Dissecting a cracked expansion tank cap -- 9445462

[jreed]

I've had a couple of these expansion tank caps for the coolant system crack over the years. The most recent one cracked last month when I overheated the car a little bit due to a disconnected fan relay wire. This is a Gates cap purchased from FCP 3 or 4 years ago. I cut the cap open to see what's inside -- never looked inside one before and don't recall seeing one dissected here before. Just adding this in case it's interesting to somebody. Below shows the lid after cutting the edges and snapping the green lid in half. The cracks are visible in the center of the cap, inside the small circle.

Cap after cutting the ends and breaking the green lid apart

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Side view:

Side view with black pressure valve

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There was more to the pressure valve than I was suspecting... on the left is a frame with a rubber gasket, at center is a metal disk with white seal, black seal and tiny spring, and on the right is a black cap with a large spring that has a flattened end:

Components of pressure valve

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The large spring puts pressure on the black cap, which is pressed against the center of the green lid. It could be this force plus the heating cycles is why the green lid develops cracks over time in the center.
Welcoming others' opinions, input and experiences with these green caps. I did see that FCP has changed the listing for the Gates cap -- it's now shown in black instead of green... not sure what the differences are besides the color change. Fcp says: "Note: This is a green/grey OR black cap and the pressure rating is 1.50 bar or 22 PSI. The part has been updated with a black color, so you still may receive a green cap as inventory cycles". (from: https://www.fcpeuro.com/products/volvo- ... dot-50-bar )

[scot850]

There was a theory for a while the color meant a different pressure for opening the relief valve. Based on what FCP is saying it would suggest this is not the case, although that may be just for Gates products?? I have several new and uses ones. Only issue I have ever had is the valve leaking a little and that was on cars that came with aftermarket caps. I should have a look at the used ones and see if there are any cracks like you have seen.

Thanks for the info and dissection!

Neil.

[wizechatmgr]

I believe these are setup to break vacuum as well as ensure designated pressure. I have a couple bad ones here, maybe I'll sacrifice one instead of sending it back to FCP.

[foggydogg]

50/50 anti freeze mix coolant has a boiling point of 223*F. A 15 pound cap pushes that number to 268, which I think is plenty hot. If any of my two-decade old cars gets any warmer than that I would rather they puke on the ground to let me know they are feeling poorly, as opposed to pressurizing the system to the point where the heater core blows ( a common problem with early generation Tauruses with the head bolt issue.) I took off all the 22s and replaced them with lighter pressure caps.
There was a pretty amusing discussion on the Forum a while back that involved some confusion over the metric-obsessed and the "0.75 bar" designation of those lighter caps, with one conclusion that the boiling point was somehow reduced by using the low pressure cap. I think the actual US designation for that cap is 12 pounds.