TheSamba.com :: View topic - valve stem for homemade coolant pressure tester?

Thu Jan 24, 2008 2:57 pm

I'm trying to find a hard to track coolant leak in my van, so I'd like to put together a bicycle pump-based coolant pressure tester, rather than shell out big bucks for the correct Snap-On tester and cap adapter that Terry Kay recommends.

I figure I could either get a second pressure cap and put some kind of hose barb through it, or maybe even put something like a tire air valve through the original cap, so that it would hold pressure when I'm not pumping air into it.

Any of this making any sense?

Anyway, my real question is, Can anyone recommend a type of tire air valve that can be easily installed into the pressure cap without leaks? I'd like to be able to drill a hole in the cap, and then put on the valve with o-rings and nuts for instant success.

Then, to test coolant pressure or bleed the system, you just hook up the bicycle pump to the existing stem on the pressure cap, and start pumping!

Thu Jan 24, 2008 3:07 pm

have you thought about using a valve stem from a bicycle tire? or i guess you could use a plastic/rubber one from a car wheel/rim. you would need to remove material from the center of the stem then I guess that you would silicone it to a pressure cap.
the other thing that might work is to use a cap from a golf/jetta in place of the pressure cap. drill a hole in that to insert the valve stem through then seal the stem in with silicone. might work but might not.

Thu Jan 24, 2008 3:14 pm

I use a garden pump sprayer, remove the hand valve. and use a M/M hose barb to tie into your Coolant bottle expansion hose and pump up a few Lbs..
I use this method to pressure bleed the radiator and cooling system.. and to check for system leaks after a major coolant hose work.

the overflow/expansion tank has to flow backwards thru the Rad cap w/o any restriction so it works very well with a little pressure.

*note that if you go over the pressure rating for your cap, it will vent ... :!:

Thu Jan 24, 2008 3:36 pm

Oops!

Thu Jan 24, 2008 3:36 pm

Cut the Schrader valve out of a bicycle tube. When you cut it out, cut a circle of rubber around the valve that is large enough to act as the gasket for the pressure cap. Drill a hole in the pressure cap large enough for the Schrader valve to pass through. Insert the Schrader valve/gasket into the pressure cap and tighten it up. Done!

This works for bleed brakes too.

Thu Jan 24, 2008 3:36 pm

This may sound weird but I used the blue rubber circle from inside a soda bottle top to make a seal between the valve stem and the radiator cap. I drilled a small hole through the blue rubber circle then placed it between the car valve stem and the radiator cap with a freshly drilled hole. It was all I had but actually worked on my 1980 VW Scirocco. Very unorthodox but somehow worked.

Thu Jan 24, 2008 4:09 pm

Loogy,
Nice! Have you actually tested this yourself?
Also, I'm assuming you're talking about a schrader valve on which the stem is threaded down to the base, yes?
If so, I didn't realize they came that way.
Thanks.

Thu Jan 24, 2008 4:29 pm

Nice, loogy. thats what I was thinking, but i like the idea of using the tube as the gasket.

I dont think that you would need it to be threaded all the way down because the pressure will be blowing out, forcing the gasket into the hole. If you cut the circle of tube big enough so that it covers the whole opening of the tank I think that would give a pretty good seal. I still think that it would work best with a non pressurized golf/jetta cap, since they are just flat on the top inside.

If you find that you need it to be threaded all the way down you could use a pista valved bike tube with the shraeder vavle adapter, as most standard length pista valve stems are threaded right down to the tube.

Glenn

Thu Jan 24, 2008 4:51 pm

If you just want to apply pressure to a cold system to look for leaks, and don't need a pressure gauge attached, just remove the overflow hose from the pressure cap nipple and shoot some compressed air right into the nipple with a rubber-tipped air nozzle. A tiny burst is plenty, since there's only a little air space above the coolant level. The cap will release excess pressure automatically and then hold at 15psi, provided your cap is working properly. Then go around and look for leaks.

If you want to make a custom pressure tester with gauge, take an old pressure cap and remove the guts, keeping the rubber washer. You may need to double the washers to get a seal without the guts in the cap. Then get a few items at the plumbing store: a 1/8" brass hose barb to 1/8" MPT fitting; a 1/8" brass tee; a 1/8" MPT 0-30psi pressure gauge; and a 1/8" MPT Schrader valve. They sell the valves for air-pressure testing plumbing before use.

Assemble the gauge, barb fitting and Schrader valve to the tee with sealant on the threads. Then fit a short length of rubber hose, with clamps, between the barb and the nipple on the disabled pressure cap. You probably have to screw the cap onto your pressure tank first, then slip the hose from your tester onto the nipple and clamp it.

Then pump it up to 15psi, tap on the gauge, note the gauge pressure, and wait. If the system is tight it will hold the same pressure over time. Do it cold if you want to wait long-term so you don't end up seeing a lowering of pressure due to cooldown. But some leaks only manifest themselves with the system hot, so you can of course do the test hot, allowing that the reading will fall as the system cools.

Thu Jan 24, 2008 8:22 pm

I have some pictures of the one I built that are posted below.

Thu Jan 24, 2008 8:27 pm

Just like that.

Thu Jan 24, 2008 8:50 pm

Fri Jan 25, 2008 4:07 am

BTW that testor worked great and I discovered small leaks that I did not even know I had. I just pumped it up to 10psi with my bicycle pump and with the motor cold, let it sit and if there was a pressure drop after sevreal hours I would look around to find the leak (s).

I found all the parts in a plumbing & compressor section of a store. I think it cost me a total of about $15 to make.

Fri Jan 25, 2008 4:35 am

Great stuff, Pascal and $0.10, and nice picture. I'd seen Pascal's solution in the vanagon mailing list archives, and was going to p-mail you for just such a picture.

BUT, what about my original one-cap question? Why can't I just install and leave a schrader valve in the existing cap, and attach a bicycle pump with a pressure gauge when necessary? I haven't taken one apart yet, but it sounds from $.10's description that you can't do this without interfering with the cap's valve action. Is this the case?

Fri Jan 25, 2008 9:43 am

If you look at the inside of a pressure cap, you'll see the two valves that control system pressures are built into a removable disc within the cap (the "guts"). Go ahead, take it apart and see, you won't damage it.

If you just installed a Schrader above the disc, you could apply air pressure to the system without affecting the operation of the valves, but to get the pressure into the system you would need to block off the overflow nipple because otherwise it would just go out the vented overflow.

Since you would need to block it anyway, just pulling off the overflow hose and putting air in via the nipple is simpler. But if all you have is a bicycle pump this won't work.

The overpressure valve will release pressure above 15psi in either case, but it seems like adding the valve is a solution in search of a problem!

Neither one of these routes gives you a pressure reading to watch if the system leaks down, though. For that you need to use a gauge tester. Building the one I described, which is exactly what's pictured above (funny that it was built by Pascal! Nice job, P!) is a very cheap and simple solution. This is one of those rare instances where one of the unusual design aspects of the Vanagon works in your favor; it's so easy to attach a pressure device to the overflow hose nipple rather than needing to come up with a special cap to work with a store-bought pressure test tool.

Fri Jan 25, 2008 10:15 am

Thanks, $.10.
For leak testing, I already (sort of) took your earlier advice and that of danfromsyr: I bought a cheap bicycle pump from Wal-Mart, cut off the attachment end, stuck the newly bare rubber hose onto the expansion tank cap nipple, and gave it a couple of pumps. Immediately I found two leaks, both from ends of hoses at old screw-type clamps, one of which looked pretty rusty.
I'm a new convert to spring clamps!
Thanks again.

Fri Jan 25, 2008 10:22 am

Not to be argumentetive (that's not my way) or to be an extreme cheapskate (also not my way) but just for the sake of discussion, if a guy were to sacrafice a pressure cap by drilling the center right out of it, including the above mentioned valves, and using a cut up inner tube as I described, provided there were no air leaks in the bicycle pump guage or Schrader valve connection, wouldn't this accomplish the same thing? It would eliminate the valves and the overflow nipple from the equation leaving just a pressurized system. Am I missing something? Leaks in the tire pump and Schrader valve area would be my concern doing it this way. Just thinking out loud!

Fri Jan 25, 2008 10:48 am

I just had a 'it might work' idea. what if someone were to put the bicycle type valve on the coolant tank itself? leaving the cap out of the equation altogether. that way, you could pressure test whenever you wanted and it was 'built-in' to the system.

I'm thinkin', where the red dots are, a valve could be installed? See pic:

Fri Jan 25, 2008 7:30 pm

Well all you have to do to pressurise the system is get it hot anyway.

Fri Jan 25, 2008 9:42 pm

Another good question is, "How do I install a coolant pressure gauge?"