I can't believe it YES another PROBLEM this time its BAD "THE AIR BOX BLEW UP"

jk911 · 09-08-2002, 03:13 PM

I don't know if my car is jinxed or what, but just when I got it going I was going to drive it at night for the first time.. guess what happens.

I put the key in the ignition turned the motor over and ....BANG..... smoke was pouring out of the enigne bay. I went straight to the air box and moved it "YES it was spit in two pieces "

I have no time to work on my car anymore but I don't want to pay big $$$ for the repair . I will try and replace it with the engine in the car by taking the maifolds off and re-instainng it puting the manifolds on one at a time from cyl 3 &6 then 2 & 5 then 1 &4 .

Is there any reason why these backfires into the air box occur ?

Does it have anything to do with valve adjustment ? The air box looks original "No pop off valve" I don't know why it happened to me 27 years later.

Could this explasion have caused any other damage ? i.e to the sporto etc.?

Kevin Stewart · 09-08-2002, 03:26 PM

yep pop off valve,Kevin

Wayne 962 · Wayne 962's Garage

CIS is notorious for backfires, especially when the system isn't 100%. You can replace the box with it in the car. We have the air boxes on our site - new they're about $325...

-Wayne

jthibault · 09-11-2002, 04:00 PM

I just replaced a blown air box on a 1973.5 911T without removing or lowering the engine. It is possible, but its very tight working on the fuel distributor. Removing and installing the throttle body is much easier. I recommend that you purchase a pop-off valve and install it in the airbox prior to replacing the airbox on the motor. Also, be sure to buy 6 new rubber boots for the manifold intakes (they connect the intakes to the airbox).

The replacement process is time consuming, but not difficult. Took me all day. You can get away with only removing the intake manifolds from #1, 4, 5, 6. Loosen the nuts on #2 but you dont have to remove completely. #3 can stay in place. I'd also get new manifold gaskets since you'll have the pipes disconnected.

This is also a good time to clean out deposits on the throttle body and fuel distributor. When I removed mine both were covered with black residue.

Make sure you use all new gaskets and o-rings (throttle body and cold start valve need o-rings, fuel distributor needs a gasket). All parts are available via Pelican.

See the following link for an excellent guide on replacing the air box.

http://tech.rennlist.com/

Zeke · 09-11-2002, 07:26 PM

Sorry to *hear* about your air box. BTDT.

Is there any reason why these backfires into the air box occur ?

Take Wayne's statement to the bank.

CIS is notorious for backfires, especially when the system isn't 100%.

Valve adjustment, timing and all the induction stuff needs to be spot on. Then a little weather change, and BLAM!!

jk911 · 09-11-2002, 08:30 PM

Thanks guy's

Everything is in tune exept for possibly the valve adjustment. I Know its easy and will probably do it myself. I just don't know if I will have time to do it before I replace the air box. Plus I had just changed the oil in my car. Is it possible to take the valve coveres off by just removing the oil from the bottom of the engine only or does all the oil have to come out ?

Wayne 962 · Wayne 962's Garage

You need to get yourself a copy of 101 Projects - these questions are clear in there...

You don't need to empty your oil to adjust your valves. Also, make sure that you put the pop-off valve in your new air box before you install it. Again, instructions are in 101 Projects...

-Wayne

jk911 · 09-12-2002, 01:54 AM

Wayne, I purchased your book in the same week I got my car (April this year), from this little automotive bookshop in Sydney Australia. Your book is more popular than you probably think !

I thought that when you remove the lower covers that oil will pour out. Am I wrong ?

My car has the turbo lower covers as you recomend but there are still oil leaks. Could new gaskets possibly fix this.

Oh and I wouldn't dare start her up again without a pop of valve. Not after this ...... I learnt my lesson the hard way.

Langers · 09-12-2002, 04:44 AM

Quote:

Originally posted by jk911
Wayne, I purchased your book in the same week I got my car (April this year), from this little automotive bookshop in Sydney Australia. Your book is more popular than you probably think !

Motorbooks in Cremorne?

Jim Smolka · 09-12-2002, 05:46 AM

Back when I had a 3.0 in my car, a PO install a SS air box. It was made somewhere in FL. Unfortunately, I do not have any further information...

RarlyL8 · RarlyL8's Garage

The '78SC engine I just sold has an aluminum box on it. the pop-off valve is a burst disc. Not the toilet flapper looking thing but a true pressure spring operated disc like you would find in a radiator cap. Really a class design. Cost $500 years ago.

I don't know what year your car is, but now would be a great time for upgrades like '78/9 runners and airbox with POV.

I just sold my last setup for $100, so they're still a few around and they're not too expensive. Good airboxes are becoming rare, age of the CIS components is causing backfires in 20+ year old vergins, but they are out there. I was able to repair one box that had simply pulled all the screws out and not cracked anything. Be sure to get a box with matching runners, or have them measured. Each year is a bit unique and built specifically to fit the runner size.

cab83_750 · 09-12-2002, 01:58 PM

Rarely Late,

Maybe it is time for everyone to learn how to repair plastic! I own a crotch rocket and I bought a plastic repair kit. I have not dropped the bike yet, but Boy, I always fix everything plastic with my Repair Kit.

Sunroof · 09-12-2002, 04:59 PM

The cause of my airbox explosion was a mixture TOO LEAN.......
YOU MUST install a pop off valve or it can, "get ya, and then get ya again".

Regards
Bob
73.5T/CIS

pwd72s · 05:25 PM

Quote:

Originally posted by Sunroof
The cause of my airbox explosion was a mixture TOO LEAN.......
YOU MUST install a pop off valve or it can, "get ya, and then get ya again".

Regards
Bob
73.5T/CIS

Oh yeah, it can "get ya again". Aren't we all so happy that Porsche keeps on "improving" things"? Like the "improvement" from MFI to CIS? My theory is that cars didn't get better because of Government regulation, but in spite of them. I mean...the Cayenne is a GREAT improvement, considering that it can lap the 'ring faster than other cars of Porsche's past, even a 550 Spyder... "My what a brave new world..."

stlrj · 09-12-2002, 05:49 PM

It didn't matter if the mixture on my 74 was rich, lean or right on... it still blew up with an almighty bang when you'd least expect.

What really surprises me was that there was never a recall for this defective design as you would expect from any other manufacturer. It seems they finally got it right in 81 when they introduced a modified airbox with the cold start mixture directed into the individual manifolds with steel tubing. I have never heard of one of those exploding.

Cheers,

Joe

s_wilwerding · s_wilwerding's Garage

Quote:

Originally posted by stlrj
It seems they finally got it right in 81 when they introduced a modified airbox with the cold start mixture directed into the individual manifolds with steel tubing. I have never heard of one of those exploding.

Cheers,

Joe

OK, does that mean it's not necessary to install one one on post 81 cars? I finally installed a POV on mine - it made it 140,000 miles without. Two sides - I figured if it made it that far, it'd probably go a lot longer. On the other hand, $40 is a lot less than $600, if you can even find a new airbox. So, I guess it was a good decision.