Cracked airbox it seems JB Weld maybe
 

Well since my smoke test revealed smoke around the 4,5,6 side runner hoses, I’m pretty confident my airbox has a slight crack. As evidenced by these photos:

http://forums.pelicanparts.com/uploa...1565197464.jpg

http://forums.pelicanparts.com/uploa...1565197504.jpg

http://forums.pelicanparts.com/uploa...1565197562.jpg

Pretty likely this is what caused my rpm to stay constant after removal of my oil cap.
So, new airbox or the Porsche rookie mechanics greatest weapon: JB weld.

Which one?

Cracked CIS airbox..........
 

Mike,

The pictures you posted showed the cracks along the seam or perimeter of the airbox. Time to replace the airbox with a new one. They are now less than $250 brand new. You are not saving much by fixing this 40 year old airbox. Even if you are able to seal it, how long do you think it would last? Well, it is a decision you have to make.

Tony

That was the reason for me doing my ITB swap originally. Which led to finding broken studs and rebuilding the motor...but you've already tackled that so maybe worth considering an alternate induction setup.

Epoxy is $8...

I'm not disagreeing with Tony, but if you want to try the fix, there are threads on this. John Walker has recommended a particular type of adhesive that seemed to work for him.

Hi Tony,

I'm just shocked I found it, being I'm the dullest tool in the shed and all. ;)
Prices I've seen on new ones is $450 or so, I'll shop around.
Car performed okay with the crack, I'm going to see if my fix stops the smoke then I'll decide from there. It's like a scavenger hunt ;)

And all joking aside, I appreciate the advice and comments with various opinions on the solution.

I had a leak in mine along the seam. Split it apart and glued with JBWeld. That was over 25 years ago and it's still together, leak free. YMMV.

I replaced my cracked air box for two reasons
1- it was cracked......
2- I no longer wanted a POV , which my cracked one had.
So I bought a new one, for less than $200.
Even though it appeared extremely well made I decided I could make it a little stronger by removing all the screws and replacing them with small nuts and bolts I dont think this box will blow open, but who knows.
Had I decided I wanted to keep the POV I would have done exactly what rdimarco did and I would have used JB weld after thorough cleaning of both mating surfaces. (Wire brush even). Then I would have replaced all the screws with nuts and bolts. I feel confident that would have been more than satisfactory.
I opted against the POV because I think they could be a source of air leaks. I tested three POV's by shining a light inside the opening. In all cases light could be seen under the flapper.
Maybe they were all defective.
What I might suggest, if you want to use the POV, is to find a way to add more preload to the lifting lid. That might be as easy as placing a 1/8" pce of plastic between the closing spring and the top of the flapper.
Good luck.

I'm not opposed to buying a new airbox, but JB-Weld has worked for me. Classic time vs. money bet. This time I won, but I'm glad it's worked and I didn't have to revisit it.

I’m not opposed to buying a new one either, but my genes want me to attempt to fix this one and use the savings on 3 shares of NVDIA instead :). Must admit first application let another leak be seen. Maybe it’ll be fixed airbox for a while, until I see if my rebuild holds together ... and then PMO’s. Or an “airplane” (model only).

I would fix that with laminations of epoxy and dynel cloth. If you prefer you could also use fiberglass, carbon fiber, or kevlar.

It's a lot of work but it will be successful. These two threads that I posted might give you some idea.

http://forums.pelicanparts.com/porsche-911-technical-forum/1030689-cracked-evaporator-housing-fix.html

http://forums.pelicanparts.com/porsche-911-technical-forum/1034692-air-control-box-repair-ventilation.html

I've done a lot of this kind of repair and in my experience any attempt to 'just glue it' is not going to be a permanent fix no matter what glue you use. JB weld is basically a thickened epoxy with additives (you can make it yourself) but epoxy alone is not going to seal those cracks permanently You need to add an impregnable cloth to give the repair strength.

Thanks Robert,

Interesting how this problem which the Porsche CIS troubleshooting manual flowchart said: “a fix was not required” has compounded. Thinking my JB weld fix (without the cloth) is not the best way, but the ship has sailed. And will see how long it lasts for a data point (maybe). Via testing of the oil cap rpm. Still with 6 cars, getting lots of miles on this car in many pieces will be a challenge.

Airbox prices
 

The cheapest I could find any of the SC air boxes (late 38mm or early 44mm) were a bit over $400, and on some sites they were nearly $500.

Found a 38mm for $275 just now.

I have researched this in the past. Searching my name and airbox may yield results.
Short version is next to nothing will patch these airboxes. You could get lucky, which is worth a try... cheap and relatively easy. Patching N/A is much easier than patching a cut up pressurized airbox.
I found some stuff on Amazon which sort of works.
Best assurance of no leaks is to get a new airbox.

Brand new CIS airbox.........
 

Installed this brand new CIS air box for a PP member several weeks ago and cost less than $250.

http://forums.pelicanparts.com/uploa...1565354893.jpg

http://forums.pelicanparts.com/uploa...1565354893.jpg

Tony

My post might have sounded like I don't like JBWeld and that's not the case. It's a very good product. The problem with 'store bought' epoxies or resin based adhesives is shelf life. The epoxy I use for marine repairs is potent for about 2 years, give or take a year. Polyester resins 1 to 2 years max. And storage temperature has a big impact on those ranges.

For small repairs, I used to buy the little tubes of epoxy sold in hardware stores because they were convenient. In recent years I've had more and more problems with those tube adhesives not curing fully -- even JBWeld. It's really frustrating to do a nice repair that winds up a gooey mess that never hardens!

I'm only guessing, but with our 'global economy' I imagine that epoxy products can be roaming the world at varying temperatures for who knows how long. Stores go out of business, inventory gets sold, stuff sits in warehouses, not to mention sitting on my shelf until needed.

It's best to test any epoxy or resin based adhesive you buy before applying it to the repair. Just mix up a test batch to make sure that it cures fully.

Also, epoxies don't catalyze well in tiny batches. The techs at West Systems told me that mixing small volumes of epoxy on a flat surface prevents the molecules from really getting to know one another. Best to mix even small amounts in a container, like a little medicine cup. I still mix small batches on flat plastic, but I make sure I squeeze out a healthy dollop that's about 3 times what I need. Just make sure there's some depth of 'A' and 'B' so you can stir thoroughly!

Also, epoxy mixed with 'fast cure' hardeners are far more failure prone than 'slow cure.' The epoxy we use for marine repairs takes 7 to 12 hours to fully cure. Believe me, if we could use epoxy that cures in 5 or 15 minutes, we would! I do use 15 minute epoxies but tend to avoid the 5 minute ones.

Lastly, the above is based on my experience using various products. My daughter wants to be a chemist, but I am certainly not one!

don't forget the pop-off valve (toilet-lid valve)

can't deny this, though for that bit of a repair I wouldn't go space odyssey scientific and not use it/do it JB-weld-style

Since we're on the subject, I might as well ask. Would there be a market for air-boxes made in carbon fiber?

Making a mold from a donor box is time consuming but not difficult. I thought about doing that when I repaired my evaporator box. One shop where I buy supplies, makes molds to do short runs of parts for the Navy. Their work is impeccable!

I'm not sure it's something I'd want to get into, but I am curious to know what you guys think. (If this is too much a thread derail just ignore me).

BTW: Carbon fiber is over-rated. In many of the applications where I've seen it used it's really not necessary. Carbon fiber and Kevlar are strong, but not a whole lot stronger than fiberglass. They are lighter than 'glass, so great for making canoes, tennis rackets, and race car parts. But they're also brittle and prone to cracking. Fiberglass is the hands down winner for toughness and longevity. The fiberglass shock mounts I made for my truck are as strong as steel and will outlive the frame by decades.

Flojo,

I wouldn't either and don't. I just thought I'd try and be thorough.

I guess I need a better search engine...

I fear the price as a show stopper.

$250 installed, I'll take two! :)
Are these 44 mm ports for $250?

Clarification.......
 

Mike,

$250 was the price of the new CIS airbox to replace the cracked one. I can not post the name of the source in this forum. The engine was a ‘83 SC US spec with the -2R intake runners (34-mm ID). The new airbox is now identified using the OD measurement for spec.

Tony

Edit:
Looking at the invoice, it was $273 not $250. My mistake.

Thanks Flojo, you're probably right about the cost. There goes my dream of buying a 959 with the profits from my carbon fiber air-box venture! ;-)

Now I’d pay big bucks for a titanium air box ... just saying ... but it would need to be a package deal with a titanium exhaust system :cool:

Okay, so with two passes of JB weld my airbox passed my smoke test with no leaks. Hope this does it so my rpm goes down with the oil cap removed. As I said the car ran well before this anyway. But I’ve got the CIS on the operating table so on went mr JB.

Airbox for my runner size is about $480.

Do you have the large port box?

I think so, they measured 44 mm. 78 and 79 are special :)

So are all of the Euro's from that period thru 83…

There’s some videos on YouTube fixing plastic with screen mesh and a small iron. Just burn the mesh into the plastic. Harbor freight sells the heating iron. Permanent fix...

This is all covered in a couple of other threads.

The best epoxy I've found for the plastic used in the CIS airbox is 'Loctitte Plastic Welder'. It actually does bond to the surface (not just stick).

The plastic used din the CIS airbox is not a thermoplastic (it's a thermoset) and it doesn't respond well to attempts to repair with thermal welding.

Fan,

Your comment on thermoplastic and thermoset plastic answers some things I've encountered and wondered about. If my understanding is correct, thermoplastic is the stuff they make kayaks out of -- is it called 'roto-moulding?'

If so, in my experience nothing sticks to that stuff! I've used heat to repair small cracks in a friends kayak. So far it's never opened up, but I always worry about it. It may be just psychological, but I have trouble trusting a repair that I haven't added material to.

Hi Mike,

I'm working on your titanium air-box and exhaust, but I'm afraid we'll have to add a few zeros to the estimate price! :-)

My vote would go to a new airbox. For $250. what is your time worth fooling around with repairs?

Sure, anything can be fixed but for how long will the repair last?

Rotomolding is a manufacturing process. You drop a hot glob of plastic into a warm mold and then spin the mold to coat the surfaces and make the shape. Most kayaks of this type are made from polyethylene, which can be welded for repairs.

Polyethylene doesn't allow bonding to most adhesives. There are some that will bond but difficult to find.

The world of plastics is much broader than one paragraph on pelican parts 911 technical forum.

piscator,

Basically, thermoplastic plastics are one that set up as they cool, after being formed with heat. Heat will soften them. Think car model plastic. Thrmoset plastics give off heat as they cure. Unless subjected to exteme heat, they will not melt with heat. Think a cooking pot handle.

Patkeefe is correct. there's more to it than this, but that's the basic difference.

JB Weld handles heat well up to 500 degrees F, and there's a high-heat version that goes considerably higher. For low-temp applications, West Systems G/Flex is less brittle than most epoxies and is a fantastic 24 hour epoxy.