Honda Civic / heater issues

Daughter in laws car, 2003 Civic, ~180,000 miles. Previously had overheat problems, replaced head gasket, timing belt, water pump, thermostat and hoses (+ a few other things) No longer over heating, but there is no heat coming from the vents unless the car is over 2,000 rpm. Hoses to the heater core are both warm/hot, so flow through heater core seems to be good. Also AC has same issue no cold air unless over 2,000 rpm. Is this the control to the blend door? Are they operated by vacuum and with a leak, need higher vacuum to operate? All suggestions on what to attack next are appreciated. My son and I will tackle this Thanksgiving.


This is somewhat of a stab in the dark . . . you’ve still got air pockets in the cooling system.

Does this engine use one of those coolant bleed screws, which you have to loosen, while filling new coolant?

Where is the pressure cap located . . . on the radiator, or on the reservoir?

Could be you have good level in the reservoir, but a little low in the radiator?

I remember a few vehicles I had to park with the nose up, so that I got all the air out while the engine was warming up. I was battling air pockets, and they went away when I did that. I guess the idea is that the pressure cap is literally supposed to be the highest point in the system

Been there done that, this car is a PITA to burp, but got the air out. Since the problem is with AC and heat I am thinking it is the actuator for the blend door or if it is pneumatic a leak somewhere.

2001-'05 Civics with the SOHC 1.7l do not have a bleed screw. The W/P is located low on the block, and the thermostat has a bleed hole that mist be installed “up” to insure proper venting.

With engine cold, top up the coolant level in the radiator and fill the coolant reservoir to about half full, and drive the car, warming it up and allowing it to cool several times. Then, check the coolant level in both the radiator and the reservoir, and post back.

This type of cooling system requires the warmup/cooldowns to work.

I could be wrong but pretty sure it’s a servo motor so no vacuum operation. Perhaps you have two issues, a remaining air pocket affecting heat output and low refrigerant affecting A/C operation? That’s the only way I can envision motor rpm affecting both if the blend door is electric…

…stubby finger edit …that must be installed…

We have heat.

One of the hoses for the heater core had a tight bend in it and was partially kinked. (we had replaced the original pre-formed hose a while ago with standard tubing from a Mcparts store) At low engine speeds it was enough to block the flow, at higher RPM’s the pressure was enough to overcome the restriction and get hot water through. A pre-formed hose was not in stock and time did not allow us to wait to order one. We put a ½ inch copper 45 elbow into the hose at the kink point and reinstalled it. (it is a tight fit and is not going anywhere) Burped etc and we have consistent heat. Now to tackle the AC and see what is up with that. Since the blend doors seem to be working it may be low refrigerant, that will be the next long weekend visit project.

Merry Christmas, Happy Chanukah and Happy New Year to one and all!

Good to see you got the car’s heater working. It’s definitely the season to have good heat in the car. I had a similar solution for a dishwasher plumbing problem where I lived once. I used the same copper elbow/hose clamp technique for a tricky route under the counter requiring a sharp bend for the rubber drain hose from the dishwasher to the sink and it worked fine.

Congrats on a job well done.
And a happy holiday season to you.

Many years ago on my '72 Vega I had a heater hose that was poorly routed. It chafed on the engine. I was in the AF at the time, and I had the airframes shop custom form an aluminum tube to my design to replace the hose with. Worked like a charm.