Bang! There Goes the Airbox

[island911]

in my experience, backfires, on start, can happen by advancing the timing too much. Usually a rookie type of mistake, though.

[juanbenae]

recently fire mine up and it went and blew again, it did not sound right though. it then would not start on subsequent attempts. i pulled the air box top to find that the pop-off valve had been blown out of the airbox! the epoxy failed, leaving a nice indentation in my k&n filter. i was all angry then.

Thanks for all the input. I've found a used airbox w/ pop-off valve already fitted for $100. I'll be having a go at it in a week or two. I'll aim to drop the engine a few inches &, while I'm at it, replace the oil pressure switch, thermostat O-ring, throttle linkage rod bushings, crankcase breather gasket... anything else? Also maybe remove the AAV, & check the AAR operation.

cheers

[john walker's workshop]

remember on my previous post when i suggested removing the engine on 80-83? that wasn't an idle comment. you'll find out why if you try to do it in the car. not that it can't be done, because i've done it, but it's a real pain. just a friendly warning.

[autobonrun]

A blown airbox was the very first repair I had to make on my car about 6 months after I bought it in 1984. It happened after the temperature had dropped considerably.

If you normally crank your car in a warm garage and the repair shop stored it outside, perhaps the warmup regulator let some extra fuel pool inside. Just a thought, probably a wrong thought though.

[Gunter]

If you consider just a partial drop, make sure that the transmission input shaft does not get stuck/bent in the tunnel opening. Tighten all the clamps and inspect the hoses behind the airbox. The intake runner boots are $3.80 a piece (Pelican) if you consider replacing at least the right side which is so hard to reach. There is so much stuff in the back: Frequency valve, deceleration valve, AAV, AAR. You can set the timing yourself with a stroblight connected to the 25Amp fuse inside the left rear second fuse holder. The 5deg. BTDC mark is next to Z1 on the pulley. If your mechanic tried to set the mixture, then most likely the lead plug has been removed from the fuel distributor and a looooong Allen wrench will do the job. It's great that you found a good used box. Make sure that everything is sealed. Good luck.

[Doug Zielke]

Originally posted by john walker's workshop
.....not that it can't be done, because i've done it, but it's a real pain. just a friendly warning.

Good luck not dropping the motor. This is the maze of plumbing that JW is talking about ('81SC):

[McDaniel]

Early on after I bought my '74 911 I blew the box.
Not knowing any better I bought a replacement and a pop
off valve and replaced it without doing an engine drop.
Takes a little patience and a warm place to work...but it
can be done.

[Bob's Flat-Six]

Andrew: Sorry to here of your bummer. Sound's like you got the new/ used box at a good price though.
Glad my mechanic installed a pop valve with a recent service. I had no idea the hassel to replace an airbox until reading this thread.
Cheers and good luck

[pnzr911]

Did the replace an Airbox because it was cracked about 6 months ago. Yikes! I tried to do it without dropping the engine. NO way! On my 83 the only way was a full engine drop. Once I did that I went aheaqd and fixed all the old oil leaks and replaced all the rubber and vacuum hoses. Don't kid yourself this is a BIG and messy job. At least on my engine it was. Good luck. Remember to support your tranny or you will damage your shifter shaft seal like I did. Good luck!

[Sean Hamilton]

Originally posted by Hladun
Putting in a pop-off valve does protect your airbox, but take out your AAV and you'll stop startup backfires entirely. Do you really want everyone in the parking lot looking at you each time you start your car?

Yup, it's what I did. Cost me nought except for some skinned knuckles getting at the rotten little bugger!
It's now plugged with good riddance to the cold start surging and backfiring.
Still, I now miss the attention I used to get in the parking lot