How to Diagnose Heater Problems in Your Vehicle

The principle of operation is simple; the heater is like a small secondary radiator. When you're having heater problems, much of the problems that might exist with your radiator might also exist with your heater. Just as the engine's primary radiator transfers heat from the engine to the environment, so does the vehicle's cabin heater. Hot water from the engine circulates through the heater core and air flowing through the core picks up the heat and transfers it to the vehicle's interior. The principles of operations are simple, but the electrical and mechanical devices that make it work aren't that simple. Many devices come into play in making it work. Fortunately, most heater problems are easy to diagnose and repair. In fact, you can repair most heater problems yourself.

Low Coolant Level

The most frequent cause of heater problems is a low coolant level. The heater depends on an adequate flow of hot engine coolant to produce adequate heat. Check the coolant level. If the engine has been running for a while, let it cool down before removing the radiator cap. Wear work gloves and squeeze the top radiator hose for pressure before removing the radiator cap. If there's still pressure felt, wait until the coolant cools down even further. Hot coolant can spray out and badly burn you. Wear heavy gloves and wear safety goggles. A full-face shield is best to wear if you have one.

Insufficient Water Flow

The second most frequent cause of heater problems is insufficient water flow through the heater core. In newer vehicles that are five or fewer years old, the most common cause of insufficient water circulation is an engine thermostat that isn't opening properly. Depending on where the temperature sensor is located on your car, the temperature gauge may be registering in the red or in the cold. Many cars have the sensor in the thermostat housing and because of that the gauge will be in the cold range while the engine is actually overheating. As a rule, thermostats are easy to replace and you can do it yourself. Check your vehicle owner's manual for specific details on your car model.

Partially Obstructed Parts

Partially obstructed water jackets, heater hoses or a damaged water pump, water control valve and heater core are all potential culprits of heater problems. In older vehicles, any one or a combination of these problems could be the cause of you heater problems. This is where the detective works begins when you are narrowing down the suspect parts.

  • Prep the Engine: With the engine cold, remove the radiator cap. With the coolant level topped off, place a metal clad candy thermometer in the filler neck.
  • Check the Temperature: Start the engine and let it warm up. On older makes and models, the temperature should be 180 F. On newer makes and models the temperature should be around 205 F. Don't let the engine idle for too long with the pressure cap off because it will boil over.
  • Diagnosis: If the temperatures are low, there's a problem with the engine's cooling system. Suspect a thermostat stuck close, a blocked radiator core, obstructed engine water jacket or a defective water pump.
  • Watch the Gauge: With the proper temperature reading here, replace the radiator cap. Watch the temperature gauge. If the reading stays in the normal range, you can feel confident that the cooling system is fine. The chief suspects now become obstructed radiator hoses, the heater control valve and the heater core. Feel the intake hose. A hot intake hose on the engine side of the control valve narrows the heater problem down to a heater valve that's obstructed or not opening, or an obstructed heater core.
  • Check the Valve: With the engine cold, disconnect the heater hoses at the engine. With the heater valve, open the temperature control and set it to the hot position. Shove a garden hose nozzle in one of the heater hoses then turn on the water. No discharge from the other hose confirms a malfunctioning heater valve or an obstructed heater core. On most cars today, the heater valve is controlled electrically, so you will need to have the ignition switch turned to the on position to conduct this test. You should also check the heater fuse before condemning the valve or heater core. If the blower motor wasn't running, the fuse should be the first thing you suspect as the cause of your heater problem.

Once you have diagnosed the possible cause of your heater problem, you may want to complete the repair or take your vehicle to a professional mechanic. If you choose to take it to your mechanic, you'll be able to save the mechanic some time and yourself some money by telling them what you have done and what you suspect the problem to be.

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