air box pop off valves - myth??

[dickster]

i have just been requainting myself with the superb peter zimmerman book - the used 911 story.

in there he suggests that alot of aftermarket stuff (improvements or upgrades) for the 911 is a waste of time and money.

one of the items he particularly picks on is the air box pop off valve. there are porsche improved airboxes that are available he says (and airboxes should be periodically replaced anyway) and running with a pop off valve can actually harm your engine without you knowing - via airleaks.

i have heard alot of you SC owners recommend the pop off valve - is this book too old to be relevant now? do you care?

it doesnt affect me, i am just doin my bit for carrera/SC relations........

[Kahuna]

Ah, one of the more user friendly and fun aspects of SC ownership.

Didn't have one on mine, cost me around a grand, installed a pop off and never had another issue despite some horrendous backfires due to some other CIS issues.

[Superman]

With all due respect to Mr. Zimmerman, I'm not going to be replacing a $400 air box as though it were an alternator belt or something. I expect it to serve my needs virtually forever.

On one occasion, shortly after my popoff valve installation, an air box explosion blew the popoff clear out of its hole. Apparently, the mating surface of the valve needs light sanding in order to 'grab' the epoxy. I'm sure it saved my air box.

[William Miller]

The newer Airboxes have an internal manifold for the cold start valve.
This may be the upgrade that was written about in the book. I have not read that book. My car is an 83 euro (sort of the end of the line.)
The PO installed the pop off valve many years ago. I had a problem with it because the hinge pin came half way out. I fixed that (Your supposed to put epoxy on each end to keep that from happening.)
Now that it has some age I believe the o-ring is not sealing as well as it once did.

So you can say for me it has created some problems.
Kahuna, was yours one of the later air boxes that exploded or the old style?

I'm on a quest to find a replacement o-ring. Nobody seems to sell it. I can find a close match but not an exact match.

Does anyone have any paperwork on the pop off valves or a way to contact the manf. Who makes them anyway?

[dickster]

super,

wow, i had no idea they cost that much.

he does give an idea service intervals in the book, the later models are supposed to last up to around 120k!!

is this not a well read book here?

he is part of (or was) Red Line Service - a well respected outfit?? are they still around even??

i dont doubt the wisdom of using the valve.........

[banjomike]

I have the airbox with the update (it is some internal plumbing which directs the fuel more directly into the runners) and most mornings I get a little backfire when I accelerate from that first stop sign.

Three liter owners, join me in singing the praises of the pop-off valve!

Hallelujaaaah...


the occasional check to insure the the valve is seating properly is so easy. Hell if this thing prevents me from replaceing the air box once it will have paid for itself... $1000/$50 = 20 times!

[RickM]

What size and diameter is the O Ring?

[Pettybird]

I pretty much memorized that book, and I looked at that part with great skepticism. I need a fuel pump check valve or something, so on some warm starts the car sounds like popcorn at just above idle... Check the condtion every time you pull the air filter, I suppose. My PO's PO (the guy I bought it from didn't know a thing about cars... I asked when the spark plugs were changed last and he asked 'do they do that with an oil change?') put the popoff valve in the newer airbox... The line about checking which is which by the holddown screws is right. I've got the Phillips head, and '79s should have straight heads. Quick check that's useful. That book is a good read, though.

-Doug

[Danskman]

I was Parts Manager at a Porsche dealership in the early 80's and have literally seen airboxes blow up in front of my face. The car owners were really pissed off that this item would break apart in a milisecond with a backfire, and cost them a fortune to repair. We would install a new airbox with a pop-off valve 100% of the time.

[dickster]

ok, dont shoot the messenger

well, zimmerman says that an airleak at the pop off valve can cause lean running conditions..........that was the point of my post, to bring that to SC owners attention - just in case.

well, when i'd read it, it occured to me that airboxes blow because of age?? hence periodically changing them. perhaps?? or do new boxes blow?

mike

thats interesting. thanks for sharing. makes you wonder about the validity of books out there.

[vash]

i am the proud recipient of the "mother of all backfires". thank god for my pop off valve, and the air filter for cushioning it's take off!

[Pettybird]

I'm sure it happens occasionally, but that's a vacuum seal, not a pressure seal. It's pulling CLOSED all the time, except for the backfires. I'm not shooting anyone, but I don't see the cost/benefit analysis listed in that book to be in the owner's favor.

If you're concerned about leaking, check with the carb cleaner test by spraying it directly. You'll probably get false readings from that, though, because some will invaraibly get past the sensor. Maybe you could push down on it? As far as a lean condition, I can't imagine it being any worse than leaving your oil cap off--that's got a vacuum line to it, too. You don't see anyone running around screaming that THAT seal needs to be serviced...

Also, under any power, that thing's gonna get sucked flat. If there's debris built up enough around it to prop it open, shame on you for not running an air filter. Chances are, most CIS cars are running a little rich or a little lean simply because they haven't been adjusted lately.

I liked the rest of the book, though, and it was VERY helpful when I bought my car.

-Doug

[Superman]

Rich, we're taking shots at the book, not you. No, air boxes do not 'wear out.' Yes, there can be air leaks at the popoff, but not if they're installed and checked properly. There are dozens of excellent places for air leaks to develop...the popoff is just one on that list, and is not necessarily very high on the list. Vacuum hoses and breather hoses probably top the list. Heck, an air leak in the engine might be located in the trunk (vacuum booster).

Cliff, yeah, when air boxes get excited, they really go off. When mine did, the popoff made a nice dent in the air filter. If the filter and cover and engine lid had not been in the way, I'd guess the popoff would have achieved at least a hundred feet of altitude. Probably as impressive as a spud gun.

[vash]

oh-o! superman knows what a spud gun is. info like that in the hands of someone that can build things can be scary fun i watched a pronghorn antelope almost get knocked unconscious with a lemon, powered by just alittle bit of hairspray! what is this book that everyone is talking about? i wanna read it.

[joecrum]

I hesitate to ask for fear of embarassment, but how does the pop-off valve work? It seems that it allows air into the airbox but not out.

[vash]

no it doesnt allow air to move anywhere. it is a door that only opens one way. but when it is closed, air is not moving through it.

[JTO]

LOL You know Cliff, I need to get to know you better. I appreciate your sense of humor. But, doesn't knocking a pronghorn over with a lemon constitute a form of "take"? Check the regs.
LOL,
Troy

[joecrum]

Makes sense, but when there's high pressure in the box, it seems like it would force the valve closed. I think I don't understand what a backfire really is.

[Pettybird]

Originally posted by joecrum
I hesitate to ask for fear of embarassment, but how does the pop-off valve work? It seems that it allows air into the airbox but not out.

Before I got my car, I was confused, too. The valve lets air our of the lower part of the box, into the upper part. Air flows past the sensor upwards, down to a throttle body, into the lower box, where it goes out into the runners. You gotta stick your head in there, and then the light goes on. Go to the CIS page on pelican and look at the exploded diagram for more clarification.

-Doug

[joecrum]

Unfixed, Doug, Noah,

Thanks for your responses. I believe I understand now (I love learning things about my car).

In keeping with the post, I too have thanked my pop-off valve on several occasions.

Cheers,