The 308/328 Coolant System – Anatomy of the Expansion Tank Cap

My 1972 Alfa Romeo Spider has a non-pressurized expansion tank and a cap on the radiator. My 1991 Alfa 164S and my 2001 Land Rover Discovery as well as my F350 have pressurized expansion tanks with screw on caps and an overflow relief tube to ground. They do not have caps on the radiator. The Ferrari seems to have the same type of system as these, the expansion tank just happens to be metal rather than plastic and uses a quarter turn radiator cap rather than a screw on cap.

Using logic and my new-found wealth of knowledge on radiator cap science I'm struggling to understand the purpose of a "dangling vent valve" as it would apply here.

 

"Of course that cap is vented. Why do you think it has the slotted holes in it? That is just covering the vent."

I don't think so...Yeah - they do LOOK like vents and I guess they serve some purpose along those lines but my wife's MB cap has those same "vents" but it does not allow air back through - at least I could not suck any air through the cap by mouth which I can on my 328 cap. So it seems to me, as others have said, that the cap for 3x8's should NOT have a vent that allows air back in under vacuum. IF thats true, most (all?) of us are using the wrong type of cap. Whether it WILL actually make a difference re spitting/air in the system, I can't say but it would certainly explain why the OEM systems didn't have these problems when new but do now since, over the years, the correct type of cap may have been replaced by the incorrect type of cap.

I did some more research and came across a Fiat site and they make a big point of using the "correct" Italian cap rather than a typical parts store replacement because, they say) the replacements do not function properly in the system. http://spiderroadster.com/994057price.htm Coupled with KCabs info, my conclusion is: we're doing it wrong...

Yeah, maybe all this IS a bit OTT but, heck, if it ends up "resolving" the common complaints re air/spitting that would be a good thing!

 

Stant used to make nice caps, but now they're MIM plastic junk. I prefer the Behr caps myself.

Also, if your header tank is puking, there's probably more going on than having too much coolant inside. Start by venting out any air pockets in the system.

 

It's not as if I didn't have anything else to do today but since it was just laying there I decided to take a look at the Stant cap. Considering the weight of the dangling valve and the clearance around the opening filled with the shaft and it's ability to pass relief in the positive direction my take is that this is just a cheaper way to make the same thing because someone figured out that there is no need for a spring. The moment pressure begins to increase in the tank that thing is just like the spring loaded one, it's sealed and forever remains so until pressure in the system drops below atmosphere.

That would at least explain why nobody has ever seen any difference between the two but I'm thinking the disadvantage to the dangling valve type is that when all is equal the system is exposed to outside air. I don't think that's good, the cooling system should always be closed. The dangling valve would be okay if there was a head of coolant above it, such as an expansion tank. I think that's what they are designed for, an expansion tank beyond and above the radiator cap such as I have on my '72 Alfa.

 

The OEM cap is exactly as JohnK specifies, it has the reverse normally-closed valve which allows air in. At least on my 328 which has the original "4-ear" cap.

Not being from the USA I have never heard of Stant. But their business model appears to be to raise the status of radiator caps to that of rocket science. Not surprising if thats their only product.

I am not sure why anyone would not use the OEM type which are inexpensive.

 

Well, being from the USA I'd say that most of us "never heard of Stant" either before getting here. I mean it's not like "Chevy" or anything. Plus, as far as radiator cap technology goes I don't think Stant has any of it's own designs, they just make reproductions of the OEM caps. What I've gotten out of this discussion is that I'm going to use the original (apparently non-vented) cap that was on there when I bought the car. FWIW the Superformance cap is identical to that.

 

I have one from Superformance and the original, they are both identical and vented (ie the reverse normally closed vent).

Its possible they could have changed the caps over the years from unvented to vented.

 

No.

 

FWIW, Stant has been making cooling system (and other components) for over 100 years. For most of my automotive life, if you bought a replacement radiator cap or thermostat in the US, you bought a Stant if you wanted to be sure of the quality. Otherwise you bought one from the car dealer. Nowadays, as noted, some (all?) of their radiator caps are made in Mexico. Being made in another country doesn't make something bad. If it did, why does anybody buy cars/other products that are not made in their home country? Admittedly, the MiM thermostat that I currently have on my 328 does seem that if the metal was any thinner, we'd call it tin foil!

Back to the subject...perhaps we (I) are/am overthinking all this. If the OEM Ferrari Tstat was, as claimed, the same construction - vent valve to open under vacuum - as we all seem to be using, then that pretty much ends the discussion. As I have often said, I never heard that any owners of new 3x8s back in the day had to take their cars back to the dealer every few weeks for air bleeding! So this thread got me thinking that MAYBE the reason for that had to do with the Rad cap. But if the OEM cap had the typical type of vented cap, that seems to eliminate the idea of the "wrong cap" causing the much-discussed air bleeding.

BUT...I still wonder about the comparison with the 328 vs my wife's MB expansion tank cap which does not seem to have a vacuum vent and basically has the same sort of pressurized expansion tank setup as my 328.

 

Mention was made of the Ricambi-stocked Stant cap. They also show this one which looks very different on the underside than the Stant. Does it work the same way? If it does, it appears to be an unnecessarily complicated way to provide venting under vacuum. The underside looks very similar to the cap on my wife's MB which, as I have mentioned, I cannot get to admit any air by sucking on it. No, not very tasty! https://www.ricambiamerica.com/expansion-tank-cap-radiator.html

As I mentioned earlier, the lever on the Stant that Ricambi sells will hit the deck lid so it can't work for my 328 at all.

 

Thats the same as the original one, except as they correctly mention, the ones Ferrari supply are square-ish and the one listed is the more usual 2-ear.

The slots are for the reverse vent. Superformance sell the ones which look exactly the same as original and also have the slots.

 

Well, I'd buy one of those caps today just out of interest but we are leaving TX (and the 328) in two days, bound for Bath, UK. We'll be there until 1 November. When return I'll purchase one to satisfy my curiosity.

 

Why not buy the original type from Superformance while over here? BTW if anywhere near Wimbledon (SW London) give me a shout

 

Yeah, I could do that! Maybe I'll take over the option on the Expansion Tank Cap museum that Brian is abandoning! If I visit Wimbledon during our five months in Bath, we can discuss expansion caps over some pints!

 

Here's what Maranello Concessionaires provides.
My car came with a 2 ear when I bought it and it's currently got a 4 ear on it. It's been spitting a bit requiring replacement. Just more fuel to the radiator fire. Image Unavailable, Please Login Image Unavailable, Please Login Image Unavailable, Please Login

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Since this thread is talking coolant stuff. can anyone tell me why my 1985 308 has two thermostat gauges? One in the center console, and one next to the fuel gauge. I thought one was for the oil temp, but both gauges have the water symbol on them. Although they do not have the same temp ranges on the gauges themselves. thanks

 

The temp in the main console is coolant. The temp in the center console (next to the clock) is oil.

 

so should they have different symbols in them? one for water and one for oil?

 

Thanks for the clarification Thorn. Still learning all kinds of stuff about this car, and have SO much more to learn. hahaha

 

To be honest I’ve never noticed that. Because of the relative positions of the two gauges it is not possible to simultaneously view both of them and I suppose Ferrari took a gamble that no matter how pernicious the internet became it would still be impossible for Google to find an image that clearly shows the two in the same picture. honest, I looked...

 

hey Brian

yeah now I just have to try and find the correct manual & year that came with that car. when you said I should start my own thread? what kind of thread were you talking about. Like, "Looking for 1985 308 Manuals"? hahaha

what Pics were you looking to see? I hope none of me because I am no prize. hahahaha

 

There seems to be some nomenclature confusion in this thread. A vented/closed system cap has a rubber gasket at the top of the neck (position "A" in the first post) in order to create the seal which allows vacuum to pull coolant from a reservoir as the system cools. So for systems with a coolant recovery tank a vented cap is required. However, there is absolutely zero problem with using a vented cap on a system without a recovery tank, so for our purposes the vented/non-vented discussion is irrelevant.

However, the operational mode of the vent valve is arguably of importance to us. According to descriptions in this thread, the OE cap is of the full pressure type, meaning that the vent valve is closed unless sufficient vacuum exists to open it. A full pressure system is what most of us are familiar with-- the system pressure begins building immediately as the coolant heats up. In a partial pressure system the vent stays open until the pressure in the system gets high enough to force it closed.

This difference in venting is illustrated in post 4 of this thread and explained in greater detail on this site: http://www.allpar.com/fix/engines/cooling-caps.html

Apparently (I say apparently because I have not looked at one) the Stant 10331 is a partial pressure cap, meaning that it has a weighted vent valve. If anyone knows for a fact that this is incorrect please set the record straight.

So the OP (Brian A) got it right in the end by concluding that the Stant 10231 is the factory style replacement (except for the fact that Stant caps don't crack after a short time in service). But it seems like there's still a lot of confusion between vented/non-vented and full pressure/partial pressure caps. Since our cars don't have coolant recovery tanks the vented/non-vented discussion is 100% moot, but since the factory caps look to be of the full pressure persuasion it would seem logical to stay with that type of cap.

Great thread and big kudos to Brian A for sorting it all out. I've just ordered a Stant 10231 to replace the craptastic factory cap on my 348. Many thanks for the many pics posted here, which helped confirm my decision.

 

So how does pressure build up if the vent is opened?