Solving Automotive HVAC Problems

Clogged Heater Core

Every now and again, I find a heater core that gets a restriction and causes the car to not have heat or not enough heat. This is not the first thing I check for; in fact, it’s pretty low on the list of things to check. Heater cores don’t normally clog up on their own. It’s usually caused by an outside force.

Before I get too deep here, a heater core is like a small radiator located inside your dashboard. Coolant from the engine passes through this and a fan blows air across it. The warm air you feel from the HVAC vents is a result of this process. If there is a problem with the heater core or cooling system, it will affect heater performance.

One of the main causes is not using stop-leak products properly. If you’ve had a coolant leak that you’ve attempted to fix with stop-leak and you have no heat or reduced heat, you might have a blockage in the heater core causing the problem.

Another cause is corrosion. If you run straight water in your cooling system or your cooling system is full of scale and rust, your heater core can clog up. While the engine is cool, remove the radiator cap and inspect the coolant. If it’s a nice color (could be green, blue, orange, or perhaps even yellow) and free of contaminants, the heater core is not the first place I would look. However, if it’s full of rust or looks brown or black, you might have other issues causing problems with the heater core. If you do see these contaminants, you might consider flushing the cooling system as well as the heater core to prevent issues in the future.

A good test to see if your heater core is clogged is to feel the hoses going into it in the firewall. Look at the firewall or bulkhead for two hoses close together. These are normally the feed and return hoses for the heater core. With the engine warm and running, feel the hoses. If the heater core is working properly, both hoses should be warm or hot to the touch; one might be slightly cooler than the other, but not by much. If one hose is hot and the other cool or cold to the touch, then you might have a clogged heater core. If that’s the case, try flushing it out. Sometimes you can force compressed air and cleaner through a heater core to clear the blockage. I would suggest you try this first before attempting to replace a heater core. The heater core is part of the HVAC system in your dash. It might take a great deal of work to replace it, so try to clear a blockage before you attempt a replacement. Here’s a video showing the process.

Everyone always asks me what cleaner I used in that video. It was CLR. It worked pretty well.

One last note: Jeeps are famous for clogging heater cores. I don’t know why, but I hear about it a lot.

Leaking Heater Core

I’m just gonna come out and say it: A leaking heater core sucks. One of the symptoms of a leaking heater core is wet carpet on the passenger side of the vehicle where the heater core is most often located. Another common symptom is that the windshield fogs up when you use the defroster. It might even leave an oily film on the windshield and smell like coolant when you activate the defroster.

If you have these symptoms, you likely have a leaking heater core. The repair of this can run the gamut from very easy to OMG I want to shoot myself. The easy ones are normally accessible from the engine compartment. The difficult ones are located inside the dash and you need to remove the dash and the HVAC system in order to access them. As if that isn’t difficult enough, you will also need to evacuate the AC system because you’ll be disconnecting the evaporator when you remove the HVAC unit.

Before you say “stop-leak,” remember the above paragraph. One temporary fix I’ve done a few times is to bypass the heater core altogether if it’s leaking. You can do this by routing the inlet heater hose to the outlet. Just disconnect the heater hoses from the heater core and either join them together or rout one back to the engine. This creates a loop for the coolant to flow back to the engine. Your engine won’t know the difference; the heater core is just there for your comfort and doesn’t affect engine performance at all. You can run it like this for as long as you like. This way you don’t have to deal with that leaking heater core right away.

Heater Control Valves

It’s worth mentioning that many vehicles have what is called a heater control valve, which is used to regulate the flow of coolant through the heater core. This shuts off the flow of hot coolant to the heater core during the summer when you’re trying to run your AC. You wouldn’t want your cold AC air to pass through a hot

heater core before it enters the passenger compartment, so this heater control valve regulates coolant flow into the heater core. If this valve has an issue, it can cause no heat or AC that doesn’t cool enough.

The AC not cooling enough is very rare. In fact, there is a blend door inside the HVAC system that regulates air flow past the heater core and AC evaporator. I’d look at the blend door before I looked at the heater control valve for a problem like that. It’s more likely you’d have a problem with no heat if the heater control valve failed. I’ve seen these valves controlled by cables, vacuum, and sometimes electronics. Consult your service manual on how to inspect and service them. A quick test to see if it’s working is to feel the hose before and after the valve. If it’s the same temperature after the valve, it’s open and working fine. You must have the heat on when doing this test. If the outlet is cool or cold and the inlet is hot, look to the valve for the issue. It might not be the valve’s fault though. Be sure to check what controls the valve to be sure it’s working properly. If the valve doesn’t get a signal to open, it won’t open.

HVAC Controls and Air Flow in the System

It’s going to be difficult to cover everything here. HVAC systems vary as much as makes and models do. That said, here are some of the basics when it comes to the movement of air in the HVAC system.

The HVAC assembly is located inside your dash. As we’ve established, it contains the AC evaporator and heater core. One makes cold air, the other warm air. The next thing the HVAC needs to do is distribute that air into the passenger compartment. It does this with a series of vents, doors, and seals. As you make different selections on your HVAC controls, you might hear different things moving around inside the dash. These are the doors I spoke of. They help direct the air to the desired location in the vehicle as commanded by the driver. These doors are sometimes controlled by cables, vacuum motors, or electric motors. If there is a problem with one of these motors or the doors they control, you won’t have airflow where you want it.

There are so many different configurations of this that there is no way I can cover it all here. I can offer a couple of tips, however. The first is observation. If you believe you have a problem with the HVAC duct system, put your head under the dash and work the controls. You can sometimes see the different components move around as you do this. If you’re looking for a problem, work the controls that

pertain to that problem and see if you can put your eyes on the cause. It can sometimes be as simple as a cable that slipped off.

Another thing to watch out for is obstructions. I have kids. They are probably the number-one cause of this. They think it might be a good idea to see if a pen fits in the vent, or they might have a favorite toy they put on the dash and it’s small enough to fall down the defrost vent. These items can sometimes get wedged inside the HVAC system and cause problems with the movement of the doors in the system. If this is the case, you might have to remove the HVAC assembly to remove the obstruction. Sometimes you can sneak in there with just the right tool, so be sure to try that first.

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