Air Box BOOM!

[BBAY1]

All:

So....I was going down the road minding my own business when, during hard acceleration I suffered the re-occurring sudden engine cut out (still trying to determine whether it is fuel or ignition related). I am now leaning toward ignition as I heard a loud back-fire and a sound as though the mufflers had blown off. I pulled the now non runner to the curb and tried to restart. The engine cranked laboriously with no start. I looked at the engine and pulled the airbox cover and air filter. I notice two screws loose in the air chamber downstream of the air filter. I also noticed the the airbox upper and lower portion were separated and the lower portion was no longer in contact with the plenum. The pop-off valve seems intact though the spring seems inordinately robust.
So, after, "Why me? I need to address the backfire problem, why did not the pop-off valve pop-off? Could I have ingested debris and or loose screws up through the air control flapper and into the engine?
Any pointers on removal of the air box? I hope to re-use it if it is simply a matter of the attaching screws pulling out of their respective threads.
All advice welcomed.
Regards
BB

[Zeke]

I'd like to comment with more experience other than, "It happened to me, too." However, mine was in the driveway and not under acceleration. So, I assume there was more fuel to ignite in the box and you got a good one. You'll have to see how the pop off was installed; maybe someone glued it with the wrong glue. AFAIK, it has to be sealed tight, but not too tight so it will let loose when the time comes. I think the purpose of the pop off is to relieve pressure so the box doesn't break, but once opened, you have to reattach for proper engine function.

AFA reusing your existing box, you could epoxy the thing if it's damaged only in the way you see it. You'll have to get down there and check it all out. I've not heard where anything went into the cylinders after an explosion, but how long the engine kept turning before you clutched it would have something to do with the possibility.

That's the best I can offer.

[Jcon]

It happened to you because you own an SC with CIS injection. I swear it will happen to all of them. I'm with MILT. If the box comes out and the damage is as you've described you can epoxy the box together. When mine blew up it blew the pop off valve right out breaking the surrounding plastic. The Porsche dealer epoxied it, put it back together and it ran fine. You do want to check it closely for air leaks. The more prudent and expensive approach is to get a new airbox. As for the backfiring it could be any of a hundred things. If the cut off is very sudden on / off it's in the ignition.

Good Luck - CIS sucks, but the SCs are great cars I miss mine.

Jeff

[bigchillcar]

+1 epoxy is your friend..a new airbox is $$$ (not your friend).

[BBAY1]

Thanks for the advice. I need all the 'friends' I can get.

Is the typical removal method involve removing the 4 rubber collars from the box the the inlet manifold and removing everything as a unit. It would seem to me that epoxying it back together off the car would be the preferable way to go.

[bigchillcar]

Thanks for the advice. I need all the 'friends' I can get.

send some 'friends' my way if you ever get into a 'jam' with too much of it...err, them! and yes i'd definitely remove the airbox for this job.
ryan

[SC-targa]

Originally posted by BBAY1
Is the typical removal method involve removing the 4 rubber collars from the box the the inlet manifold and removing everything as a unit. It would seem to me that epoxying it back together off the car would be the preferable way to go.

There are six rubber boots, one for each cylinder. I don't believe you ever stated which model 911 you have. 1980-83 CIS systems require at least a partial engine drop to remove the airbox.

Regards,

Jerry Kroeger

[gregg911]

Sorry about your situation!!

Just last week, I went to start my 80SC and got a little happy with the pedal before she was warm and boom!!!

Same thing that you experienced except my popoff valve blew out of it's opening and was laying in the airbox. I re-epoxied it back into place and it started right away the next day. I was afraid that the box blew also but that wasn't the case. These things must develop some pressure when they backfire to blow an expoxied valve out of it's hole.

Sorry to hear about your situation. I'm sure that once installed that your new or repaired airbox will have a valve installed.

[BBAY1]

Sorry I failed to mention the model...'82SC. Hmmm...partial engine drop, eh? Man, that can't be good. How 'partial, is it? BTW the pop-off valve looks good as new..it just sat there looking good while the rest of the world blew up around it.
BB

[Jcon]

Actually a partial Engine drop isn't that bad. For the most part you're only removong the bolts on the engine mounts that are at the far back (close to you ) corners of the engine compartment. With the engine supported by a floor jack you can lower it a couple of inches to get the clearance you need. This is the time when you need to get on line and order a Bentley SC repair Manual. It will have pretty much everything you need and since you are driving a 25 yr old car you'll be using it again.

Jeff

[bigchillcar]

yeah, a partial is just what jeff says..not a biggie..just for additional clearance you need for removal.

[JP911]

A partial engine drop would make things more manageable, but a complete engine drop would make it easy; you would be able to inspect the airbox from all angles without removing it from the car.

[bigchillcar]

already knows the box is blown. i'd do the minimum required, partial drop, remove airbox and repair if not too damaged. it's gonna have to be removed anyway..

[JV911SYDNEY]

sorry to hear about this.

it happened to me just before christmas. i had a pop-off valve...sitting on the bench in the garage

did a complete engine drop, found a 2nd hand airbox and was back on the road in 3 days

[BBAY1]

Thanks again for the input, teammates. The engine partial drop was as easy as pulling the two rear motor mount bolts. After it was done, the full pull probably would have saved time as I had a bit of learning curve reachingblindly to detach all the hoses, elec, connections etc etc behind the upper air box. The box itself seems undamaged save the threads in the lower portion. It has all the appearance of having been epoxied together at one time. It seems a bit of a chore, but the box could be re-installed as a unit so long as I can get to the clamps for all 6 rubber connectors.
I am imagining all sorts of clever (i.e. I think they're clever) ways to preclude another blow out. Most involve some spring loaded attachment method to allow the box to separate and spring back together. I suppose if the pop-off valve would work this would not be necessary. As several of the steel fuel injector collars pulled free of the aluminium intake runners, it better take a good look at all the bits in the area. I suppose replacing all the injector O Rings is mandatory.
Thanks again guy. Hope to be up and running soon.
BB

[bigchillcar]

good luck with the epoxy! maybe take some pics and post so that newbies can see what happens..

[holtjv]

Originally posted by BBAY1
T...
I am imagining all sorts of clever (i.e. I think they're clever) ways to preclude another blow out. Most involve some spring loaded attachment method to allow the box to separate and spring back together. I suppose if the pop-off valve would work this would not be necessary. As several of the steel fuel injector collars pulled free of the aluminium intake runners, it better take a good look at all the bits in the area. I suppose replacing all the injector O Rings is mandatory.
Thanks again guy. Hope to be up and running soon.
BB

Make sure to add quick release fasteners to the engine lid grill; that way you can catch the airbox on its way down. ;D

[JohnKo]

Can this thread tolerate a stupid question? I thought the whole reason for installing the pop-off valve was to prevent these kinds of 'events.' If it's doing its job, why is it still possible to blow up the airbox? Shouldn't the offending backfire or whathaveyou just vent through the valve, or are we talking about an event so severe that the pop-off valve can't handle it fast enough? TIA

[bigchillcar]

not sure johnko..i was lucky..i had cis for 6 years, with no pop valve and never had a problem, even though i had several minor backfires. i wonder if it's a function of the spring being too tight in some cases?

[Scott R]

Originally posted by JohnKo
Can this thread tolerate a stupid question? I thought the whole reason for installing the pop-off valve was to prevent these kinds of 'events.' If it's doing its job, why is it still possible to blow up the airbox? Shouldn't the offending backfire or whathaveyou just vent through the valve, or are we talking about an event so severe that the pop-off valve can't handle it fast enough? TIA

I lost an airbox early on on my 3.0, I had a pop off valve however the explosion was so violent in the airbox that it still blew apart, even though the valve was operational. In my case I had a cylinder that would not fire, fuel backed up and BOOM!