low pressure OEM style coolant cap available?

[DanHoug]

this talk of blowing senders out got me thinkin' and Googlin' to no avail...

is there a low pressure version of the Blau cap available? it seems the pressure tank deforms right around the OEM cap pressure of 15psi and many have blown the sender or other hoses. i'd love to drop my system pressure down to 7psi.

i know the RMMW aluminum tank is the answer, with the cap PSI of your choice.

-dan

[borninabus]

the black caps are lower pressure.
what pressure does your system run at?
_________________
88 Van WBX, A/T - 13 JSW TDI 6M/T - 2012 Touareg TDI Sport

[DanHoug]

have a new blue Blau, so whatever pressure the OEM one runs. anyone have a PN on the black one?

-dan

[yiucycle]

Just remember if you run lower pressure cap, you also lower your coolant boil over temperature too and overheat your engine.
_________________
1980 2.8LV6 Fiero ASI/Riviera Vanagon
"there is no point in being embarrassed of all the stupid shit i done and will continue to do"

Stop dead photo links! Post your photos to The Samba Gallery!

[Ahwahnee]

The cap is somewhat (crudely) adjustable by removing the fitting that holds the valve and spring together. Weakening the spring will lower the operating pressure. Trial and error - best done with an old spare cap.

I did not know that the black caps were lower (my 84 came to me with a black cap, possibly original).

[DanHoug]

at a 60% glycol solution, we have the following boil temp/pressures:

16psi = 273F
8psi = 253F

given that the design of the pressure tank is flawed with respect to withstanding pressure, i'd say you run greater risk of a system failure by blowing the tank/sensor than you do dropping the BP 20F.

interestingly, the ruptures seem to occur at idle suggesting the idle speed recirculatory volume is too low. plenty of cool coolant in the rad, just can't bring it in fast enough.

soooo.... a slightly fast idle along with a low pressure cap seems to make sense. or maybe just a fast idle is enough. a snap action spst thermostat, bonded to a coolant hose, and tied into the PS switch might not be too stupid.

-dan

[tencentlife]

I have seen systems where coolant at the heads was actively spot-boiling at idle when the cap held only 6psi. As yiucycle noted, peak coolant temp is regulated by system pressure. In fact, the glycol only provides 1/3 of the elevation in boil point, system pressure is responsible for 2/3rds. There's a reason it's commonly called "anti-freeze" rather than "anti-boil", the ambient boil-point increase in a 50/50 mix is pretty small.

You'll see people who claim they run a no-pressure system with glycol/water (as opposed to straight glycol, which can work because straight glycol boils at over 360ºF.) and they claim it never boils over, so all is good. Right? Wrong. Just because the overall mass of coolant isn't boiling enough to keep expelling liquid from the system doesn't mean it isn't actively spot-boiling at the heads (to believe that, one would need to believe that coolant temp is the same everywhere in the system, but of course if that were the case there would be no cooling effect whatsoever). Vapor formation at the heads radically lessens the amount of heat being absorbed by the coolant, so coolant temp is actually lower, while the heads are actually running hotter. And the coolant's boil point is now only about 225ºF, rather than the 240º+ it would be under containment, and the critical temp runs right in that range. And because of the very uneven cooling, the heads develop stresses which can hasten cracks and warping.

But the gauge says the engine's running cool, even cooler than normal, so all is well. NOT. But it doesn't keep blowing coolant out the filler. SO WHAT? It only has to expel the small amount the vapor bubbles displace one time, and after that, unless the boil rate increases, there will be no more liquid lost. But don't the vapor bubbles entrain with flow and collect in the top of the radiator, and then displace coolant out of the filler? Of course not, the bubbles are water vapor (aka "steam") which turns back into liquid water as it cools, and the radiator is real good at cooling things. Contrast that with a fire ring leak where the combustion gas mixture, although partly water vapor, is mostly inert N and other trace gasses which will remain gas, that's why fire ring leaks displace more and more coolant, but spot-boiling doesn't.

Your system will actually run at about 10psi at normal temps, leaving 4psi headroom before release of pressure. Every component in the system in good nick can easily contain much higher pressures. Lowering system pressure is not how to avoid problems, having a good tight system without unduly aged components, a radiator with sufficient heat rejection capacity with fan that assists at the appropriate times, and a working cap that limits peak pressure are all that's needed. Oh, and a pump to move it all around.

I agree the OEM plastic caps can be problematic in several ways and being able to switch to a more sturdy and common bayonet-style dual-action cap is reason enough to get the RMW or similar tank. But deliberately reducing system pressure is, in my oh-so-humble opinion, just trading a headache for an upset stomach.
_________________
Shop for unique Vanagon accessories at the Vanistan shop:
https://intrepidoverland.com/vanistan/

Please don't PM here, I will not reply.

Experience is kryptonite to doctrine.

[DanHoug]

good points, Chris. the spot boiling at the heads, with it being reabsorped as the steam cools freaks me out and is total justification to keep the pressure up.

maybe we just need a coolant booster pump that is tied to the throttle idle switch! actually, i'd prefer the simpler approach like i had on my old Fiats... a throttle lock knob next to the choke knob that allowed you to set the idle at whatever you wanted.

-dan

[Wildthings]

Personally I would rather boil a bit of coolant from time to time than to blow a hose or fitting. Over the many hundreds of thousands of miles my family has driven these vehicles, I am sure I have dumped my coolant at least ten times when some hose or fitting or tank blew up. Heads crack no matter what happens, so I am not worried at all about any slow damage that might occur when compared to the instant death that very likely will occur when a coolant manifold or other piece fails all at once.

From memory I have had the following parts fail:

3 hoses, two being large and one small one for the oil cooler
2 coolant towers/manifolds
2 or 3 times that ends blew off of the coolant pipes
2 cracked pressure tanks, one failed suddenly and cause extensive leaking
1 expanded pressure tank where the cap blew off
1 level sensor blew out of tank

For myself I will run a low pressure cap from this point on as I have done for the last two years on my 83 1/2, thus the worry about a sudden loss of coolant is greatly lessened.

On the '91 that I converted to a Subaru maybe ten years back now and replaced the long pipes with stainless steel, zero leaks so far.

[tencentlife]

[djkeev]

I wired in a switch where I can turn on the A/C system yet turn off the compressor clutch.

This gives me air movement in the back of the Van and turns on the low speed radiator fan..... simple to do and very effective!

Dave
_________________
Stop Dead Photo Links how to post photos

Ghia
http://www.thesamba.com/vw/forum/viewtopic.php?t=392473

Vanagon
http://www.thesamba.com/vw/forum/viewtopic.php?p=6315537#6315537

Beetle
https://www.thesamba.com/vw/forum/viewtopic.php?t=482968&highlight=74+super+vert