I have a 1993 civic dx that has a 1.6 vtec engine from a 95 si. I recently replaced the water pump due to a leak as well a the timing belt and water pump while I was in there. my heat went from great to fair. I have went back and changed the thermostat to a 180 degree instead of the 170 factory as well as blowed/flushed out the heater core and checking the mixing valve with no difference. I noticed tonight that the bypass hose that hooks right behind the heater hose on the thermostat housing gets very hot after running a few minutes while the heater hoe only warms slightly. Im getting around 100 degree heat out of the vents but it doesn't seem to be warming the interior of the car nearly as good as before the water pump replacement. Any ideas??

 

You need to get all the air out of the system. Park facing uphill, or at least on level ground so the radiator is the highest point, and make sure the radiator is full to the top. If not, pour coolant into the radiator and run the engine with the radiator cap off and the heater slide on full hot. Keep adding coolant if the level drops. As the engine warms up, the radiator should start to overflow, put the cap on at that point and test drive until fully warmed up. Then park and wait for the engine to cool and repeat the whole process if necessary.

Your car may have a bleeder valve on the fitting where the top radiator hose connects to the engine. Opening that valve while you are filling the radiator will help the process.

Also check that the push-pull cable operating the water valve on the firewall is working to open and close the valve as you move the slider on the dashboard.