Heater Core???

[Michael Gates]

I always seem to find interesting problems that no one has seen before.

I think I may have a clogged heater core, I’m looking for someone else’s thoughts. 92 Toyota Pickup, 22RE 4cyl, manual, 2WD.

When I leave the HVAC temperature selector on cold, the engine operates in the normal temperature range. When I move the selector to hot, the engine starts to heat up. Also, I can hear periodic gurgling coming from the heater core, like there are bubbles moving through it. There are no coolant leaks into the cab, or onto the ground, near as I can tell. The truck has always consumed a liter of water every six months or so, but I’m guessing it’s either escaping as vapor through the overflow bottle or it has a pinhole leak into the combustion chamber somewhere. It never actually gets to full-on overheat, but it heats up almost to the red at the end of the dial. If I let it ride up to the far end of the temp gauge, then switch the HVAC temp selector back to cold, it drops back down to normal operating temperature within a few minutes. I know this selector is not supposed to be for engine temperature, it’s supposed to be for cab temperature, so I’m having trouble figuring out why it’s affecting operating temperature. My best theory is that it’s causing cooling restriction in the engine, which decreases the pressure in the cooling system and causes it to heat up. Before I go tearing out the dashboard to replace the heater core, I’d like to see if anyone else thinks I’m on the right track. Considering the fact that it’s doing anything at all, that’s telling me that the control valve appears to be working, so the only thing left is the heater core, right? Additionally, before I realized that the HVAC switch was having an effect on the issue, I replaced the thermostat and radiator cap, topped off the coolant and overflow and bled the system. Coolant is all new and clean. None of this seemed to affect the problem.

Topic

[Bryan Carter]

Does the air blow hot when the heater is on? I ask because gurgling from the heater core and no heat is a classic symptom of air still being trapped in the system. I know, I know… you said you already bled it, I’m just putting it out there.

If the air comes out volcano hot, the problem might actually be with your radiator, and not the heater core. Sometimes when radiators get clogged, they’ll run fine under normal conditions, but overheat when the heat is on. During normal driving, there’s enough pressure in the cooling system to drive coolant past the restriction since it has no easier path to travel. But when you put the heater core in the loop, the pressure drops, and you now are supplying an easier path for the coolant. The problem is that your heater core is way too small to dissipate all the heat your engine is creating and it starts to overheat.

So check out your radiator before setting out to swap out your heater core.

[Michael Gates]

Thanks for your thoughts.

The air is hot when it comes out when the heater is on. I hadn’t considered that radiator issue, but that makes sense. The radiator is only a year old and the coolant has always looked clean in it, but who knows? I’d considered taking it out and running water through it when I did the thermostat just to make sure it’d pass without any issues, but I decided to take the half hour approach instead of the half day approach on that one.

I also suppose it’s possible that there’s air caught in the heater core, but I did run it for quite a while with the cap off (let it come up to operating temp, etc…) to bleed the air out. To the best of my knowledge there’s no purge valve on that engine, so I’m not sure how else to try to flush that out.

I was also wondering if the heater core does turn out to be clogged, what the odds are that I could blast a clog out if it with a hose nozzle? If the fins are as small as a radiator I don’t see that working, but I’ve never actually messed with a heater core before. I have taken the dashboard out of this truck, done a wiring project inside it and reinstalled it in one day when I replaced the dash a few years ago, so I feel like the project is doable.

[Ingvar]

1. It does sound like air lock with air trapped inside the heater core.
2. have engine at normal operating temp. Open heater core and let coolant run. Locate and touch by hand both heater core hoses. Are they same temperature?
3. Locate lower radiator hose. Is it same temp as upper one? If not, you have obstruction somewhere.
4. Not sure about your truck, but many engines have air bleeder provision, usually in t-stat housing, or anywhere at the system highest engine point, that needs to be opened during bleeding procedure.

[EricTheCarGuy]

+1 on air in the cooling system. If your heater core was clogged, you wouldn’t get good heat. Also, the heater core has nothing to do with engine cooling. If you bypassed it the engine would not know the difference. The heater is there for YOUR comfort, not the engine. That said, I’d recommend you bleed the air out of the system and recheck for the issues. This video may be helpful.

You might also find useful information here.

http://www.ericthecarguy.com/faq/what-to-do-when-your-engine-overheats

Keep us posted.

[Author]

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