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