I bought this test kit to try to find out where my car is leaking coolant. I want to hook it up to the thermostat housing and first pump it up to around 15 psi, just below the 16psi that will open up the radiator cap. Check if pressure drops and visually inspect if I see any coolant leaking. Next I'll pump it up to above 16psi, just to see if this will open the radiator cap and coolant gets in the overflow bottle. All this with the thermostat removed, BTW.

Is this a proper way to test the cooling system?

You need the T-stat adapter to test Fiero w/ most Testers sets for same reason in https://www.fiero.nl/forum/Forum2/HTML/145505.html Coolant Pressure Testing for Fiero.

You use same "end" to fit the rad on top of the adapter so can test in back.

While can test the Rad Cap on the car... May make a mess or have other problems.
Also "Valid" test for 15-16PSI Cap "pops" around that to vent pressure on 18PSI max test system. ⚠️ Warning: Do Not allow the system to reach 19-20PSI!
This is why a "proper" Cap test require another adapter that's basically "tube" to test the cap on the tester.

The "tube" has 2 ends that same small end as T-stat adapter.

E2A--> Stant 12552 cap adapter
Example: www.amazon.com/Stant-12552-...dapter/dp/B001DCF0QI

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The Ogre's Fiero Cave

[This message has been edited by theogre (edited 04-30-2024).]

Yes, the kit comes with a GM cap which should fit the t-stat housing (although I haven't tried it yet).

Hard to tell from Amacon link as just say 2 GM (Cadillac)
Most tester kits don't have T-stat adapter even most Stant kits.

Pull the left 2 metal adapters & look under them.

Because There is/was larger Rad Cap to fit Heavy Trucks like Semis. If that big adapter = small ones next to it on the bottom.... Likely won't work on Fiero T-stat "tube."

Correct adapter look very similar to T-stat cap but likely won't spin like Stant units so be careful turning it on/off. If you lube it, use only silicone or brake grease.

Again, if you test w/ Rad Cap on, max pressure = rad cap pressure that should = 14-16 PSI. Rad Caps are not very accurate even when new.

I think somewhere in the past "they" just "bumped" the PSI rating for the Caps to be @ Max PSI. While many other docs still say 15PSI. Even most Coolant Marketing & other numbers for boil over protection etc still use 15PSI.

⚠️ That gauge switches colors @ 15PSI but worse turns Red well over 20PSI... Pushing > 18PSI on many vehicles including current models is not a good plan.
I updated test page w/ newer Stant ST270 gauge & what the marks mean. (I think Motorad is selling same thing since the buyout.)

[This message has been edited by theogre (edited 04-30-2024).]

quote Originally posted by Cliff Pennock: Yes, the kit comes with a GM cap which should fit the t-stat housing (although I haven't tried it yet).

yeah those usually work, be careful with the adapter as it likes to break the rads when you don't have it completely perfect, also a likely leaking area If it's from the back (~ 5 fieros now) is from the water pump outlet hose and only became an issue when hot which was extremely annoying to find