[alfaloco]

My son's gtv6 recently popped off the plenum while I was doing some work on the power windows, starting and turning off the engine from time to time. I wasn't even in the driver's seat, turned the key to start it and POW!
After some choice adjectives in Spanish as is my custom, I decided to get serious about this phenomenon.
No it doesn't happen often at all, but when it does happen it's always a PITA to loosen the clamps getting access to them with a slender screwdriver, resetting the plenum and then getting access into that tight space again to tighten the clamps. I don't like living with problems, if I can resolve them.
So I began to think about it, which is usually kind of dangerous.
I researched low pressure relief discs, conventional pressure relief valves, and the spring loaded disc that is sold for the 911 flat 6 engine which addresses the backfiring habit and prevent cracking of the airbox. I looked at back fire protection for propane-fired engines like forklifts. Unfortunately we are restricted by space limitations and the ribbed design of that cast aluminum plenum anyway. Realizing that any of several methods might work, I finally decided on a pop off cap fitted to a large-diameter hose barb. Like an oversize vacuum port cap. Will this be large enough in cross-sectional area to reduce the explosive pressure enough to keep the plenum in place? Heck, I don't know! I didn't do any calculations. I didn't have enough data for that. But I just decided to go with as large a fitting as I could and relocate the check valve and hose for the power brake booster.
So please just refer to the photos and you'll see that I bored out that port to 18 mm from the stock 16 mm and installed a large hose barb fitting with as large an ID as possible. And I relocated the power brake hose over to the other side using a 1/4 inch pipe thread fitting and 3/8 diameter hose. I also used a plastic check valve in place of the original steel threaded check valve that was installed into the plenum.

 

[alfaloco]

The black cap is a molded pvc chair leg cap, a hardware item, that slips snugly onto the hose barb. My vacuum reads normal at idle, so no leaks are apparent. And yes, there are a couple of spare caps in the glove box. Hopefully, this may relieve enough of that instantaneous pressure so the plenum will stay clamped down. And I tightened the hose clamps on the intake tubes very securely, too. We'll see... maybe one of these days.

 

[dancia]

I like it. It's worth a try.
Man against car, who will win?

 

[alfaloco]

I like it. It's worth a try.
Man against car, who will win?

That's how I felt... it was worth a try. It just might work.

 

[cda951]

Having installed a number of the aftermarket pop-off valves on customers' CIS/K-Jetronic injected Porsche 911s, I must say that they are a fix for a symptom rather than the cause. The root cause is an improper or unevenly distributed air/fuel mixture during cranking and/or starting; this is often down to a combination the imprecision of a single cold-start injector and old sensors/switches/actuators/wiring. A sticking auxiliary air regulator and/or a slightly flaky thermo-time switch combined with a dribbling cold start injector might be all it takes to cause a large pop-back within the intake plenum.

In the CIS 911s, the plenum section of the intake manifold is combined with the lower half of the air filter box and is made of plastic. Therefore a good-sized pop-back could split the entire plenum and cause a large vacuum leak. And it is tricky to replace the 911 CIS airbox without engine removal! It wasn't long before the aftermarket stepped in and offered a spring-loaded pressure relief valve as shown here:

Porsche 911 CIS Pop-Off Valve Installation | 911 (1965-89) - 930 Turbo (1975-89) | Pelican Parts DIY Maintenance Article

No 1973 1/2 - 1983 911 should be without this very important addition. The CIS cars backfire all the time, don't blow up your air box, install a Pop-Off Valve.

www.pelicanparts.com

Of course, the function of the valve is only as good as the quality of installation, I have seen numerous vacuum leaks from sloppy installations, and even the entire valve blowing out from an intake pop-back.

In Porsche's case, they devised an updated CIS airbox/plenum than incorporated internal steel manifold pipes to evenly distribute the fuel from the cold-start injector (visually identified by Phillips head screws to fasten the airbox halves versus the slotted pan-head screws of the original versions). This eliminated about 90% of the issue.

Now, I am not suggesting that you rig up some sort of internal manifolding for your GTV6 cold-start injector, but I am providing food for thought. If you have had a persistent issue with blowing off the manifold during cold starts, the root cause is likely down to old, worn components of a primitive fuel injection system. That vacuum cap might do the trick, but it is also possible it will only partially do the trick. The problem is it is a difficult condition to test for!

 

[alfaloco]

Once again Chris-- well stated! Thanks for that background. The $25 relief valve is likely the single most reasonably priced item you can buy for a Porsche!
I would have preferred a disc type spring loaded poppet, with a vacuum seal, directly on top of the plenum. But that lovely cast ribbing has to be machined down flat, and a hole bored, to accomplish that. I have no doubt that size of a poppet would have enough cross-sectional area to work as designed. That's do-able, but it wasn't practical for the time frame I was working in.