CIS Air box, unusual pop-off valve repairable? (74)

moneymanager · 02-20-2008, 12:43 PM

My 74 2.7 has what appears to be an aftermarket aluminum air box and an unusual aluminum pop-off valve which looks quite different from the usual valve you see. I assume, perhaps wrongly, that it came with the box. The pop-off valve itself leaks. Anyone know if this is repairable, if any parts are available? Thanks, Jim

Porsche_monkey · 02-20-2008, 01:03 PM

A picture might be useful here...

moneymanager · 02-20-2008, 01:14 PM

Good idea. Here's a pic:

ossiblue · 02-20-2008, 04:03 PM

I've never seen one like that, but I assume there's a spring in the conical area at the top that holds a rubber seal against the opening--or some variation of that. You might try unbolting the unit and see, upon disassembly, what parts there are and if you can find or fabricate replacements. Most likely the rubber seal has hardened/cracked/warped, or the spring has weakened. Just guesses, but someone may have direct experience with this type of valve
and know of a supplier.

john walker's workshop · 02-20-2008, 04:25 PM

if the box is metal, then you don't really need it.

moneymanager · 02-20-2008, 04:56 PM

Armed with this suggestion, I took an 11mm to the little device and pulled it apart. No rubber inside, just a spring loaded shaft with a circular plate milled so that a chamfered edge sits snugly against a matching shoulder. Other than cleaning it, or putting a washer under the spring to up its tension, not clear there is much to be done without making the seal worse. Pressure inside the box pushes against the spring which loosens the seal and allows relief.
John...I'm afraid the aluminum box would pretty much blow itself apart without some kind of pressure relief in the event of blowback. Or something else would have to give elsewhere. Thanks for the help.
Jim

911Freak · 02-20-2008, 08:00 PM

I have this EXACT same one!!!

Never any issues with it...

ossiblue · 02-20-2008, 08:48 PM

Just a thought--is the leak from the valve or the outside area around the valve body?
Also, you might try to resurface the metal seat with valve grind compound and gently rotate the plug in the seat. I think your idea of increasing spring tension or lubricating the plunger shaft may also be a course to try. Sounds like a pretty simple mechanism so barring some undetected warping, it should work. Keep us posted

moneymanager · 02-20-2008, 09:07 PM

Good thoughts. I'll check the seal between this unit and the box. And a bit of valve grinding compound on the seal might just do the trick, if I can get it all apart so I don't end up dropping abrasives into the cylinders! Thanks, Jim

moneymanager · 02-22-2008, 07:25 AM

The device looks like it is glued to the box pretty securely, and the use of valve grinding compound would seem to require pulling the entire air box so as to avoid leaving grit in the intakes. So I guess I'll wait for another opportunity to try these fixes. But the spring will get a thin washer to increase tension a bit, and the whole unit can be cleaned. Maybe this will help.

Porsche_monkey · 02-22-2008, 07:48 AM

Quote:

Originally Posted by john walker's workshop

if the box is metal, then you don't really need it.

So just glue it shut. And make sure it has a good seal to the air-box.

Porsche-O-Phile · Porsche-O-Phile's Garage

Interesting design. I actually like the construction of that thing more than the conventional one. I'm always a bit paranoid that the thing is gonna' come apart and the metal spring part is going to get ingested. I check mine semi-regularly as a result of my paranoia.

911Freak · 09:14 AM

As I posted above I have this exact same Pop-Valve in my 80SC Air Box...

When my mechanic did the install he used a silicon bead around the base to ensure a good seal thru heat cycles etc. also he used the same "non-hardening" sealent around each of the top halve bolts..as he told me "a saftey measure incase the screws back out over time"

I'm not sure about the gringing compound either..if you want PM me and I'll give him a call for you and find out exactly the brand product he uses...

Correctly mentioned above; there isn't a "rubber seal" according to my mech. he "services" these by doing the process mentioned above; sanding the "metal seat" with valve compound, it needs to be completely flat/true or it will not seal..Over time these can become deflected, resulting in leakage.... Not sure if it's a heat cycle thing but I'd doubt it would be...

He has been installing this valve in this manner since the '70's and hasn't had an issue with them coming apart in the air box or the srews backing themselves out...He swears by this modification..

I bought a Pop-off Valve from our host a couple of years ago and never installed it, none of my local PCA memebers needing it..so if any one want to buy it from PM me and let me know...

moneymanager · 02-22-2008, 12:30 PM

thanks, 911Freak, for the offer. As I don't want to pull the airbox at the moment, I've just put it back together with a good cleaning and a thin washer to add a tiny bit of tension to the spring. Now if I can just get the battery to stop losing charge after 24 hours, I'll try to fire it up and see whether my 1/2 fix made any difference.