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 engine overheat (quickly).
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 get become hot to the touch within a few minutes of starting the engine, the thermostat may be defective and needs to be replacement.
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 fna used on many front-wheel drive cars. A failure in the temperature switch, coolant sensor or the 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 you 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 you may want to check the radiator cap under pressure tested. A week spring inside the will cause coolant loss through the out the overflow or expansion tank.
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.
Exhaust Restriction Causes Engine Overheating
If you have restricted the exhaust resulting in back pressure you can cause your engine. 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 blades 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 reduce heat exchange
Air being trapped in the engine compartments
Flow restriction caused by too few or too small a tube which flat lines the flow rate
Tired spongy hoses which result in hose collapse at higher rpm
Cavitation in your water pump
Impeller blades corroded away
A core clogged with sediment or lined with scale which impedes heat exchange