.
Bad Thermostat
The job of your Civic’s thermostat is to regulate the temperature of the engine. It does this by allowing water to enter the engine when it is getting hot, and by keeping coolant from entering while the vehicle warms up.
Sometimes when a thermostat goes bad, it stays stuck open. This allows the coolant to continuously enter the engine. When this happens it’ll feel like the heat is barely working, or you may notice that it’ll work if the vehicle isn’t moving, but if you get on the highway it stops working.
If the heat is blowing, but blowing cold, it has something to do with the coolant getting to the heater core, or the heater core itself.
Want to make sure all of the air is burped out of the system after working on it. As in another thread yesterday we showed a technique of drilling an 1/8" hole in the thermostat on the mopars that lets the trapped air out of the engine when filling the cooling system. Mopar Small Blocks take 4 full gallons to be completely full. Check your coolant capacity and make sure the system in completely full of coolant.
If your Civic has a built in temperature gauge, take a look at it and see if it is in the spot it is normally in after a bit of driving. If it looks like it is colder than normal, that’s a pretty good indication that there is something wrong with the thermostat.
