[Tchernobyl]

Hi All - when my radiator came back, refurbished, it came with a round cap which apparently is "for cars with no expansion tank", whereas the original one, which was more squared, is listed as "for cars with an expansion tank".

What could possibly be the difference?

By the way, forgive my lack of mechanical knowledge but when I unscrewed it, I heard a vacuum noise, followed by the sound of liquid going further down the drain. Is this normal? I also noticed faint marks on the inner side of the hood, drawing the round shape of the cap, as if fumes came out at some point (even though, trust me it was very well tightened and I had to struggle a bit to unscrew it). Odd...

 

[spiderserie4]

in the photo above of the two tapes from "my alfa", the obvious difference is written on the caps: 0,5 as opposed to 0,7 bar
you need a 0,7 bar cap, doesn't matter what shape it is, but the square type with the red warning sticker is likely the correct type for an 86 injected.

basically, if the round one you got with the refurbished radiator says 0.7 bar then that's fine I'd say.

 

[Tchernobyl]

0.7 confirmed. Thanks!

 

[Mauricio101]

By the way, forgive my lack of mechanical knowledge but when I unscrewed it, I heard a vacuum noise, followed by the sound of liquid going further down the drain. Is this normal? I also noticed faint marks on the inner side of the hood, drawing the round shape of the cap, as if fumes came out at some point (even though, trust me it was very well tightened and I had to struggle a bit to unscrew it). Odd...

NO....You need the correct cap. Your cap is not allowing pressure out into the expansion tank, then back in when the coolant cools and contracts. Correct is .7 bar or 10psi with vent.

 

[Gubi]

The cap should have two seals and a vacuum relief valve. The bottom seal holds the coolant in the radiator via spring pressure and seals at the bottom of the neck. When the cooling system pressurizes, this opens and coolant can flow to the overflow tank (which is unpressurized).

The top seal seals around the top of the neck. When the system cools down, this seals the neck so that the contracting coolant in the engine can pull coolant back from the overflow tank into the engine. There is a poppet valve in the bottom of the cap that allows for vacuum release via this backwards flow (it's the springy bit in the diagram below the orange section).

Make sure your cap has the right pressure rating, both seals, and the vacuum relief.

 

[Tchernobyl]

Thanks for taking the time to explain how it works. Buying my 1st Spider a month ago clearly was also a good way to improve my "automobile" culture.
Here's a pic I just took of the cap. Looks like it has the two seals design but not quite sure about these 3 openings at the bottom... this seems to contradict the principle of a valve, right?

 

[Gubi]

That looks right to me. As long as it doesn't say "no vacuum relief" on the cap then it almost certainly has vacuum relief.

 

[Lantheaume]

I’m dealing with radiator stuff on an ‘86 Alfa and to show you the WRONG CAP here’s some photos of what was on my car.

Sent from my iPhone using Tapatalk

 

[Tchernobyl]

Noted. Thanks everyone!

 

[0die]

The cap should have two seals and a vacuum relief valve. The bottom seal holds the coolant in the radiator via spring pressure and seals at the bottom of the neck. When the cooling system pressurizes, this opens and coolant can flow to the overflow tank (which is unpressurized).

The top seal seals around the top of the neck. When the system cools down, this seals the neck so that the contracting coolant in the engine can pull coolant back from the overflow tank into the engine. There is a poppet valve in the bottom of the cap that allows for vacuum release via this backwards flow (it's the springy bit in the diagram below the orange section).

Make sure your cap has the right pressure rating, both seals, and the vacuum relief.

View attachment 1653205

Great info. I never knew there was a difference in the internal workings of a cap. But it appears it's really 3 seals. Doesn't that poppet valve have it's own small seal?

If the cap is not marked, how can you tell for sure? Can you squeeze the cap hand enough to open the poppet valve so you "know" there really is one there? Or should you just be able to see the 2-part seal for the poppet?