Expansion Tank Cap Basic Questions
 

Just trying to figure out some things related to expansion tank caps.

1. If the inner seal of the cap is compromised/deteriorated, what is the effect? I'm assuming lower of system pressure and potentially loss of coolant but wanted to confirm.

2. If the cooling system has a recovery tank that is in good working order, will that prevent a bad expansion tank cap from resulting in a loss of coolant?

3. Is there a certain risk in replacing a bad cap on an older cooling system in that the increase of pressure might cause something else to fail?

Answers
 

Answers:

1. A pressure cap that doesn't seal tightly will prevent the system from pressurizing as much as the cap is supposed to allow. For the most part, the purpose of a pressurized cooling system is to prevent the coolant from boiling at normal operating temperatures, especially at high altitudes. Using antifreeze also increases the boiling point. If the coolant never gets over 100C you could probably operate without significant loss of coolant even with a damaged cap.

2. The recovery tank may prevent loss of coolant depending on how bad the pressure cap is. However, coolant return from the recovery tank is based upon vacuum in the cooling system and a damaged pressure cap may not allow a vacuum to form.

3. There's no way to know whether pressurizing a system that hasn't been pressurized in a long time will cause leaks. Try it and find out.

Jeremy

I can't really explain why, but in every case where I've had a car with a bad pressure cap, it's also made the coolant temps run hotter. My best guess is localized boiling in the cylinder head slowing heat transfer, but I don't know if that makes any sense.

for #3 i would say if you dont pay attention and put a 20psi cap on there you will find a weak link the system is supposed to operate at 14psi almost every single replacement cap i have found has been a 20-25psi not 14 like the car needs, I was having coolant leaks due to to high of pressure... now i have a 13psi cap and it appears all of my leaks may be gone, still watching... i also have lower temps as well not having leaks...

So it sounds like the 1.4 cap that's recommended makes for too much pressure?

A pressure cap is used to prevent the system from being over pressurize. There are many parts that might be fail due to the over pressure.