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Why Is My Engine Overheating?

Why Is My Engine Overheating?

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.

Dieseling Down

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.

Comments on this post (1)

  • Jul 07, 2020

    I know you guys are in the business to sell water pumps and not trouble shoot peoples engine cooling issues but if you are interested and would like to offer up some help I would appreciate it I have the following problem.

    I have a 1963 Ford Galaxie with a 390 V-8 and its running hot maybe you can help I have done the following.

    1. This engine was rebuilt back in 2001 and it was a quality rebuild at the same time I started my family so I had to stop working on this project that is restoring the car I would start the engine every few months just to run it but this last few months now that I have some time I am starting to work on it again but like I said it is running hot.

    2. I had the radiator rebuilt back in 2001 water is a little rusty looking I live in southern California near the beach so I don’t run anti-freeze just water with a rust inhibitor.

    3. Water pump was new when motor was installed it was an NOS part if I can remember.

    4. When running motor the temp gage in the car would slowly increase the needle would move all the way to the right after about ten minuet’s or so.

    5. The water in the expansion tank with the radiator cap off so I can see did not seem to be moving or flowing usually after the thermostat opens up the water in the expansion tank will drop a little water pump was turning over no issues with that so I replaced the thermostat (160 F) the old thermostat was in a failed state that you could physically see the plunger or center shaft had pulled out of its sleeve and jammed so it was never going to open again.

    6. OK so I install the new thermostat (160 F) coil part of thermostat into the engine but we have the same thing engine temp gauge starts reading hot all the way to the right although I can’t say the engine feels that hot or smells hot and that’s driving it around the block a few times but I think it is running hotter than normal.

    7. So I remove the new thermostat and put it in a pot of hot water on the stove I measured temperature and after about 160 F the thermostat started to open so it is working so I put thermostat back in the engine but we have the same thing we start running hot again.

    8. I replaced the temperature sending unit that goes into the intake manifold with new but the same thing we are still running hot I just wanted to make sure.

    9. OK so I continued to trouble shoot I checked the initial ignition timing I was running at about 10 degrees advanced (BTC) at about 650 to 750 RPM auto transmission in park timing would advance with RPM increase after about 2500 RPM we would have about 30 degrees total advance which means that the vacuum and centrifugal advance mechanism’s seem to be working I know a retarded initial trimming can cause over heating also an ignition timing that does not advance with RPM can cause over heating just for the record dwell was about 24 to 25 degrees the book says set at 27 degrees but what I am running should be good for now.

    10. OK so what’s next do I replace the water pump ? I am not sure but I still don’t think the thermostat is opening.

    Any help will be appreciated. Thanks Danny Kubisch

    — Danny Kubisch

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