Vented vs. Non-Vented Radiator Cap

[karlos]

When running a non-pressurized coolant recovery tank, it's my understanding that the radiator cap should be of the vented type to allow the radiator to draw the coolant back out of the recovery tank during cool-down. But there are many conflicting opinions on this. Given that I'll be running an unpressurized recovery tank, I believe the two caps shown below are the correct type: the 10231 on the radiator (16 psi vented cap), and the 10203 on the recovery tank (0 psi cap that just prevents the coolant from sloshing out). Comments/suggestions anyone?

Thanks.


http://thefactoryfiveforum.com/attachment.php?attachmentid=55207&d=1466572422

[wallace18]

Pressure cap on rad pipe, vented on resv tank.

[rich grsc]

The recovery tank should be vented to the air, not using a pressure cap at all.

[mikeinatlanta]

Haven't checked you part numbers, but you are correct. On a coolant recovery system, the rad cap (or fill cap at system high point) needs to allow return flow on cool down or the system won't work. Tank should be vented, so cap style is pretty much irrelevant.

[karlos]

Thanks for the replies, all.

After doing a little more digging, I find that there are two different types of vented caps. I pulled the following quote from the website linked here: http://www.allpar.com/fix/engines/cooling-caps.html



The pressure rating is the parameter most of us are familiar with, but there are other important choices to make when selecting a radiator cap. One of them has to do with how this pressure/vacuum equalization is approached. In a “full pressure” cap, the vent valve is spring-loaded into the closed position. It only opens when a vacuum sufficient to overcome the vent valve spring develops and pulls the valve open.

A full-pressure system begins building pressure as soon as the coolant begins to heat up, and retains it long after coolant outflow would cease to push closed a free-hanging centre valve. This protects against localized boiling in the system, but at the cost of increased physical stress on the entire cooling system—hose junctions, seals, solder joints in the radiator and heater core, etc—because they’re under pressure more of the time, including during system temperature changes.

In a “partial pressure” cap, the vent valve is free-hanging and weighted into the open position; it closes only when system pressure rises high enough to cause a volume change that can overcome the weight and push the valve closed.



I picked the Stant 10231 shown above because it's clearly marked as a vented cap. As it turns out, it has the free-hanging vent valve and is therefore a partial pressure cap. I'd actually rather set my build up as a full pressure system, mainly because that's what I'm more familiar with, as that's what the majority of modern cars use. So a Stant 10230 is a better choice. Still a vented cap, but with the spring loaded vent valve that produces a full pressure system.

Turns out there's more to the humble radiator cap than meets the eye...