This is a super quick post that can be summed up with the post title. Sorry to not have a ton of content or testing or whatever. I just wanted to get this out there and hopefully save folks from these cheap silicone radiator caps.
TLDR at the bottom.
Back story – Sharka, my 95 NA Miata, eats radiator caps pretty quick. One lasts about 6 to 12 months and then it no longer holds much more than 8 psi, leading the car to run hot. I have no idea why this happens.
I’ve got a fancy pressure tester that lets me test both the cooling system pressure and a radiator cap itself by switching adapters and screwing on the cap. It’s this small one from Mityvac if anyone is interested. With it, I can verify there’s no leaks in my cooling system and see just how much pressure a 16 PSI cap (for example) can actually hold.
I always use OEM rad caps from Mazda. I buy them in bulk every couple years and get the 16 PSI/1.1 BAR units that were stock on the NB. They’re good caps, they just… get eaten. My NB never ate one. They lasted years on Bucky. Just Sharka.
Anyways, I decided I’d try one of these silicone ones from amazon. Instead of a rubber sealing surface, they have silicone rubber. They’re supposed to last longer and not degrade like rubber. Neat, huh?
No, not neat. Far from.
I installed this cap and overheated on the very next drive. It wouldn’t hold any pressure at all and was brand new.
There I was, a couple miles from home at a gas station, coolant rushing out the overflow bottle and peeing the ground by the pump. Once things cooled down a bit, I changed the cap with a spare I keep in my trunk tool kit and went home.
Yeah, I keep a spare rad cap in my trunk.
Inspecting the silicone one at home, I saw this tiny slice in the silicone. It wasn’t there when the cap came out of the flimsy box. Installing the cap once must have torn the silicone and caused it to fail. I verified that it no longer held pressure with my trusty Mityvac.
Let me say that again – installing this cap once made it fail.
Back to OEM rad caps for me.
TLDR: Silicone radiator caps can tear on the first install and become worthless.