CIS vacuum leak found, advice needed

[Johner]

Hi

I've been tracking down a CIS issue for the last week or two. I did a smoke test and found smoke coming from round the airbox pop off valve. I assumed it was the seal, but found the whole thing was loose and just pulled out. The obvious thing to do is refit it. Any recommendations on a good adhesive that people have had good results with? Are there any other options? My airbox is an 83, so may be less prone to airbox backfires. Is replacing the pop off valve with a blanking cover viable?

Thanks

John

[Robey5]

FWIW - the pop-off valve was added as a stop-gap to when a CIS backfires. Not “if” …. But “when” as I understand it. If it were my car, the first thing I’d do is bust out the Dremel and clean out the hole with a shop-vac pulling as much of the crumbs and such out while ya do it. Then, I’d buy a new pop-off valve from our host, and install it with a generous amount of 2-part epoxy like JB weld (and let it sit overnight to dry).

Consider yourself lucky after this job is “done” if you have only THAT which was leaking.

In my experience, these air boxes have gotten harder to get at a reasonable price. I suppose you may be one of the lucky ones to find a metal version that someone made in the classifieds if you choose to go that route, but - keeping it simple: replace the pop-off valve with another pop-off valve for this repair.

Good luck - and lets see some pics of this repair!

[MarkWard]

3M makes a product called 5200. It takes a week to fully cure, but will cure. Amazon sells it in black. Clean up all residue, scuff the gluing surfaces, and follow the directions.

JB Weld would also work, but I believe the 5200 will remain more flexible. I’m sure others will have their brew recommendations. Good job finding the leak.

[EC900]

Poor original installation, improper epoxy prep and two-part mix or too many backfires.

I’d call PP and see what “epoxy” they recommend since they sell the valves and provide tech article.
That said a new surface prep will be required, whether this can be accomplished without taking air box off but the old epoxy will need to be removed.
It’s important to know the makeup of the parts you’re gluing together, epoxy works wonders but doesn’t set up properly if it isn’t mixed correctly. Epoxy sticks to most everything but needs “teeth” to hold, if the air box is ABS, polymer based or the ubiquitous polypropylene needs to be determined when using adhesives other than what’s recommended. The terms “plastic and epoxy” are used loosely and often generic.

https://www.pelicanparts.com/techarticles/911_pop_off_valve/911_pop_off_valve.htm

[Johner]

Thanks for the input and link. As suggested in some of the tech article comments, I'm thinking of trimming back the ribs on the airbox so the flange sits flush with the top of the airbox. This creates a much better surface for the adhesive to bond to. The 3M 5200 looks like an interesting alternative to epoxy.

It does look like the root of the problem is incorrect prep of the parts though; the old adhesive just peels off and there's no sign of either surface having been scuffed up prior to bonding it. The valve has been in the car at least as long as I've had it (7 years), probably much longer. I don't think I had any backfires and the valve was still in place. My guess is that the adhesive joint just degraded, maybe with lots of heat cycles and flexing over time, and gradually got worse to the point it failed

I will recheck for leaks once I've fixed it as it may well have been hiding other leaks during the smoke test.

John

[fanaudical]

The airbox is not ABS.

Here's a previous thread summarizing adhesives used for other airbox repairs:

http://forums.pelicanparts.com/porsche-911-technical-forum/853053-air-box-repair-myth-reality.html

My recommendation: Use Loctite "Plastic Welder" two-part epoxy. Clean all surfaces very well and lightly abrade them (scuff them up) prior to application of the adhesive.

[Johner]

Thanks, there's some good information there. I 'm looking forward to getting this fixed and seeing what else is leaking!

[rwest]

Make sure you put it in he right way so if it does pop, it won’t hang up on the filter and not shut.

[Johner]

Thanks for the reminder, it's easy to get that kind of thing wrong when you're rushing to work with adhesives.

With regards to the adhesives mentioned, I couldn't find loctite plastic welder, only lctite plastic bonder https://www.loctiteproducts.com/en/products/build/epoxies/loctite_plasticbonder.html. Does that look like the same product?

For the JB Weld I find https://www.jbweld.com/product/plasticweld-syringe or https://www.jbweld.com/product/plasticweld-syringe. Any thoughts on which would be the better product for this application? Or is regular JB weld what people have used?

John

[997at]

Since your airbox already has a valve installed, I agree with the remove, clean, and put back in with adhesive advice. It might be worth double checking that the airbox is still intact at the seams. Check the screws to see if they are still tight. If they are not, then the box likely has been blown at some point along the seams and you will want to replace it.
I chased CIS leaks etc. for a year or so after first getting my '82SC. After fiddling around and never getting the car to run perfectly, I elected to completely dismantle the CIS system and inspect, test, repair, replace every single component. When it came to the airbox, my old one had a loose pop off valve and also loose screws around the seams. I bought a new box but DID NOT install a pop off valve. If you system, hoses, devices, etc. are all working properly and the CIS is tuned, the car will not backfire. In fact, what can happen over time is that pop off valves become loose, introduce unmetered air, and actually create a backfire that might then blow the box if it's a big one.

[Old H2S]

Check, check and do it again BEFORE you put it in with epoxy. The valves are crap now and I have gone through 3 in the last MONTH to get one that does not leak..

[Johner]

Thanks, I did give it a basic test using a Mityvac, a bit of flat plastic packaging and a liberal coating of vaseline. I could apply a vacuum of 60mbar/17 inHg and it would hold vacuum for about 15 seconds. I also filled it with water and left it overnight and no leaks. Given the crude test setup I think it is probably ok to put back in, but if you can suggest another test method I'll give that a try too. Where did yours leak from?