installed new rad cap and now runs hot!

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mach1chick

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71 429 ....runs good, has always run cool (gauge never ran much above 180) . The old rad cap was rusty so I bought a new one, the one called for which is a 13 lb. Now it runs hot (was around 230 when I shut it down), and a lot of coolant came out of the overflow hose.

Should I switch to a lower pressure cap or higher? Or???

Thanks for any help!

 
K

Kit Sullivan

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I think the factory cap is rated at 16 lbs.

Basically, for every pound above ambient pressure, your cooling system's boiling point is raised around 4°.

A 16 lb. cap will raise the boiling point from approximately 212° to roughly 270°.

The fluid will "hold" and carry the heat to the radiator to be released into the surrounding air only so long as it is in a liquid state...below its boiling point.

If it is allowed to boil, it can longer hold the heat and will spontaneously release the heat.

That is why cooling systems are pressurized. Coolant at ambient air pressure cannot move enough heat away from the engine before boiling.

The 13 lb cap increases its boiling point to about 260°...10-15° lower than a 16 lb cap would.

Remember though, a higher-pressure cap puts more stress on the entire system: hoses, clamps, radiator, heater core, head gaskets, etc...

A high-mileage degraded engine might not be able to handle that pressure.

Many cars overheat due to mismatched fan shroud and fan combos, or missing fan shrouds.

 
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mach1chick

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So I should go to a. 16. Lb cap? The car had no other problems with the cooling system with the old cap. It didn't even burp any coolant out of the overflow hose.

Thanks for your help!

 

matrixx

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Yeah 13lbs sounds a bit low.

Sent from my SCH-S960L using Tapatalk

 

Mach1JB

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Should i use a 16LB cap on my 351CJ also ???

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c9zx

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The Ford shop manuals for 71-73 specify 12 to 16 PSI cap. It sounds like a bad thermostat to me if the only thing you did was change the radiator cap.

Chuck

 
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Kit Sullivan

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Yes, the other components should be checked also including the thermostat. Don't just assume the cap is the issue.

 

droptop73

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Are you using straight water or coolant? Is the system full, no air bubbles? Factory gage or aftermarket?

 

OMS

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71 429 ....runs good, has always run cool (gauge never ran much above 180) . The old rad cap was rusty so I bought a new one, the one called for which is a 13 lb. Now it runs hot (was around 230 when I shut it down), and a lot of coolant came out of the overflow hose.

Should I switch to a lower pressure cap or higher? Or???

Thanks for any help!
Do you still have the old cap?

 

mach1chick

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I threw the old cap away. Should have known better!

If the thermostat or other component were faulty, wouldn't it overheat or run hot regardless?

 

c9zx

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Yes, you would think so. While I don't really believe in coincidence, something else could have failed at the same time the cap was changed. Did the car sit unused for a period of time before or after the cap was changed? Even if the cap were defective and held no pressure, if the operating conditions and the ambient temperature were the same, it shouldn't affect the operating temperature of 180 degrees. It would affect the temperature at which the coolant boils (it would lower it). A proper mix of water and anti-freeze raises the boiling point of the mix about 6 degrees making it about 218 degrees at sea level. A 15 psi cap would raise the boiling point about 30 degrees. Consider the design parameters and condition of the cooling system before deciding more pressure is better.

Chuck

 
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