We get a lot of calls asking “why is my engine overheating?” Over time we have compiled a short list of common Causes of Engine Overheating. We add to this list from time to time so check back as needed.
Stuck Thermostats Cause Engine Overheating
Your thermostat is usually located in a housing where the upper radiator hose connects to the engine. It controls the operating temperature of the engine by blocking the flow of coolant from the engine to the radiator until the engine reaches the desired temperature. When the thermostat opens, coolant is permitted to circulate from the engine to the radiator. Unfortunately if the thermostat sticks (usually caused by a steam pocket under the thermostat due to incomplete filling of the cooling system or coolant loss in the system) no coolant can circulate through the radiator and the engine quickly overheats. You can check for a stuck thermostat by carefully touching the upper radiator hose when the engine is first started and is warming up. (Watch out for spinning fans blades). If the upper radiator hose fails to become hot to the touch within a few minutes of starting the engine, the thermostat may be defective and needs to be replaced.
Air Pockets in the Cooling System
Air pockets will develop after the cooling system has been drained and refilled. If an air pocket is present, the coolant temperature sensor output will swing wildly as it’s alternately exposed to air, then coolant, then air. Many thermostats come pre-drilled with a small hole and "jiggle valve" designed to prevent air pockets by allowing air bubbles past the thermostat, so they can reach the highest point in the cooling system
Defective Fan Clutches Cause Engine Overheating
A fan clutch is often used to improve fuel economy on rear wheel drive vehicles with mechanical i.e. belt driven fans. The clutch is a viscous-coupling filled with silicone oil that lets the fan slip at high speed and reduces the parasitic horsepower drag on the engine. If the clutch slips too much though the fan cannot turn fast enough to circulate the air and cool the engine. Over time that silicone fluid inside the clutch breaks down and may leak out. If you see oil streaks radiating outward on the clutch or the fan can free spin the fan with your hand (when the engine is off!) the clutch may be compromised and need replacement. Play or wobble in the fan due to wear in the clutch also signals the need for a new clutch and will destroy you water pump bearings.
Electric Fan Motor Failures Cause Engine Overheating
A temperature switch or coolant sensor in the cooling system cycles the electric fan used on many front-wheel drive cars. A failure in the temperature switch, coolant sensor or relay that send power to the 12V fan motor will lead to engine overheating. If the fan motor itself goes bad, the fan won't work and the limited airflow will also result in engine overheating.
Coolant Leaks Cause Engine Overheating
Loss of coolant in the cooling system due to leaks in the radiator, heater hoses, water pump, heater core or engine freeze plugs will reduce the coolant levels and result in rapid overheating. A flashlight and a keen eye will help isolate the leak.
Radiator Caps can Cause Engine Overheating
If you cannot find the leak but still experience coolant loss you may want to check the radiator cap under pressure. A weak spring inside the cap the will cause coolant loss through the the overflow or expansion tank. Caps must seal around the top lip on the radiator neck and hold vacuum as well, otherwise coolant will never be recovered from the overflow tank. If the system can’t hold pressure, coolant will boil in the cylinder head and next to the cylinder walls, forming vapor bubbles, and the engine will quickly overheat.
Internal Coolant Leaks Cause Engine Overheating
Still can’t find the leak then brace yourself as there may be a crack in the cylinder head or block, or a leaky head gasket allowing coolant to escape into the combustion chamber or crankcase. This is bad news.
Clogged Radiators Cause Engine Overheating
Clogged radiators and heater cores can impede coolant flow rates and create hot spots in the core. This reduced cooling capacity will result in overheating as the radiator cannot fully dissipate the engine heat. Comparing the temperature of radiator inlet and outlet hoses provides some indication of coolant flow, but a clogged radiator will exhibit the same signs as a lack of pumping action.
Exhaust Restriction Causes Engine Overheating
If you have restricted the exhaust, resulting in back pressure, you can cause your engine to overheat. Blockage may be in a plugged catalytic converter or compressed pipe. Test your intake vacuum and exhaust back pressure to rule this out.
Failed Water Pumps Cause Engine Overheating
Not maintaining your coolant system pH can corrode an aluminum or steel water pump impeller to the point that the blades are eaten away. You must replace your water pump.
What causes my engine to overheat on the highway?
Highway speed overheating is more rare due to higher coolant flow rates and better air flow less likely because at highway speed you have higher flow rates from higher engine rpms. Causes of higher speed overheating may include;
- Too dense a fin pattern in the core creating a “block of metal” which will not permit air flow
- Objects blocking airflow e.g. winches, ac condensers, intercoolers, screens etc.
- Too shallow a fan shroud which may trap air on the back side reducing the source sink temperature differential and heat exchange
- Air trapped in the engine compartments
- Flow restriction caused by too few or too small a tube may flat lines the flow rate in the radiator
- A tired spongy hoses which result in hose collapse at higher rpm
- Cavitation in your water pump
- Impeller vanes corroded away
- A radiator core clogged with sediment or lined with scale which impedes heat exchange
- A blocked water passage
If your engine seems to keep running after you turn your ignition off, it is referred to as dieseling down. The classic example is the Griswold’s family truckster in the movie Vacation which seems to run and after they get out. Potential causes; (1) incorrect timing (2) running too rich caused carbon build up in the ignition chamber that then holds heat and ignites the un-burnt fuel (3) a spark plug that retains heat and causes un-burnt ignition (4) a sticking (and still hot) carburetor that causes combustion in the piston chamber or (4) oil gases from the engine crankcase can provide ample fuel for dieseling.