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Blown air box on start up !!! Help Please

Just tried to start my 1976 2.7 911, was running ok when taken out 4 years ago, put new set of plugs in, adjusted tappets, set points.....

Primed fuel system by lifting airflow lever in air box

Turned key fired up first revolution ran nice for about 2-3 seconds the stopped, did this a few times, so decided to give it a little remote throttle (next to the handbrake), tried starting again and BANG backfire, air box has visible cracks in it and screws popped off...

Any ideas what could have gone wrong ?, could not having filter & cover on air box cause it ? or can you over prime using the air flow lever ? or remote throttle ?

Not a great day

Any good How to threads re replacing air box ?

Old 11-13-2019, 04:49 AM
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Same thing happened to me...it's a good time to reseal everything from the intakes up. There was one for sale yesterday here https://forums.pelicanparts.com/porsche-911-used-parts-sale-wanted/1043078-cis-airbox.html ,and yes there are several write ups on how to proceed on this site....will have to look for them:<)
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Old 11-13-2019, 06:14 AM
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Not having the cover or filter probably didn't cause it. I don't know where you are located, but if you're in one of the many areas in this current unusually early cold snap, the cold air might be a part of it. Your cold control pressure may be too high, causing a too lean mixture @ low temps. Resetting your points may have changed your timing slightly, as well. Also, your WUR may be a little out of spec, which wasn't apparent when you put the car away.

You might want to consider a popoff valve in your new airbox. I know it's a "belt and suspenders" solution, but I like the added security for those "just in case" situations.

If you do a search, you'll probably find some info on airbox replacement. There may be something in the "How To" section or Wayne's book.
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Old 11-13-2019, 06:15 AM
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I have CIS also although i do have a 3.0 litre ..

i NEVER " prime the fuel lines lifting air plate .. " you are dumping raw fuel into the intake runners when you do this. this will just pool in there and vapourize and then bang..
i store mine all winter .. even when restarting, i just fire her up and never have a problem ..
if i want to prime the lines i remove them from the injectors first and put them into gerbers baby food jars or something similar.

the good news is with the new airbox it has a plenum insider it to help distribute fuel . although i think its connected to the CSV and i dont think you have it if you have the hand throttle. i think the CSV and thermo time switch replaced this.

I personally am not a fan of the pop off valve. Even when i replaced my air box ( for an unrelated reason ) i never pout a pop off valve in the new one and it has been fine for over 12 years.
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Old 11-13-2019, 06:36 AM
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I don't know if this really helps but when my 79SC sits for a few weeks to months I prime the fuel system but not the engine.

I use a fuel pump relay with a toggle switch installed to run the fuel pump. When the fuel pump is manually turned on you can hear the pump strain when it "charges" the fuel system from the tank to the fuel filter.

This way I think the fuel fed to the engine helps a more even regulated pressure so a no temporary lean condition exits. Before I started doing this I would get a few scary pops thru the intake.

Does this make sense? I already had the relay/toggle switch for CIS tuning/testing/repair.

Terry
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Old 11-13-2019, 07:06 AM
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air box

If you have trouble finding an air box, let me know.
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Old 11-13-2019, 07:17 AM
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Down the fault tree farther is the possibility of a sticking fuel injector
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Old 11-13-2019, 07:40 AM
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Thanks everybody, just got engine out of car, just need a good how to!?
Old 11-13-2019, 07:44 AM
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I think this goes with out saying but you should seriously consider a pop off valve in your new air box. Mine has saved me at least a few times. Our host carries them and has a great walk through on how to install them.
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Old 11-13-2019, 07:50 AM
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Past post worth reading:
https://forums.pelicanparts.com/porsche-911-technical-forum/689763-blown-airbox-replace-prevent-while-im-there.html
Old 11-13-2019, 08:29 AM
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Quote:
Originally Posted by theiceman View Post
i NEVER " prime the fuel lines lifting air plate .. " you are dumping raw fuel into the intake runners when you do this. this will just pool in there and vapourize and then bang..
Doesn't the fuel distributor stop fuel being being pumped it the intake runners ? if not then this could have been the reason for the big Bang (split it several places and fixing screws popped off).

Would excess fuel still be in the intake runners if the engine had run a couple of times for 2 - 3 seconds?

I still can't find a How to, anybody !?

Cheers
Mark UK.
Old 11-13-2019, 11:20 PM
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Lots of info here.

https://forums.pelicanparts.com/porsche-911-technical-forum/758788-cis-troubleshooting-dummies.html

stlrj already posted the relevant thread. https://forums.pelicanparts.com/porsche-911-technical-forum/689763-blown-airbox-replace-prevent-while-im-there.html#post6865184 is what you need.
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Old 11-14-2019, 12:20 AM
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Lots of great stuff in the threads and will be very useful but no How To.
Old 11-14-2019, 12:44 AM
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Thumbs up

https://forums.pelicanparts.com/porsche-911-technical-forum/1034111-1980-cis-airbox-replace-repair-questions-writeup-help-appreciated.html
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Old 11-14-2019, 04:53 AM
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https://forums.pelicanparts.com/porsche-911-used-parts-sale-wanted/1043078-cis-airbox.html
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Old 11-14-2019, 06:22 AM
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That 82 airbox has the same size ports as 74-77 and 80-83 US.
Only 78/79 US had bigger ports in the US.
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Old 11-14-2019, 02:02 PM
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I followed this advice on a different thread !

Quote:
you may simply need to prime the injectors. Remove the air cleaner, turn on the ignition, and gently lift the plunger inside the intake. You should hear the fuel pump come on, feel resistance, and in a second or two, hear the injectors squeal. If all that happens, go back and try to start the car.
BE AWARE See this thread Blown Air Box on start up !!!

Diagnosis so far from this Forum and local specialist here in UK, say the reason for my blow air box is because I followed the advice quoted above. Priming this way you are in danger of putting large unknown quantities of fuel in to your intake runners, give the right conditions on cranking BANG.
Old 11-14-2019, 11:47 PM
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Feel your airbox pain. Mine blew on initial startup after a drop and trans rebuild several years ago. I was so p*ssed off that I had it flatbedded to a local shop and let them replace it. New pop off valve installed and no more issues other than a feeling of lightness in my wallet.

FWIW, all sleeves and seals were installed in the intake system and it already had a pop off valve that I had installed. Supposition is that I had a slight leak somewhere. Upside is that you will have a shiny new airbox and not an old, brittle one
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Old 11-15-2019, 05:40 AM
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Believe it or not, the factory did come up with a solution...the 3.2 Carrera. Unfortunately, starting a CIS engine will always remain a game of russian roulette, turn the key and hold your breath.


Cheers,

Joe
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Old 11-15-2019, 09:09 AM
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Are you looking for a how to replace the air box? It is really very simple and can even be done with the engine in the car and a partial drop. If you have the engine out and have the parts it should not take longer than an afternoon.
I've done it several times with the engine in the car but never made a DIY.
Lots of guys here will help walk you through it plus you should have a Bently manual

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Old 11-15-2019, 11:58 AM
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