Rick's Free Auto Repair Advice

Car overheats — common causes

Car overheats—6 most common causes

Here are the six most common causes of car overheats condition

  • Radiator fan not working
  • Faulty thermostat
  • Low on coolant due to leaking hose, radiator, heater core or water pump
  • Faulty water pump
  • Clogged radiator
  • Damaged head gasket

Inoperative or wrong speed radiator fans can cause car overheats condition

Most late-model vehicles have several electric radiator Radiator fan motor, blade, and shroud for Ford Mercuryfans that run at different speeds depending on the cooling needs of the engine. GM, for example, uses two fans that can run at two speeds. If you’re driving with your AC on, the fans run at high speed at all times. With the AC off, the engine computer determines whether to run the fans at low or high speeds depending on how well the low speed is removing heat. The most common cause of GM radiator fan failure is a bad relay. There are three relays, so check all three. Here’s a link to how to check them.

Chrysler uses a different method. Their electric radiator fan relays run the fan at variable speeds depending on how much cooling is required. A failed radiator fan relay is the most common cause of engine overheating. See this post on how to diagnose and fix a Chrysler radiator fan relay.

Other carmakers use variations on those two approaches, where relays control the fan speed or a variable type relay is used to provide varying fan speeds based on cooling needs.

cheverolet avalanche thermostat

Engine Thermostat

A faulty thermostat can cause a car overheats condition

Engine thermostats operate with a pellet filled with a mixtures of wax and copper particles. The wax mixture melts at higher engine temperatures and expands. The expansion a pin outward which opens a valve, allowing coolant to flow. Over time, the rubber sealing gasket wears out and the molten wax seeps into the coolant. At that point, the thermostat stops working, leaving it in a permanently closed position, preventing coolant flow, resulting in your engine overheating.

Low coolant due to a leak will cause a car overheats condition

Check the coolant level in your coolant reservoir. If it’s low, start by checking for leaks.

Leaking radiator or heater hose

The most likely cause hose leakage is old age. Radiator hoses are coolant coating on hoseunder constant pressure and over time the rubber deteriorates. That’s why you should check the condition of your hoses regularly, especially on an older car.

Leaking water pump

I’ve seen water pumps last for 300K miles and I’ve seen them fail at 100K miles. What’s makes the difference? Simple. Neglecting coolant changes or using the wrong coolant is the single best way to damage your water pump. See this post on water pumps and see this post on coolants.

Leaking radiator

Neglecting coolant changes or using the wrong coolant is

clogged car radiator

Corroded radiator tubes

the main reason for clogged radiator passages and internal corrosion that causes leaks

Failed head gasket causes a car overheats condition

An engine head gasket can fail in several ways

• It can leak coolant into the oil.

This causes the oil to look like a chocolate milkshake. If left in that state, the oil/coolant mix will destroy your engine, but it does not create a car overheats condition.

• It can leak oil into the coolant

Since oil is lighter than coolant, you will see an oil slick in the coolant reservoir. However, the oil film will coat the radiator fins, reducing cooling ability. While the oil in the coolant won’t destroy your engine, it can cause a car overheats condition.

• It can leak coolant into the combustion chamber

When coolant leaks into the combustion chamber it will burn along with the air/fuel mixture, causing white smoke from the tailpipe. The head gasket breach between the combustion chamber and the cooling system can also force hot combustion gasses into the cooling system. This will cause, the coolant to boil, cause the coolant to overflow from the coolant reservoir and cause a car overheats condition. In fact, this type of head gasket leak can cause your temperature gauge to rise into the hot zone in just a matter of 1-2 minutes.

Neglecting coolant changes allows corrosion to set in and that corrosion builds up in coolant passages, restricting coolant flow and causing engine overheating that can damage the head gasket. Neglecting oil changes causes the oil to build up excessive amounts of acid that attacks gasket materials.

head gasket leak

Typical head gasket


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Posted on by Rick Muscoplat

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