In online discussions about replacement coolant pressure caps for Vanagons, people often talk about blowing, quacking, making kazoo sounds, etc. as confirmation that their new cap is OK. Some of these folks are undoubtedly referring to a short, funny little video that’s made the rounds, where an older bearded dude sucked and blew thru a cap’s outlet nipple and declared one of the caps was good because he couldn’t suck air out the nipple and it made a rough kazoo kind of sound when he blew in, where the other one flowed air freely in both directions, so it was junk. Well he’s right on the latter part but the video is highly misleading because what is shown is in no way a functional test of one of these caps, and if you put your trust in such a test you may be in for some surprises. And when those surprises crop up, many folks will be climbing the walls looking for the cause because they believed that they credibly tested the new part, and since it passed the old bearded guy’s test, the problem must be somewhere else.

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Coolant tank pressure caps as well as radiator caps of the last 40 years or so provide two separate and critical functions: one is to contain the pressure within the cooling system created by expansion of the coolant as its temp increases. This raises the boiling point of the coolant the same way a pressure cooker raises the boiling point of the cooking water to cook food faster. In an engine this suppresses spot boiling in the hottest parts of the engine, namely the cylinder head or heads, as well as suppresses harmful cavitation (low pressure-induced boiling) at the water pump. About 2/3rd’s of the boil point increase is due to pressure, only 1/3 is from the higher specific gravity created by mixing the water with ethylene glycol, so pressure containment is critical to the function of the cooling system. Most caps are designed to contain pressure up to 1 bar, which is atmospheric pressure at sea level, about 14.5psi. Above 1 bar the pressure containment valve releases coolant to the reserve tank to limit total system pressure. Pressure is for all practical purposes nearly the same at all locations within the system at all times.

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The cap’s second function is to allow coolant to be drawn back from the reserve tank when the engine cools down again and the coolant inside the system contracts back to its normal volume. That is regulated by a second one-way valve that opens in the opposite direction at about 1 psi differential.

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By containing system pressure the cap establishes the highest temp the coolant mix can reach, above which it will boil. By allowing excess liquid to be let out when hot, and then to return with cooldown, the cap regulates the total volume of liquid in the system, and keeps the liquid in the system liquid, because gases are relatively poor at absorbing and rejecting heat.

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All the video test showed was that neither of the two separate valves in the cap was stuck wide open, as one was in the bad cap, and that the return valve required some small amount of pressure differential to open (the kazoo sound). When the fellow in the video sucked on the cap and couldn’t draw air, all he confirmed was that the pressure containment valve was able to contain a few psi pressure differential, because 5psi is about all the vacuum a normal human can casually induce with their mouth. But the pressure containment valve must not break open until ~14.5psi (1 bar), and at normal running temp the cooling system will be around 10psi, so he’s a long way from proving that cap will contain high enough pressure to raise the coolant’s boil point to the expected working temp.

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When the cap releases at too low a pressure, that sets the upper temp limit of the coolant, its temp can’t go higher, it will instead simply boil faster. So, when a cap isn’t containing the spec pressure, surprise! the engine doesn’t overheat. Or it just seems not to. The problem is, the gauge is only reading the coolant temp as a proxy for engine temp. Since the coolant will be spot-boiling in the cylinder heads and surfaces have to be wetted with liquid coolant, not steam, to effectively transfer heat, the actual engine temp may be increasing out of control. And if the gauge sender isn’t in liquid coolant and is instead sensing the temp of a stream of vapor bubbles, it will read even lower. But drive on, the gauge says everything’s OK!

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The kazoo test is at least little closer to saying something, since the return valve opens at only ~1psi, so it takes little pressure to blow it open by mouth. But it’s the pressure containment valve that provides the most critical function and most often fails, and is sometimes not up to spec in brand new caps. That must be tested on a pressure test rig with over 1 bar pressure applied to confirm it holds to that level.

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I supplied caps to my waterboxer engine customers and bench tested every one, so I’ve tested hundreds of them, new and old. In my experience, for Vanagons the Blau branded caps have the fewest sub-spec caps when new, so that is the only brand I would supply or recommend, other than a dealer-bought VW branded cap.