Pretty sure that's not how a pop-off valve is supposed to pop off — what now?

[bford]

For the first time in 1.5 years of early 911 ownership, my car is in a non-operational state.

Up to now, my 73.5T Targa has been the model of reliability. Always starts up on the first try. I believe it only has backfired two or three times during cold start. Two days ago, I went to start it using the same cold start procedure I always do (lightly touch the accelerator while pulling up on the hand throttle, make sure the pedal engages, turn the key half way, listen for the fuel pump, wait 5-10 seconds, crank it with foot never touching the pedal). There was a loud backfire, and then the engine wouldn't start. Oh no, I thought, did I blow up the airbox? Can't be, the PO had a pop-off valve installed.

I raised the lid and opened the airbox, and saw the following:


So basically, the pop-off valve blew off entirely. On its way up, it dented the air filter:


The pop-off valve drill hole and valve itself have a lot of solid epoxy residue on them:


So I need some guidance here. The airbox itself appears to be intact, no cracks. What do I need to do?
• How do you clean the hardened epoxy off the existing drill hole and valve?
• Or do I need to just get a new valve?
• Has anyone else had this happen? Is this indicative of a "bad job" installing the pop-off valve the first time?
• Please don't tell me I need a new airbox.

Thanks.

[Frank Beck]

Common problem. It happens quite often; especially if the surfaces weren't cleaned especially well.

As long as none of the screws popped out your box it's most likely fine.

The hole should clean up easily with a razor blade. A new valve is only $50. If you can't get all of the old epoxy off I would recommend replacing it.

[911quest]

Yep I have even had them shoot across the shop.

[Mr9146]

Yep I have even had them shoot across the shop.

HA! Haven't had that happen yet, but I suppose it's something to look forward to.

[bford]

Common problem. It happens quite often; especially if the surfaces weren't cleaned especially well.

As long as none of the screws popped out your box it's most likely fine.

The hole should clean up easily with a razor blade. A new valve is only $50. If you can't get all of the old epoxy off I would recommend replacing it.

OK, good to know, thanks. This seems like a design flaw in the pop-off valve product. Why are we gluing it in place instead of, say, putting threads on it and screwing it in place from the underside with a gasket in-between?

[mohrgan]

OK, good to know, thanks. This seems like a design flaw in the pop-off valve product. Why are we gluing it in place instead of, say, putting threads on it and screwing it in place from the underside with a gasket in-between?

It really is a crappy design!

[Frank Beck]

OK, good to know, thanks. This seems like a design flaw in the pop-off valve product. Why are we gluing it in place instead of, say, putting threads on it and screwing it in place from the underside with a gasket in-between?

Screwing it in place from the underside? Huh?

The airbox is screwed and glued together. WE are gluing it because we cannot get to the "underside". The guy who invented this product and brought it to market did FAR more than Porsche EVER did to fix this problem.

All Porsche did was DENY that there ever WAS a problem.

I'm guessing that your engine popped back through the intake at some point so this valve, even though it broke loose, still saved you about $1500 in parts and labor. You should be very thankful that all you have to do is glue it back in.

[edmayo]

On the last design iteration of the airbox Porsche finally solved the problem. They used a 'steel spray tube' cold start system that injected the fuel into the aluminum runners, not in the plastic plenum. When you glue it back in make sure you orient the flap correctly, hinge faces rear of car. Reverse it and the flap hits the aircleaner instead of opening.

[Frank Beck]

I've seen the ones with cold start plenum blow up too Ed!

[Eminence Gris]

Screwing it in place from the underside? Huh?

The airbox is screwed and glued together. WE are gluing it because we cannot get to the "underside". The guy who invented this product and brought it to market did FAR more than Porsche EVER did to fix this problem.

All Porsche did was DENY that there ever WAS a problem.

That's not entirely true. There was a recall in about '77 where Porsche put braided spark plug wires on CIS cars to prevent induced sparks on the wrong plug. (One imagines a debate between engineers at Porsche over the backfiring problem and having them compromise on the solution.)

I know because I bought a T from a dealer that had my car (used) on their lot when the recall happened. They chose not to do it and I won a new airbox in small claims court after it blew up.