How to Troubleshoot an Overheating Engine

What You'll Need
Heat resistant gloves
Pliers or screwdrivers
Fluid container/bucket for draining
Replacement fuses
Wrenches/sockets/ratchets

An overheating engine in your car can damage the motor in major ways, resulting in hefty repair bills. If your car starts to smoke, check the engine right away to find the cause of the problem. Your main line of investigation will be the cooling system, which includes the radiator, water pump, thermostat, intake/output hoses, and cooling fan.

Step 1 - Check Engine Coolant Level

The first step is to check the coolant level. If the fluid level is low or completely dry, running the engine can cause major damage. There are two places you can check your levels: the coolant reservoir, and the radiator itself.

Give the engine a minute to cool down if it's been running. It's a good idea to use heat tolerant gloves to keep your hands safe during this process.

Many coolant reservoirs have a helpful mark to indicate a low level and inform you of the type of coolant you will need to purchase. The coolant level in the reservoir tank should be between the marks labeled hot and cold or full and low, depending on the make and model of your vehicle.

The coolant level within the radiator itself should be about an inch below the radiator opening. If you can't see any fluid inside the radiator, squeeze the hoses on either side of the radiator to see if they may be holding fluid.

If the level is low, there may be a leak—either into the engine or onto the ground below. Fill the radiator and coolant reservoir immediately with the appropriate coolant before driving the vehicle again to prevent further engine damage.

green fluid pouring into a car coolant system

Step 2 - Check the Engine Thermostat

An engine thermostat includes a valve that opens when the engine warms up to a certain temperature. Once the temperature is reached, the fluid is released into the system to absorb the radiating heat from the engine, moving it away to the cooling radiator system.

When the engine thermostat is not working, it may stop the coolant from getting to the radiator, causing the fluid to heat up and contribute to engine failure. Another possibility is that the thermostat valve might be stuck in the open position, never keeping the fluid away from the engine long enough to cool down.

To determine if the engine's overheating issue is caused by the thermostat, remove the hose that connects to the thermostat (loosen the hose attachment with pliers or a screwdriver) and drain the fluid into a container to replace into the system later. Next, remove the thermostat (find a parts manual for the engine model if necessary for specific directions), and visually examine the thermostat for damage.

You can test the thermostat functionality by putting it in a pot of water and turning up the heat. The thermostat should open up just before the water reaches boiling temperature. If the thermostatic valve has not opened by the time the water is boiling, it should be replaced with a new part.

Step 3 - Inspect the Electric Fan

An electric cooling fan also helps lower the temperature of the coolant, which prevents overheating. When the fan isn't working, the coolant stays hot, which causes the engine to overheat and fail.

To test the electric cooling fan, be sure the car is turned off (disconnect the battery to be as safe as possible) and attempt to spin the blade manually. If it doesn't spin easily, the fan will need to be replaced.

If the fan spins easily, start the car, then turn on the air conditioner. The fan should turn on within a few minutes. If it doesn’t, you'll need to change the fuse for the fan to identify the first of the possible electrical failures.

If a fan fuse replacement doesn't work, the next step is to check the wiring and electrical components. If the electrical system is connected and functional and the fan still isn't working, it should be replaced.

a car radiator fan

Step 4 – Test the Radiator

The water pump is the mechanism that forces the coolant into the radiator. The heat is then removed from the coolant via cooling fins in the radiator tubes. When the radiator is not working properly, the heat stays in the coolant, and can result in the engine overheating and even cracking. This is a gradual process, and is usually noticed in a moment of strain on the car, like when it's driving uphill in hot weather.

To test for a radiator problem, let the engine cool down and look for fluid leaks around the radiator body. Check for bent, broken parts, or holes in the radiator. Then, remove the radiator cap and examine the interior of the radiator. If there's a large amount of dirt or debris in the system, signs of physical damage, oily chunks of yellow/brown/green/orange in the fluid or around the rim of the opening, or a lack of fluid in the radiator itself, the radiator may need to be replaced, flushed, or filled.

Thick, Oily, Chunky Coolant - Flush the radiator out either manually or with a specialized flushing machine. Check the engine block for any signs of damage.

Damaged or Leaking Radiator Body - Replace the radiator and hoses.

Leaking Coolant Near Drain Plug - Replace the plug if possible. If the leak continues, replace the radiator and hoses.

Step 5 – Examine the Catalytic Converter

The main emission control device on a car is the catalytic converter. While it may seem unrelated, this system essentially allows the engine to breathe.

When the catalytic converter isn't working correctly, it can clog the exhaust system. This, in turn, can cause the engine to overload the cooling system with the backed up gases which attempting to escape. This can lead to an overheated engine, even with a healthy radiator and cooling system.

If there is a problem with the catalytic converter, the engine is also likely to lose power while idling.