BANG! There goes my SC airbox, but why?
 

'82 SC with 55k, owned for 13 years since 40k. Never had a backfire or cold starting issue, not even once. Went to start and battery was very low, just barely turned over. 1st crank was for 2 seconds. 2nd crank lasted 1 second and then backfire. Ambient temperature wasn't very cold, 60-degrees maybe and I did not touch the gas pedal when cranking.

So my question to all you veterans out there is, could a low battery condition be the cause of the backfire?

I am trying to decide between replacing airbox with or without a pop-off valve. Given the completely trouble-free history, I'm leaning towards without, but if the consensus is the backfire could not be due to the low battery, then a problem in the CIS may have just shown itself and it might make sense to get the pop-off to protect the new airbox while I troubleshoot the system.

Thanks for your thoughts. Btw, I discovered glovebox light was left on which drained the battery.

Charles

blown air box on CIS engines is a historic issue, it happens just the way you describe out f the blue after years of fine service, usually on a cold start in moderately cold weather. Fuel ignited in the air box, usually the air box is damaged beyond repair. There have been several theories as to the cause, an early rec was the shielded wires to eliminate any possible crossfire.

Replace it w/ a new one and do install a pop off valve for the next time it happens. Also replace all the seals and check the 7th injector

Good reading for you, please pay particular attention to posts # 10 & 11...

http://forums.pelicanparts.com/porsche-911-technical-forum/689763-blown-airbox-replace-prevent-while-im-there.html?highlight=airbox+pop+valve

There is absolutely no functional reason why you would decide to replace your airbox and then NOT include a popoff valve. Unless you want to buy a third airbox down the road!

You're right, Peter, good reading. When I was shopping for my 911 and many were SCs, I always wondered about the pop-off valves that everyone had installed, especially regarding leaks. You see, even if the valve were considered to be a Good Thing®, I have very little faith in other people's installation skills, and always worried about leaks. Seems like a valid concern now that I read that.

Peter - thanks for that link. It was a very good read. As a new 911SC owner (2 months) I appreciate the vast knowledge out there.

I am interested in the original question, would the weak battery tend to be part of the problem? Would the slow cranking speed affect the fuel air mixture? I wonder if this is partially the cause, since at that low a cranking speed the air would not be moving fast enough to atomize the fuel leading to a lean mixture to the cylinder, and also a rich mixture in the air box and or intake runners.
As for the pop off valve, since my car is a 1973.5 911T, the first generation cis, I installed one. Like anything else, it requires a little attention to make sure the spring cap seals and the epoxy doesn't leak. A band aid, sure, but if it prevents a blown air box once it is worth the cost to me. Like anything else, if it isn't maintained it could be the cause of the backfire and blown air box. Key thing seems to always come back to proper maintenance of the complete system.

Yes, I think that's correct.

Whenever you crank a CIS car (or a carbed car) without it firing, you are getting vaporized fuel distributed, but not burned. This vaporized unburned fuel is a potential source of an unintended ignition/backfire.

You'd see it all the time in the old days, when people would crank and crank hard to start cars - with a backfire resulting.

"..no functional reason.."

Not exactly true.

Newer factory replacement airboxes have a manifold inside that evenly distributes the fuel during cranking to each of the 6 intakes. Porsche fix for the problem.

His car is an 82, which came from the factory with that updated airbox.

They don't blow up as much as the earlier ones, but they do still blow sometimes. His is proof of that.

Thanks everyone for the numerous and helpful replies.

@ Pete - I have been reading everything I can on the subject and your post in that other thread was exactly why I am undecided.

@ Arne - I think you hit the nail on the head; knowledge of proper installation and periodic inspection of a pop-off valve is the only way to minimize it becoming a source of unmetered air.

@ E Sully - I'm hoping the slow crank was the cause and not some new gremlin.

IF I was able to replace JUST the airbox and not disturb anything else, I would feel greater confidence that the system would perform trouble free as before. OF COURSE that's not possible and furthermore I will be replacing numerous things while I am in there. (thanks to everyone's contributions to bsimonson's thread)

It seems to me that the difference between successful CIS reassembly and tuning and having another backfire before achieving that tune might depend on one's level of familiarity and experience with CIS. I will follow the steps & advice diligently, but I rank as "noob with potential" SmileWavy on that scale, so a pop-off might save my butt if I mis-step. Can any of you who have been through the airbox replacement comment on the likelihood of a backfire during the first start up & subsequent tuning process?

You are over-analyzing this. The slim chance of a vacuum leak vs. having to buy (and install!) another airbox? Are you some kind of purist that will be bothered by an aftermarket part that's hidden under the air filter? :)

I am curious who came up with the pop off valve? Instead of trying to fix the problem (distribution channels, ground to hell wiring etc.), Porsche should have just admitted that the car backfires into the air box occasionally and found a way of preventing damage ...

G

^ It's the fear of the unknown! LOL I'm sure once I roll up my sleeves and get into it, it won't seem so daunting.

I vote for putting in a pop-off valve. Save yourself a headache in the future.

Jim has done an excellent job with cis info. Read through this and if you want to set up your system buy the gage set up and adjust your system before starting.
CIS Primer for the Porsche 911

Install a popoff valve into the new airbox, on the bench, before you put it in. Easy Squeezy!

Does anyone know for certain sure that a pop-off valve will provide enough QUICK/VOLUME pressure relief or is it just wishful thinking...?

Given the rarity of these events....

I've had plenty of incidents over the last 23 yrs w/ my car to say the popoff valve definitely works.

Once w/ a perfectly tuned engine w/ a fully functioning WUR, a wire from the CDI came off just prior to starting. I was in the downtown core @ the time. The ensuing BANG reverberated through the quiet of the sunday morning. It was probably the loudest thing I've ever heard and scared the ship out of me, as well as several bystanders, and probably woke many who were sleeping in the neighboring condos.

I'm convinced the popoff valve must have worked or I probably would have a very expensive coffee table base right now.

The pop off valve will work depending on how big the backfire is. As Peter Zimmermann's link shows, any backfire can cause some damage, and a large enough one will still break the air box.

The one on my SC was installed backwards, meaning the hinge was pointing towards the front of the car. If the pop off valve is slammed open, the lid edge would get stuck on the air filter, leaving it open. On two occasions I had to lift the air filter to close the valve again after it had opened. This means 2 times in 3 years I would have possibly had a blown air box.

So - I learned two things: The pop off valve works and is necessary. The pop off valve needs to be installed with the hinge towards the back of the car. ;)

G

I've had cars with and without popoff valves. I've had the popoff valve work as designed and I've had one blown out of its seat. In both cases, the air box integrity was not compromised. It's a game of percentages, a big enough backfire will break the air box in any case.

Thanks for the pointer to setting up the CIS.
As for the pop-off, it appears to be a wise choice, but like most things, is not completely fail safe.
Anyone else have any thoughts as to whether or not the slow cranking could have resulted in the backfire?

Also Porsche has made changes to the air box over the years to reduce the risk of a backfire. Those of us with an early CIS (like a 1973.5) lack these improvements. The pop off is more important to us as a result.

But then again, I am sure Pete has forgotten more than I will ever know.

http://forums.pelicanparts.com/uploa...1349836888.jpg

here is a picture of the manifold in the intake on a 81 sc. it shoots the fuel farther down the intake runners to help prevent air box detention.

Same thing happen to me. Install pop off with new box.

Pop off valve
 

Hi, I am interested in this, as I was only told about this few days ago, because I am changing my HT leads which have the braiding fitted. Obviously the new ones do not. 1980 SC. It doesnt look like I have any mods done already to the airbox.
Is there a link which shows the position of this valve, and also what does it look like.

I mentioned the braiding to the supplier of the replacement HT leads, he said it was because in Germany, owners complained of rodents biting the leads when parked up.....The mind boggles!

Thanks

the blowing of air boxes is from intake valves being open and spark or other source of ignition igniting the fuel/air mixture in the intake manifold increasing internal pressure. nothing to do with the spark plug wires. more of a design failure and a quick fix. Backfires happen a lot in carb engines (the cis mimics carbs on cold start by injecting fuel into the air way upstream). pop-off valve is the best bet.

While the martens do chew wires in Germany, this certainly is not the reason they were braided with metal on the outside. The shields are carefully grounded towards the engine / chassis. Only electrical reasons would warrant such grounding: backfires and radio interference are the only two reasons you'd do that. I have heard both.

That said, you are okay with the non shielded wires, the wires are not the root cause for the backfiring. For saving yourself from backfires, you want the pop off valve. Check under your air filter, you may already have it: http://www.pelicanparts.com/techarticles/911_pop_off_valve/911_pop_off_valve.htm

I have to get the patient into the OR before I can begin disassembly, so I haven't had any opportunity to handle the airbox. From the factory, are the top & bottom held together with just screws (permitting separation) or are they glued or sealed as well? If just screws, presumably there is some sort of gasket between the halves?

Top and bottom are glued together with unobtanium glue....
Drop the engine and remove the entire cis system.

Pop off valve is time honored solution. While I haven't been into my SCs lately there may be a simple engineering solution. My Alfa GTV 6s would backfire when I'd stop to get my key out for my mailbox on leaving my condo in the A.M. Engine not yet warm and restarted "Kaboom!" the intake would blow off at the rubber tube connectors. As I recall there are bolts that lock the SC intake system in place, while the Alfa merely had not so fixed clamps on the aluminum plennum. Admittantly the porsche system has a lot of plastic, but what if there was a way that it could allow tubes to slip off and prevent the destruction? Pop off valve works, but maybe there is another way.

Dan

This is what I went to after 2 cracked boxes, and then after the built in pop off leaked. Hard to find, but they are out there.

http://forums.pelicanparts.com/uploa...1350105327.jpg

http://forums.pelicanparts.com/uploa...1350105443.jpg

Pop off Valve
 

Can anyone tell me where I can buy the pop off valve in the UK, along with price.

I've seen both sides of the story. In the 80's I worked for the company that held the patent and manufactured the Pop-off valve, we were also a Porsche repair shop. Every airbox car that came in either had one or got one. We shipped out 100's per week.
Then in 1995 I started working for Pete Zimmermann. Pete explained as he does in his book, that a properly tuned, well tuned, 911 doesn't need a pop-off valve. Well don't you know that 17 years later I haven't replaced one Red Line Serviced cars air box that was equipped with a late model air box, but I have had to rebuild a few pop-off installed motors for burnt cylinders obviously caused by air leaks/ vacum leaks, at the pop-off valve. Late model refers to SC and earlier replacement air boxes that have the cold start distribution manifold installed. Generally these air boxes have phillips head screws visible in the air filter chamber area. If you have slotted screws than you have an earlier airbox without the cold start distribution manifold.
If you have a pop-off valve, check the epoxy seal at regular intervals and also the condition of the rubber o-ring. I've also seen pop-off valve stick to the o-ring and basically be useless to there purpose. We've had a number of tow in's for no start with pop-off valve cars. These include valves not closing after, valves coming loose, or even valves being installed backwards and having the flip get stuck in the air filter screen.
Moral of the story, keep your car in proper tune and you don't need the gimmicks. Also as mentioned in "The Used 911 Story" Airboxes are a plastic/nylon material, they do get old, they do get brittle. If your rebuilding your motor, ALWAYS install a new air box, especially if you have no history, if it's over 60-70,000miles old,or it's an early style (slotted screws).:cool:

Marc, Thanks for sharing your thoughts and experiences.

Be careful when installing your pop-off. I drilled into my fuel distribution manifold accidentally when doing mine. To fix it I just cut a small piece of sheet steel and epoxied it over the hole I drilled. Annoys me that I did that, but no one will ever know... hehe

I'm not arguing w/ Marc or Pete. Like most of us here, I'm in awe of their accomplishments and credentials re these cars.

My car had just been serviced by an independent Porsche mechanic w/ a new CDI installed. During a stop somehow a wire from the box came loose and then, when restarting the car, the engine backfired massively. I'm certain that the popoff valve I had saved the airbox.

Now, maybe one could argue that the car wasn't "properly maintained" since if it were correct the connection wouldn't have come loose. I don't know. The thing was just installed a few days previously, and there appeared to be a bad connection, that wasn't readily visible @ the time of installation.

As Forrest Gump famously said, "It happens!"

In this instance I'm glad I had the valve.

I like the Forest Gump Quote.

@ jrolstin - that's good advice! I can easily imagine doing that myself.

Thanks to everyone for sharing your knowledge and advice. I'm sure I'll be posting up lots more after the motor is out and I've waded in up to my neck! Till then...

does anyone know if a 911 with a 08 82 date of manufacture would come with factory pop off? or when were they delivered with a pop off? thx