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Engine getting hot when stopped!

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Alright Guy's, Would not having a thermostat make the engine get about 220 deg. in about 95 deg. weather:confused:? This is for a guy I met that has a 1972 620 with a L Block. Would installing a thermostat possibly take this problem go away or would buying a Newer/Better radiator be the answer? Is so, What brand is realy good? Truck runs good with no other problems just the "heating" up issue.


Thanks Guys for any Info.:)


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Vehicle speed will provide enough air through the rad for most cooling. When stopped it's up to the fan to move enough air. So:


The fan shroud has been taken off and not replaced. It's important to have the shroud to increase fan efficiency.

Fan belt is glazed, slipping or loose.

Air flow restriction caused by leaves, dirt, plastic bag stuck to front of rad.

If a fan clutch has been installed on this older truck it has probably failed and does not turn fast enough.

The gage is wrong.


A thermostat won't fix this but it is a good idea to have one, as it allows the motor to warm up quicker. A warm engine is more efficient so more power better gas mileage.

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i've seen engines including a l20b have a runaway reaction increasing the engine temp with no thermostat but can not explain why. they do make a 160 temp thermostat. but if you have a 521 don't expect any heat out of your heater with one of those. try a 180 and see what happens. they're cheap and east to install.


carb running too lean will increase the temp.


my l16 was over heating last month or so and decided to swap out the water pump for fun although the pump looked and seemed fine it fixed the problem.:confused:

i also noticed that there are different sized fins in some l16 water pumps.:confused:


one more thing as mentioned above by mike, the sending unit or gauge may be off. changing the sending unit is easy. not cheat though units run about $20. if it is running hot you can tell just by feeling the valve cover and radiator.


make sure the fins on the radiator aren't bent over. maybe flush the radiator. i've heard running no anti freeze will raise the temp slightly also. good luck.

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If the steel ring around one of the the cylinder holes on the head gasket cracks or corrodes, you can get enough compression or combustion leak into the water jacket to push the water below the sender housing. You wind up reading the temperature of steam. 220 degrees sounds right with a 13 or 14 pound radiator cap.

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A compression leak will over pressurize the system for sure. This will cause the rad cap to vent coolant into the recovery can, if equipped, eventually lowering the level to where the motor overheats. Once this happens it will run hot weather you are idling or driving. He doesn't mention unexplained coolant loss, just over hot. This would suggest to me insufficient heat transfer from the rad to the air flowing through it. (misread the first post)


Lack of air flow:

Unlikely at highway speeds, however fin damage, leaves, bugs and dirt will obstruct flow. If the rad has rotted the fins will flake off and block air flow. You should be able to look through the rad from front to back. Use a garden hose to soften and rinse dirt and bugs out.


Lack of coolant flow: Soft lower rad hose collapses under high speed suction from the water pump, restricting flow. If not sure, replace it.

Rust, sediment and scale from hard water may have built up in the inside of radiator's vertical cooling tubes blocking flow. Partially drain and remove rad cap and inspect tube ends through the opening. They should be clear of any obstruction. If there are scale deposits, you may be able to use an over the counter product to dissolve the lime deposits. Follow the instructions and refill with a good quality coolant/antifreeze. Change every two years no matter what.

Loose sediment can be flushed out by removing the bottom rad hose and flushing with a garden hose. Probably best to reverse flush the block as well.

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i had my 72 521 recored at northwest radiator for i forget how much about 10 years ago. something to think about if he wants to stay original and have a new radiator at the same time. if he does, while its out bead blast it and make that brass shine. nothing cooler than a shiny brass radiator.


i put a brand new radiator in my 89 jetta and holy crap the temp went 2-3 needle widths down.:cool:


was this a recently aquired problem or has it always ran that hot when stopped? if the temp only goes up while staying still than i'd say the radiator or water pump.

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