Radiator Cap not releasing pressure - 30psi ?

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Joplin
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Last night our old Camry leaked out part of it's coolant after the radiator developed a crack in the top plastic housing (at the seam where the cooling fan bolt attaches). Looking it over today I rented a coolant pressure tester/pump to pressurize the system to find that leak. I was surprised when I also checked the radiator cap and found that the cap was not releasing pressure as I pumped it up to 30psi. As you know, normally the cap will release the pressure at it's rating - in this case 13psi. In the past all of my radiator caps would go bad because they would not hold pressure so this is the first time I've seen a cap that would not release the pressure. This cap was purchased new at the local Toyota dealer about 3 years ago and has less than ~15,000 miles on it ! Was this over-pressure the probably cause of the crack in the radiator plastic ? I'm still puzzled that a radiator cap can fail like this to allow so much pressure to build - - am I overlooking something here or is just replacing this cap with a new one the next thing to do (after repairing or replacing the radiator) ?
 
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midwest
I tested some radiator caps this summer, 2 used honda oem caps, one new honda cap and one used stant. the honda caps all let off pressure near the rated psi, the stant never let off pressure. I was using the stant cap on the car and it seemed to work fine, just not with the tester. I didnt trust it though so i replaced it with the new honda cap.
 
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NJ
Originally Posted by JamesBond
I tested some radiator caps this summer, 2 used honda oem caps, one new honda cap and one used stant. the honda caps all let off pressure near the rated psi, the stant never let off pressure. I was using the stant cap on the car and it seemed to work fine, just not with the tester. I didnt trust it though so i replaced it with the new honda cap.
I had replaced my factory cap with a Stant as part of PM at 300,000 miles. The cap was defective and let the coolant evaporate out. I thought Stant was good stuff from back in the 70s when my friend's father used them on the cars he repaired in this driveway to earn some extra money. Never again. I'd definitely replace the cap with the radiator. Might want to replace the upper and lower hoses and thermostat while the system is drained.
 
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Washington
I had the same thing on a 2011 ML550 that I bought. I have no idea how long the cap had been that way. The Mercedes cap is a two-stage cap. It did not release pressure at either pressures. There was no apparent damage to the system. I replaced the cap with a fresh one. The new one released at the listed pressures. Dave
 
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MN
Excellent reminder to check those caps once in a while----- rarely the problem, but often overlooked ----- simple check!!
 
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