Hi everyone. I think this is the correct section for this. If not, please move it accordingly.

A couple weeks ago my heater started acting up. It wouldn't blow hot air unless I was at a minimum of 2k RPMs. If I was at an idle it would still blow, but it would be cold. Now, it won't blow hot air at all. The coolant reservoir was empty, so I filled it up to under the max line. Turned the car on and let it sit awhile while running and still no heat. With the engine running I looked at the reservoir and saw no fluid moving. With the engine only running about 10 minutes, I popped the cap off the radiator and saw no fluid inside. I saw no circulation of anything.

What could be causing this issue?

'98 Civic EX. D16Y8 engine (I believe). Here is an old thread of the engine if you need to see. https://www.hondacivicforum.com/foru...ad.php?t=51857

 

There are a few things that can cause this. Check the blend door under the glove box, to see if the cable from the hot/cold control is working. Is the car warming up all the way? if not then you may need a thermostat. Is the control valve leading to the heater core working? It is in the engine bay by the fire wall on the trans side. There is a cable going to it that opens the coolant flow to the heater core. Make sure the cable is working and not broken. The heater core could be plugged up. Do you do regular service to your car ie cooling system flushes? Oh and make sure the radiator is full of coolant also. Hope some of this helps.

 

What was confusing me was that the radiator was leaking. I only noticed it after all the leaves were cleared from the street and there was a huge puddle under the car. Replaced the radiator and now the heater works fine.

 

moved to tech section.

That makes sense, actually. Low coolant from radiator leak = air in the system. Air = no hot engine coolant getting to the heater core. When you were at higher rpms, the pressure was forcing the air to circulate with the coolant, so some of the hot coolant was getting to the heater core.