Car’s heater doing a funky (SnowMiser/HeatMiser) dance.

TLDR: The heat takes a long time to get hot, then it gets super-hot for a minute, then it cools back to ambient temp for a minute or 2, then it gets hot as hell again, only to repeat the cycle.

The heater in my stepson’s 2004 Honda Civic is misbehavin’. He’s been saying “it doesn’t work”. Needing more data than that, I asked him to drive me around the other night (ambient temp about 47F).

To begin this experiment, I had him start the car and I dialed the temp selector all the way to the red. Vent selection was a very appropriate “floor level”. I kept the fan in the “off” position and waited until I saw the needle creep-up past the first line on the water-temp gauge in the dash.

The engine started to get warm in –what I would consider- a normally expected time of about 2-3 miles of 25mph to 45 mph driving. I turned the fan on at that point but the heater took another ~3 minutes to put out any warm air. It took a long time to get hot, then it got super-hot for a ~ minute, then it cooled back to ambient temp for a minute or 2, then it got hot as hell again, only to repeat the cycle.

I tried it on all vent selections, including “defrost”. Results didn’t change with any change of vent selection. I failed to notice if the compressor came on during the defrost selection and I didn’t press the A/C button on the panel, either. I also didn’t note whether we were re-circulating air or drawing-in fresh.

Any educated assumptions on what the problem could be?

I’m betting that there is air in the cooling system. Have you checked the radiator to make sure it’s full of coolant with the engine completely cold? I’m talking the actual radiator cap and not just the overflow.

Also has there been any work to the car such as replacing a hose or maybe the water pump.? That can let air into the cooling system. I hate to say it, but something like a blown head gasket or cracked head could also introduce air into the cooling system.

I’ve had a couple of vehicles which had specific instructions on bleeding air out of the cooling system.

There is an easy method for getting some of the air out. Park the car pointing up the steepest incline that you can find. Shut off the engine for 4 or 5 hours. Pop open the radiator cap and fill it with a proper mix of coolant/antifreeze. Start the engine and let it run with the cap off. See if any air bubbles start coming out. Once the bubbles stop and the fluid starts coming out from heat expansion, put on the radiator cap and see if that helps the heat.

what JerrySTL said is a possibility. it could also be the blend door or blend door actuator; the blend door is what controls the ratio of hot and cold air in the HVAC manifold.

Another thought. The temperature is usually controlled by a blend valve that mixes hot and cold air. Often they are controlled by a small electrical motor and linkage. Sometimes it’s just a cable from the temp control to the blend valve. Usually it’s behind the glove box or somewhere close to the front passenger’s feet area. See if you can find this blend valve linkage and see what it’s doing. As a temporary measure, I once blocked the blend valve to the full hot position on a Hyundai until I could get a replacement electric motor for it.

I like the thought of air in the system. I like the thought of it as the issue, not that it’s happening, of course. The engine’s been purring right-along at the correct temp and all. No variations… it gets half-hot and stays there (or so he says).

I took the cap off to find that the radiator wasn’t full but the water was higher than the coils. I was only able to add ~ 1 cup of (distilled) water. I burped the upper and lower hoses but that didn’t really give me more than another few tablespoons of room to add. I studied a bit on-line and read about the bleeder screw where the upper hose attaches to the engine. I also read the guys are parking them on a slight incline, leaving the radiator cap off and watching the burps as the engine gets warm. When it stops burping, it’s full. We may bleed the cooling system this weekend, pending the mixing door evaluation.

The mixing door sounds like a real possibility, too. I didn’t get a chance to spend that amount of quality time with the machine, but I’ll be back on it tonight. Actually, I see JerrySTL said “valve”. I am assuming there’s a mixing door/flap set-up but is there a different kind of control system that’s more like a valve? E.g. my Dodge van’s HVAC has a vacuum pot that pulls a lever to physically move a door that channels the air. Is the same thing happening in the Honda or should I expect to find something different there?

that’s the “blend” door I mentioned above. depending on how it was designed, it may be actuated by vacuum, a cable, or an electric motor. Don’t know what Honda used.

This is a classic symptom of a sizeable air pocket in the cooling system. It is not a typical symptom of the other things mentioned above.

It probably has a slow coolant leak. If the situation gets worse, the next symptom is likely to be engine overheating, which poses a risk of engine damage.

Proper bleeding and topping up of the system may well restore good heat. If it does, the next thing is to find the cause of coolant loss/air ingress.

to this point, do you notice a “sweet” odor coming from the vents when the heat is on? heater cores are a common leakage point.

Thanks for that, GaryT. We’ll work on burping the system this evening. I’ll query him closer on the car’s maint history and ask him to monitor the situation closely. Air ingress… would this come from cavitation? Could we be accumulating air without leaving a puddle on the garage floor?

I didn’t notice a sweet smell from the vents but I’ll keep my nose on it.

No. The air comes in to replace the coolant that has leaked out.

Yes. With some leaks, the coolant can evaporate before it’s visible.

Roger. Thanks again!

Ok. I removed the cap, started the engine and let it purge for ~ an hour or more. Little bubbles were coming out of the radiator for a while. I probably added around 400ml of coolant/water as the level dropped when the air departed. Sitting here in the driveway, it seems to be fixed. Tomorrow morning will be a new test.

Check and top up the coolant level before starting the engine tomorrow – if it was warm when shut off tonight, the level may go down as it cools.