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

My 308 recently spewed coolant through the expansion tank vent tube. I replaced my coolant tank cap (Stant 10231) which solved the problem.

I studied my old cap and thought I would post a couple of photos which show how (I think) a “vented” cap works. A vented cap is needed in the 3x8 system. I also have an old OEM vented Italian cap (0.9 bar) but, although it works the same way, how it works is not as obvious as for the Stant cap.

Below are photos of the neck of the tank fill pipe and of the Stant 10231 cap.

First, notice that the expansion tank neck has two flanges which the cap seals upon. I’ve marked these flanges “A” and “B” in the photos. The corresponding washers, also marked “A” and “B” on the cap photo, seal against these flanges.

The “A” flange is simple: the “A” washer on the cap totally seals the top of the cap so that no coolant can leak out to the top of the coolant tank. It is a strong pressure-proof seal.

Washer “B” which presses against flange “B” is more complex. It is pushed against flange “B” by the big spring. This is the main pressure relief system. Pressure builds as the engine warms because both the coolant and air in the system expand. If the pressure of the cooling system rises above the rating of the cap (for the Stant 10231 that is 16 psi or 1.1 bar) (above atmospheric pressure), the spring lets washer “B” leak a little bit. First the air pocket in the expansion tank is vented and then, if necessary, some coolant will also begin to pass under washer “B”. As shown in the photograph, this excess air and/or coolant then travels out through the vent tube which leads to the bottom of the car. Any liquid dribbles on the ground.

The cooling system is supposed to keep the car at a maximum temperature of 80-85C. That means that the coolant and air expands a certain maximum amount and no more. When fully warmed, all air in the expansion tank is purged through flange/washer B and the coolant fills the expansion tank to the brim.

That is why the expansion tank is only about half full when the engine is cold. The 308 Owner’s Manual says that this level “should never be 6 cm (2.3”) below the filler plug base”. I’m not quite sure what that means, but the equilibrium level is specific to each individual car anyway. My car stabilizes 4.7 cm below the top lip (Flange A) of the coolant tank (my coolant depth is thus 6.5 cm to the bottom of the tank). If I fill it more that that, the car expels the surplus coolant through the vent tube to bring the level back to that 6.5 cm cold level.

So, that what happens when the engine heats from cold to operating temperature. However, when the coolant cools again when the engine is shut off, it begins to shrink in volume. This means that the system wants to suck air/coolant back into the expansion tank.

As the engine cools back down, another vent in the cap takes over to let that air back in. This valve on the Stant 10231 is immediately below washer “B”. I’ve marked this valve’s washer as “C” on the cap photo and the vent valve itself is visible below the washer.

This valve is normally closed but it opens up when the system wants to suck in air. There is no pressure threshold to overcome like the 16 psi needed for flange/washer B. It opens at 0 psi. If the expansion tank needs a bit of air, this “vent valve” obliges. As the engine cools, as much air as is needed to return the expansion tank to 0 psi is sucked back into the tank. When the system is cold, the expansion tank cap can be opened with no release of pressure or vacuum.

I must note that this is my understanding of how the system works. Please correct any mistakes or misunderstandings I have.

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The air volume in the expansion tank is to allow for the 4% increase in volume in the water/coolant as it increases in temperature. The bubble of air acts like a spring. Overfilling the tank so that it vents overboard is not the best practice. You are just putting stresses in the system that it doesn't need and pouring coolant on the road.

The valve letting air back in is only doing work because its overfilled. In a properly filled system, the air charge is large enough that you never really have a vacuum and the system is balanced. The coolant expands, compresses the air and the system equalizes and nothing vents overboard. When it cools after shutoff, the coolant volume shrinks back down, the air volume expands and decompresses with it and in the balance no appreciable air should be needed.

The coolant volume in a 328 is 22 liters. So the coolant will expand about 800ml. The tank needs to have an adequate air volume for that expansion.

The fluid level in the tank should never be less than 6cm below the filler neck... or less than 6cm of air space.

 

Excellent. Thank you for the clarification and corrections.

If the coolant expands by 800 ml from cold, how many ml of cold-engine air space needs to be in the expansion tank to prevent the cap from venting air?

It sounds like I may be overfilling my expansion tank. It normally doesn't expel coolant, but it must be expelling and ingesting air each heat/cool cycle.

 

Have a look at the following pics, the top one is for a recovery system cap and the lower valve is open when cold allowing coolant to and from an expansion tank. This looks very much like your cap in the first post. The 308's and to the best of my knowledge 328's do not have an expansion tank where coolant is transferred to and from dependent on temperature/pressure. The second pic shows the non recovery cap where the lower valve is normally closed. This is the expansion style cap that is fitted to 308's and I'm suspecting 328's.


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I cannot find Ferrari’s specification for a “vented” or “unvented” radiator cap. However, “common wisdom” says that my original description and use of the Stant 10231 vented cap is correct. By this I mean Ricambi America lists a vented cap (the Stant 10331) as the appropriate cap and folks on FChat, such as Rifledriver, do the same.

As I understand it, the difference between the Stant 10231 and 10331 is the red lever for hot pressure relief (they are both vented for vacuum relief). (To confirm this I just order a 10331 from Amazon which will be here on Friday.) (For a cost of $6.08 which is a whopping $1.35 more than the cost of a 10231!)

Here is a link to a generic description of various coolant tank caps. The claim there is that the vented cap puts less stress on the system. http://www.allpar.com/fix/engines/cooling-caps.html

I am sticking with my Stant 10231 but I may lower the coolant level I keep in the tank. As I note in the first post, the coolant level in my car stabilizes 4.7 cm below the top lip (Flange A). You are right about how the coolant level should be 6 cm below the top lip. Thank you. I now see where Ferrari specifies that coolant level; it is the diagram on page 22 in the 1983 QV Owner’s Manual. Lowering my coolant level will give me some air gap.

As for the diagrams of the bottom vent with and without a small spring, it would be nice to have the small spring, but it sounds like it is unnecessary. In the name of science, I drew a vacuum (with my mouth) (blah) on my 0.9 bar Italian OEM cap and got the vent valve to open but it did seem like there was a very light spring (the mechanism is hidden inside the cap).

Thanks again for the comments and help understanding this system. (I am still keen on being corrected where my understanding is wrong!)

 

I think you were okay until you got to that point. Because the system is under pressure which is due to the expansion from heat, it’s not going to “suck anything in” until that pressure drops below atmospheric. But since it’s cooling down to the temperature where it started it should never go negative unless you boiled over or lost coolant. Sounds to me like you just had it overfilled. Just drive it and don’t add anymore coolant, see what it does.

 

First off, I have been thinking waaaay too much about radiator caps these last three days…

The Stant 10331 Lev-R-Vent cap arrived via Amazon this morning. This is the “common wisdom” cap that is listed as the OEM replacement on the Ricambi America website, and by many people here such as Rifledriver.

Here is a listing of the 4 Stant caps that are identical in performance to the 10331 model:
https://www.ferrarichat.com/forum/posts/144248607/
(This link is to a 140 post thread on radiator caps.) (!)

Below is a photo of my new Stant 10331. This confirms that it has the same dangling vacuum pressure valve. Apparently these valves are designed to close only when there is a significant pressure difference between the inside and outside of the expansion tank. This would be when the coolant starts to boil or, perhaps, just expands to the top of the expansion tank. This would mean that the coolant system operates at atmospheric pressure until that significant event thereby purging a lot of air and requiring re-ingestion of the air as the system cools.

If I am understanding this correctly, I surprised by this design. A consequence is that the car would probably dribble a little bit every time the coolant rises to the level to close the valve.

There ARE vented Stant caps with the vacuum vent valve held closed by a light spring. I believe that is model number Stant 10230. However, per above, this is not the model that is usually recommended by the experts for our cars.

An interesting little observation is that the Lev-R-Vent cap has these words printed on the top, “Test or replace when changing coolant.” For me, that would be once a year.

… I am officially done with thinking about radiator caps.

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Just be careful the Stant Cap with the red handle is to high the red handle will hit the deck lid ,it did on my 328 so I removed the handle and it worked fine after that,just test with a dap of grease on the handle and close slow and see if you got grease on the underside on the deck lid.

 

Same here - the lever on a lever cap hits the deck lid on my 328. I've always used a regular non-lever 16PSI 5$ cap of whatever brand from whatever parts store is closest.

 

Out of curiosity I looked at two caps I have, one I'm assuming is original and a new one I ordered from Superformance that looks identical. Neither one of them have the "dangling vent valve"

 

Which would be correct. The open vent style is for recovery systems where you have an unpressurised coolant reservoir as fitted to modern vehicles


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It really sounds like the Stant “dandling vent valve” caps (Stant 10231 or 10331) are incorrect despite them being the “common wisdom” choice. I find it particularly unsettling that Ricambi America – the Ferrari OEM parts specialist – sells them as the replacement cap.

 

I bought one of the Stants when I first started troubleshooting my issue. Didn't seem to make any difference either way to be honest but I later bought the Superformance cap because the Stant "looked" different. Of course I didn't know back then half of what I now know about radiator caps thanks to you guys.

 

I have to admit that I never knew that there are two different types of rad caps!

But after reading this thread and then doing more research, I agree that these cars should have a "non-vented" rad cap for the expansion tank to function correctly. The use of the wrong cap - a "normal" cap, which is vented - could explain the whole business of spitting/air in system that seems so common on these cars. IMO, it's ironic if one of the annoying (or charming, depending on your point of view) quirks of these cars is caused NOT by the design, but by owners (that would be us) /shops using the incorrect type of radiator cap!

 

The 308 reservoir is a pressurized tank. In my (somewhat educated) opinion, the proper cap for this type of system is non-vented, as there is no recovery tank that makes use of a vented cap. Excess coolant is expelled out the release tube, not into an unpressurized tank that accepts the excess coolant and then pulls it back into the main system under vacuum.

I acknowledge that RifleDriver (who is far, far more experienced than I am) and Ricambi recommend a vented cap - though given the design of the 308 cooling system, I do not understand why.

 

A vented cap IS the appropriate cap, but the issue is what type of vented cap.

There are three types of caps in discussion here:

Type 1: An unvented cap, which only has the "A" and "B" flanges shown in my post #1. This type of cap only has a pressure relief valve at the cap's rating of 16 psi. If so much fluid is expelled because the coolant got hotter (and therefore expanded more) than usual, the system will be under vacuum when the coolant cools and contracts again. Getting caught in a traffic jam on a hot day is an example where the system might expel more coolant than it ever had in its recent past.

Type 2: A vented cap with a spring-loaded vent valve. This type of cap provides pressure relief at the cap's rating of 16 psi but also has a vacuum relief valve which is closed as the system heats up. The vent valve opens only when the pressure inside the tank drops below atmospheric pressure as when the system cools and the contracting fluid volume causes a slight vacuum inside the tank.
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Type 3: A vented cap with a "dangling" vent valve. This is the weird one. This type of cap provides pressure relief at the cap's rating of 16 psi but also has the vacuum relief valve like type 2. The difference is that the "dangling" valve only closes when the pressure inside the expansion tank is significantly higher than the outside of the expansion tank. See my post #7. The vent valve opens again the same way as Type 2 when the pressure inside the tank drops below atmospheric pressure as when the system cools and the contracting fluid volume causes a slight vacuum inside the tank.

We are concluding that this second type is appropriate for our cars even though the second type is the type commonly used.

My Italian OEM cap has a screen at the bottom of the cap. I believe this is evidence that it is a vented cap but with the vent's workings hidden internally within the cap. I am reluctant to conclude whether it is a vented type 2 or vented type 3 just because it is so old (stamped 1996 and used until 2014).

I think I am going to buy a Stant 10230, which is a Type 2 cap, and begin using that.

I now have an awful lot of caps. I will open my Expansion Tank Cap museum as soon as I can find a patron willing to donate museum floor space.

 

I was cleaning up my writing a little bit and hit the 10 minute limit for editing.

The line below the descriptions of the three Types should read, "We are concluding that this second type is appropriate for our cars even though the third type is the type commonly used."

 

Been watching this thread and, boy you guys over think things.

The correct cap is what is being referred to here as the un-vented or non-vented cap. I have an original which was 0.9 BAR on my 308 QV. Typically it is replaced with a 1.1 BAR cap. NAPA 703-1698 is what I have on my car now. Under $6.00 at NAPA.

...edit: What Brian is calling type 2.

This one

 

I would reply that it may be that us guys have under-thunk. It may be that our common wisdom here on FChat has recommended the wrong cap for a very long time.

 

Agreed, but a vented cap is designed for a system with an overflow tank which uses that vacuum to pull expelled coolant back into the system from a third tank (where the coolant overflowed to, a tank the 308 does not have) - not air - to keep a consistent amount of coolant in the system.

 

Well, I hate to disagree but the OEM cap on my Toyota pickup, which has an unpressurized recovery (or reservoir) tank has the Type 2 cap. It doesn't matter what you are sucking back. These are two way differential pressure caps. They let stuff out when the pressure exceed the cap rating and they suck stuff back in (air or coolant) when system pressure drops below atmospheric.

 

I found this interesting: Jeeps had systems similar to ours until 1991 when they changed from an expansion tank to the overflow bottle system that most cars have used for many years. A standard "upgrade" is to convert from the pre-91 system to the later system. I am NOT suggesting that we should do anything like that - most of their complaints is about the plastic expansion tank cracking. But on various sites they speak of the same sort of problems we are discussing if the wrong type of cap is used.

I saw what is allegedly a "correct" cap on Ebay Australia and it looks very different from the normal cap we all seem to be using. But surely Stant sells the appropriate cap.

I just ran outside to look at my wife's '03 MB S500 which also has an expansion tank. The cap does NOT have a vent to allow vacuum to pull air/coolant back in. The underside of the cap looks very similar to the Oz cap. Here's the Ebay link for the Rad cp in OZ: https://www.ebay.com/itm/Ferrari-308-Gts-328-Gts-512-Radiator-Pressure-Cap-101499-R/273198975477?_trkparms=aid=444000&algo=SOI.DEFAULT&ao=1&asc=50544&meid=416f70e56b474fcfa8dbb32bd029f945&pid=100752&rk=4&rkt=4&sd=273198049445&itm=273198975477&_trksid=p2047675.c100752.m1982

 

You guys are nuts.

 

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

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So far, I think we've concluded that both pressure and non-pressure tanks use both vented and non-vented caps. Except on Wednesday.