EXPLOSIVE Airbox!!!
 

Ok, so ive had a pop off valve on my car sice i got it (which has, in fact, broken loose twice in the last 4 months). After regluing the pop off, all seemed well...until it backfired so hard yesterday, that it literally bent the steel mesh in the air filter and blew a friggin hole through the paper. has anyone heard of this happening before...holy crap
PS its a stock 74 2.7

Hi,

You didn't say whether the two loose pop-off valve issues were due to backfires or not but thats an important factor in determining the resolution to your problem. :) :)

Did the recent backfire occur when the engine was cold or hot?

The very first thing that needs to happen here is a measurement of cold and hot control pressures. You, or your shop, will need the CIS Fuel Pressure gauge setup to do this.

yeah they were backfires and both times the car was cold...thnx

check your airbox thoroughly
 

Mine backfired a few months back. My air box was damaged even though i had a pop off valve. I discovered it because the car was backfiring more than usual on start-up and when i went to adjust the mixture with the new gunson tester, i noticed that the box moved a lot and when i examined it it was cracked. Look at the slotted screws and see if they are loose and grab the box on an edge and lift it up to see if it gives...

If it is broken post wanted to buy on the parts here or ebay.

If you need to install a new one. do a partial engine drop and find my post here on cis air box.

Is the popoff valve oriented correctly? IE the hinge toward the back of the car?

Aside from verifying your fuel and control pressures, make sure your coil connections are good. I had a huge backfire this summer when one of the low tension wires from the CD box to the coil had an internal break. It caused a huge noise that reverberated through the caverns of downtown Toronto on a sunday morning. Scared me, and I'm sure quite a few others. It didn't produce any of the damage you mentioned though.

Also, make sure your points and gap are good.

I had a detnitation that blew the pop off valve through the K&N filter! would not start again till I took the air cleaner cover off and saw it. No damage to the air box but the pop off valve gave it's life. replaced it and the car ran fine. scary.

holy crap thats wild....i tried starting the car with the top of the airbox off and no AF one time...my brother was standing behind the car when the pop-off blew off and about 20 yrds into the lawn, almost lost an eye!!

good point paul. if it is the other way, would the flap hit the air filter? i will have to look at mine.

Anyone ever try to retro fit the cold start manifold into an earlier airbox?

Be sure to check the quality of the cement joint at the "front" (far side) of the Pop-Off Valve. It is invisible without a dental mirror and can lead to a lean backfire if not completely sealed. I also coated the valve flap rubber seal with a thin coat of silicone to prevent sticking.

thanks howard good point; to comment on the orientation of the pop off, yes the flap will hit the filter if not facing backwards (i found this out the hard way)

Check the new Porsche superseded airbox, it lowers the cold start injection valve into an area that prevents the explosions. Get rid of the toilet seat pop off valve

is there something else out there to use instead?

Are you talking about CIS or something else.......
 

Vmarch,

Could you elaborate a little more about this subject? What is 'the new Porsche superseded airbox'? First time I heard about this and would appreciate you sharing this info with us. Thanks.

Tony

yeah could we get more info on that? never heard of an alternative box

hmm. My 911 sc has never backfired, is it more rare that they do, or is it more rare that they don't?

Make sure the rubber seal in the pop-off valve is lubricated. Heat and non-lubed rubber can bond the top flap to the valve and it won't open up causing the valve to be "ejected" from its glued in location.

Tony, I think he's referring to this:
http://tech.rennlist.com/911/911scfaqengine.PDF

If it is, it's not really news--it's the box Porsche developed in 81 which moved the cold start injector to the runners instead of the air box. It reduces the chance of an explosion, but does not eliminate it (does nothing to prevent backfire from a lean running engine, for example). Not a good idea to get rid of the flapper, IMO.