Fixing car heat issues

Things to check first

Steve Knickerbocker:

First, make sure you have a thermostat and you're not low on coolant. If that checks out you can usually use a garden hose to blow the crud out of the heater core. I just had to do this on my Voyager and I've done it on my Dart. Sometimes it'll take quite a few alternations between heater core nipples to get clear water to flow.

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Mike Nakagawa:

Sometimes, if the coolant level gets low, air gets trapped in the heater core because it is a high point in the system. The solution for this is to jack up the front of the car (ramps), remove the rad. cap, warm up the car and keep running with the heater on high. Low pressure without the cap will allow air to dissolve in the coolant and leak out at the open cap.

Then watch that you don't get bubbles stuck near the thermostat when you close everything up. Squeeze the upper radiator hose [editor's note: CAREFULLY! and make SURE it's WELL AND FIRMLY ATTACHED first! Wear protective gear - especially for your eyes, because a bad hose can burst or spring a leak!] and fill coolant up to the neck as you release the hose. Keep doing it until there are no bubbles when you squeeze.

I just changed the heater core on my elder Spirit. Not difficult, just time consuming, particularly if you reassemble a few things out of order, and have to redo it to get some blocked part to fit. The box with the heater core and A/C exchanger is huge. My A/C was dry, so that was one less worry, otherwise you need to purge the system and make sure it stays clean through reassembly.

IMPORTANT: see our full guide to bleeding the system!

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Dave wrote: Bubbles in the coolant seems to be the #1 cause of overheating and head failure, so please read this guide to purging your radiator even if you have a mechanic changing your antifreeze.

Heater cores

Gerald Richey:

Your heater core can become clogged and leave no other noticeable problems with the remaining cooling system. If anything clogs first, it will normally be the heater core. The heater core has the smallest passages in the entire cooling system. If a "pour in" repair was ever made two or more times, depending on the pour in brand, I can nearly guarantee that at least a partial clog in the heater core will occur causing less effective heating. [Problems can occur without "pour in" repairs, too.]

If the heat control slider (the gadget you move to adjust the amount of heat/cold air coming in) loses its connection to the duct, your car is still producing heat, but it's not coming in to the passenger compartment!

The radiator cap may also not be making a tight seal.

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Gene Poon:

If your heater core is plugged, you might not need to replace it. It's a lot of work...includes discharging your air conditioning and recharging it later [not something most people have the equipment to do].

Radiator Stop Leak leaves a soft residue most of the time. Try this: swap the inlet and outlet hoses to the heater core. This will backflow it and might just remove any soft deposits plugging it. That worked for me on a Valiant. The two hoses were of different diameters, so I had to force one of them to go on, and really clamp down on the other one to prevent leaks. But the results were instant. Just a five-minute drive, and I had good heat again. I immediately parked the car, let it cool, and drained the cooling system. That Valiant blew hot from the heater any time I wanted it, from then on.

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Heater valves (Chris Zwingli)

1. If you have to replace a leaking heater valve, be careful and compare the old one with the new one if it is aftermarket. Some of the aftermarket valves are taller than the original factory part and will cause interference problems with the air cleaner housing, leading to reinforcing ribs on the air filter housing rubbing through the upper hose between the heater core and heater valve.
2. Cut the old hoses off the heater core inlet and outlet instead of trying to twist them off. Twisting or pulling off the hoses could provoke the heater core into leaking- not a happy prospect.
3. While I am on the subject of hoses, it is possible to replace the z-shaped molded hose between the intake manifold and the heater valve with plain old 5/8" heater hose-just be sure to let the engine get to full operating temperature after you refill the cooling system. The heat will allow the hose to adapt itself to the new situation with satisfactory results.

Another car heat repair possibility (by Rick Hageman)

This tip is directed towards Stratus owners, but the procedures may well work for other Mopars. It's getting colder out and now time for the defroster. You get in, start the engine, turn up the heat control, and adjust the fan speed to medium and to your surprise...no air is coming from the defroster vents. Before you call the service department or start tearing apart the dash, the likely culprit is the blower motor resistor, located under the passenger side dash. This resistor allows the fan motor to operate at multi-speeds. When this goes out you'll have two fan settings, No fan or high. To access it, remove the two plastic push pins holding the plastic panel and carefully remove that panel. You will then find two wiring harnesses attached to a connector. This connector is held in by two hex head sheet metal screws. Use an 8mm nut driver or socket to remove these and out comes a plastic connector with a birdcage and wire resistors affixed to it. Likely, the "thermal fuse" has blown due to the heat generated by the wire resistors. Auto manufacturers have located this in the plenum as the moving air helps to dissipate the heat generated by the resistors.

You can now go to your Mopar parts department (I'm told auto parts stores don't sell them), and get a new one for $20 or less. There is some debate as to whether Mopar has problems with this design and the number of failures, but I can personally attest that I've replaced 3 factory units within a few months. I'm told the downfall is using the blower on the lower speeds as these resistors generate the most resistance, thus equating to the most heat, thus failure of the thermal fuse.

I have found that by going to Radio Shack and purchasing their thermal fuse # 270-1320, a 144 degree C, $1.49 part, I can replace the faulty one. One must carefully bend the 4 tabs holding the metal birdcage, removing this, carefully bending the wire resistor to access the faulty fuse, clip the leads, and either pull out the crimped remaining leads or with a small drill, drill them out. I elected to drill them out and positioned the new fuse, crimped the leads, and then for extra support / connection, soldered the two leads to the base. As a warning note, thermal fuses, by design, are heat sensitive, so use a heat sink between the fuse and where you solder. To forego this will ruin the fuse. You can use a commercial heat sink or a pair of self closing needlenose pliers.

From what I can tell the stock fuse on my Stratus is a Microtemp SRFBWC Y9E01 TF 110C. I believe this is a 110° C fuse. Attempt to position the new fuse as close as possible to the old location, reinstall the birdcage, making certain the wire resistors are realigned and not touching anything!

Reinstall the connector, wiring harnesses, panels, and pins. Turn it on and enjoy the heat.

Note: This procedure has worked well for me, but use at your own risk.

Overheating solutions • Thermostats • Radiator Caps • Radiators • Coolant • Fans

Overheating issues and solutions | Main car repairs page

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