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CIS Pop Off Valve Installation

Pelican Technical Article:

CIS Pop Off Valve Installation

Bob Tindel


2-3 hours (let epoxy dry 24 hours)






Two-inch hole saw, right-angle electric drill, drill bits, metal drill punch, special epoxy adhesive, hammer, workbench, shop vac, sandpaper

Applicable Models:

Porsche 911 (1978-83)

Parts Required:

CIS pop-off valve (backfire protection valve) kit

Performance Gain:

Protects your 911 airbag from costly damage

Complementary Modification:

Replace your airbox
[The pictures in this article are of an airbox that was removed from the engine for clarity in illustration. It is not necessary, nor recommended, to remove the airbox for this procedure.]

     The pop-off valve, which is also called a backfire protection valve, is a good modification for 911s with the Continuous Injection System (CIS). In the event of a backfire, which is not uncommon with these cars, the force can crack the airbox. A cracked airbox will prevent the engine from starting, as it creates a massive intake vacuum leak. The airbox is expensive, and takes several hours of labor to install. Replacement of the airbox at a Porsche dealer can easily top $1,000. The pop-off valve allows backfire pressure to escape harmlessly through the air intake. This easy modification will protect your 911 airbox from costly damage.

     The pop-off valve kit includes the valve and a drilling template. You will also need a two-inch hole saw, a right-angle drill, and special epoxy adhesive. In choosing the hole saw, one with a long pilot drill, or one that is adjustable if preferable. If the pilot drill is too short, the hole saw teeth will contact the ribs in the air box floor first making drilling more difficult.

     To begin installation, remove the air filter. Carefully position the template in the bottom of the airbox, and drill the hole. (Figure 1) Note that the arrow on the template should be centered on the screw, regardless of whether the template touches the left wall of the airbox. (Figure 2) You may wish to mark the center of the hole to be drilled (Figure 3), and drill a smaller pilot hole to prevent the hole saw from moving around. (Figure 4) Make sure you hold the drill bit vertical while drilling the hole. (Figure 5)

     Vacuum the plastic chips from the airbox, and from the hole as best you can. Any tiny chips left will be drawn into the cylinders and burned. (Figure 6)

     Test fit the valve into the hole. (Figure 7) It may be too large initially. If so, lightly sand the valve to reduce its circumference until it is a snug fit, and can be pushed all the way down, flush against the ribs in the bottom of the airbox. Once you are satisfied with the fit, take the valve back out, and proceed to glue it in place.

     Ensure that the area around the hole in the airbox in clean, and free from any oil or gas. Mix the epoxy well, and apply it to the lower part of the circumference of the valve, where it will seat in the airbox. (Figure 8) Apply a thin coat of epoxy to the inside edge of the hole in the airbox as well. (Figure 9) Install the valve into the airbox, rotating it slightly to ensure that the epoxy is spread completely around the valve. Note that the hinge of the valve must be oriented on the side nearest and parallel to the rear bumper. (Figure 7again) Put a tiny dab of epoxy on each end of the valve hinge pin to ensure it does not come loose.

     Let the epoxy cure for at least 24 hours. Make sure that the valve can be opened, the O-ring is seated, and that the spring holds the valve closed properly. Reinstall the air filter, and you are ready for driving your car without worry of backfire damage.

     When you do the routine maintenance on your car, it is a good idea to inspect the O-ring in the pop-valve to ensure that it is in good condition.

A random customer added:

A tip, for your tip catalogue? I was having hot start problems , and I found that my popoff valve "o" ring was not seated well in the slot (inside air box) and was allowing a vacume leak, hence , no start, but would start well cold with the help of the cold start valve, kinda wierd , but thought I'd pass it on anyway

Figure 1

Pop-Off Valve Location

Figure 2

Setting Template

Figure 3

Marking Center of Hole

Figure 4

Drilling Hole

Figure 5

Drilling Hole

Figure 6

Pop-off Valve Hole Drilled

Figure 7

Test Fitting Pop-Off Valve

Figure 8

Applying Epoxy to Valve

Figure 9

Applying Epoxy to Airbox

Comments and Suggestions:
Neil Comments: I've sussed it finally - the operation of the valve and why the direction confuses some people! It's not been explained very well.

You are actually drilling down into a lower chamber in the airbox - into where the cold start valve is and the inlet tracks to each cylinder. It is in this lower chamber where a massive positive pressure will occur on back-fire. This massive positive pressure will blow the lower part of the air box apart! The big positive pressure therefore occurs "below" the valve on it's underside. It pops upwards to to let the pressure into the upper chamber where the airfilter it.
This is effect short circuits any backfire pressure having to pass the long way backwards passed the throttle butterfly and then the metering unit's disk to get to the same chamber and escape out to atmosphere!

My newly acquired 1981 SC doesn't have one and I'm learning still. Definitely need to buy one of these as not fitted!
November 6, 2017
  Followup from the Pelican Staff: Thanks for the follow up and sharing your experience
- Nick at Pelican Parts
Nod Comments: This was the best thread to come across. I had a pretty good backfire on start one day. And looked to see the pop off valve had blown right off. Due to it sitting on top of the fins.
I did as the thread suggests and used a dremmel on them to make the pop off valve sit flush. Worked a treat and started first time.

Thanks again for your help!
November 12, 2016
  Followup from the Pelican Staff: Thanks for the additional info and feedback. We appreciate it.
- Nick at Pelican Parts
Joeybear Comments: 1983 911 SC The car ran fine before I installed the pop off valve. Now, it starts fine, but has a rough idle, it seems to miss while driving. When you let off the gas it pops and bangs like a really flow through exhaust through the exhaust. Are these problems caused by a vacuum leak at the pop off valve, or do I have an air box that maybe has a crack. My 911 has 36,000 original miles. I know, I need to drive it more, and since I am now retired I will. Your help is appreciated. Joe
October 18, 2016
  Followup from the Pelican Staff: You have a vacuum leak where the valve goes in the plenum. Try resealing it. - Nick at Pelican Parts  
sponge Comments: I can only load one picture at a time, so here is the second photo with the pop-off valve in place.

Also, as Lois S says below on March 3, 2010: from 1980 thru 1983 there is a metal manifold under the airbox. You have to be very careful so as not to accidentally drill too deep or you will compromise the metal plate.
September 3, 2016
  Followup from the Pelican Staff: Thanks again. - Nick at Pelican Parts  
sponge Comments: Just finished the airbox pop-off valve installation. The included instructions and comparing to pictures on this website, other websites, and in the book "101 Projects for Your Porsche 911" describe installing the pop-off valve on top of the airbox fins. This just didn't make sense to me. So, I installed the pop-off valve flush to the airbag floor by trimming the plastic fins of the airbox with a Dremel. Placing the pop-off valve flush just seemed "right". I was able to secure the valve with a bead of JB Weld. Trimming the plastic fins allows for a solid contact and very "O.E.M-appearing" installation. The pop-off valve flips open and closed as designed. The engine starts and runs as before.

I am posting this because I don't see that anyone else has mentioned this.
September 3, 2016
  Followup from the Pelican Staff: Thanks for sharing your installation process and experience. These type of comments add so much to the Pelican tech community.
- Nick at Pelican Parts
Bill Comments: I had a backfire incident that blew off the pop off valve. I replaced it using high temp RTV gasket sealer the red stuff. However, I installed it hinge to the right I have a K&N air filter housing. Is this direction a problem?
June 6, 2016
  Followup from the Pelican Staff: No, it should be OK. - Nick at Pelican Parts  
ed Comments: so I just got a car 79 that on the airbox a large air filter vs a hard in theory I think the air could go back thru the wouldn't need the pop off valve or would I?
June 12, 2014
  Followup from the Pelican Staff: If you have no lid, the valve is not not needed. - Nick at Pelican Parts  
Spotcab1 Comments: I must be missing something! When the engine misfires it creates pressure in the air box, hence breaking it, but the relief valve relieves in the wrong direction to allow the pressure to escape, if I am reading the direction's properly! My 81 911SC also has a rubber plug on the right side of the air box. What is that there for? Thanks :O
February 13, 2014
  Followup from the Pelican Staff: The valve relieves the pressure to prevent damage to components.

Can you share a photo of the rubber plug? - Nick at Pelican Parts
John V. Comments: I recently purchased an 82' sc with 113,000 miles on it and the pop off valve has never been installed. Should I really be concerned with installing the pop off valve? I don't want to drill through the box and possibly hit the manifold like "Louis S" did.
February 12, 2014
  Followup from the Pelican Staff: It is a good idea to install it, but you don't have to. Drive it for a while and see if you feel the vehicle requires the valve. - Nick at Pelican Parts  
April 24, 2013
  Followup from the Pelican Staff: Thanks for the input on this one. We appreciate the help. - Nick at Pelican Parts  
Pete Comments: I have a small leak in my pop off valve, 0-ring seal seems the problem. The valve is also installed in the wrong direction. I want to replace the valve. Does anyone know how I can remove the old valve safely? Thanks
December 7, 2012
  Followup from the Pelican Staff: If it is epoxied on, you will have to cut away the old epoxy. If the housing is damaged during this process, you will have to replace it. - Nick at Pelican Parts  
MLB Comments: My pop-off valave rarely gets stuck and the fix is pretty sinple and quick.
October 4, 2012
  Followup from the Pelican Staff: Thanks for the additional info. We appreciate it.
- Nick at Pelican Parts
valued_user Comments: If there's any air leak at the new pop off valve, the car may not start. There's a key bit of advice here that is very important for success.
"Test fit the valve into the hole. It may be too large initially. If so, lightly sand the valve to reduce its circumference until it is a snug fit, and can be pushed all the way down, flush against the ribs in the bottom of the airbox."
The part ABSOLUTELY WILL BE LARGER than the 2" hole called for in the instructions that come with the part. Those instructions say to enlarge the hole with a die grinder or similar. DON'T MAKE THE HOLE LARGER. It's impossible to make the hole larger and keep it round with the airbox in the car. You'll end up with gaps that the adhesive cannot fill.
By patiently sanding to reduce the diameter of the new part, you can have a tight, concentric fit.
Once the part is installed and adhesive cured, leave the airfilter and housing off, start the car and listen/feel for leaks around the pop off valve. At idle, any air leak will be obvious. Add adhesive and patiently wait for it to cure before re-testing.
My pop off valve came without adhesive. I used JB Weld, bought at local hardware store.
May 31, 2012
  Followup from the Pelican Staff: Thanks for the input on this one, great stuff. We appreciate the help.
- Nick at Pelican Parts
Chris Comments: Just replaced the air box and pop off valve on my 77 911s, the car was running perfectly well, but the air box had previous repairs from before I purchased the car seven years ago, and thought it was time to replace before it failed. Since the new box has been fitted, the car back fires every few seconds, I have since removed the box, to see if anything was not fitted correctly, but all seemed good. Any ideas what I could have done - could a vacuum leak cause a backfire?
February 17, 2012
  Followup from the Pelican Staff: It would have to be a large vacuum leak. Can you seal off the pop-off valve and see if the problem goes away? - Nick at Pelican Parts  
Tim G Comments: I must be seeing this the wrong way. If pressure builds up in the air box after a back fire, how could the valve open? It seems to be installed backwards.
December 22, 2011
  Followup from the Pelican Staff: The top where the valve is in the air cleaner. A backfire is just that - an explosion that goes "back" through the system. So, it would backfire up through the box. The valve is indeed shown properly installed in the photos. Hope this helps. - Wayne at Pelican Parts  
Alan Comments: Quick note - make sure you install your valve facing the correct direction see the instructions and make sure to seal it completely. We repaired a 911 last summer that had a valve installed backwards - the valve would pop open and then catch on the air filter. The car would go super-lean and die. In order to get the car fired again you had to open the airbox so the valve would spring closed!

Additionally, the valve was barely sealed in, with huge vacuum leaks around it - meaning the car was running leaner than it should be. This valve had been installed by a Porsche dealership near here years ago and the car apparently never ran right since. It's run properly since the repair was redone properly thanks to Pelican providing us with a new valve.
December 6, 2011
  Followup from the Pelican Staff: Thanks for the input on this one. We appreciate the help.
- Nick at Pelican Parts
2porscheguy Comments: My 1980 911SC Targa has an aftermarket BAE-Rajay turbocharger kit installed but has the standard airbox. Do I still need to install a pop-off valve to protect the airbox?
June 8, 2011
  Followup from the Pelican Staff: I would assume your aftermarket turbo-charge has a blow off valve installed already. - Nick at Pelican Parts  
Austin911SC Comments: A series of violent backfire events have damaged my pop-off in the area of the hinge-pin. Any ideas for breaking the damaged one loose so that a fresh one can be re-installed?
Naturally, I don't want to remoe the airbox from the car.
November 15, 2010
  Followup from the Pelican Staff: I would gently cut the old one out with a saw by breaking it into a few pieces. You certainly don't want to accidentally crack your airbox by using too much force trying to remove the valve. - Wayne at Pelican Parts  
June 17, 2010
  Followup from the Pelican Staff: Thanks for the feedback. Glad we could help.
- Nick at Pelican Parts

June 15, 2010
  Followup from the Pelican Staff: It usually happens when there is a backfire in the engine. A backfire can be caused by a whole host of things, including a bad distributor cap, to putting on the ignition wires incorrectly. - Wayne at Pelican Parts  
Charlie V. Comments: Question: The pictures show the airbox base removed from the car to install the backfire valve, but the directions imply that the work is/ can be performed on the car. Please clarify. Thank you
May 10, 2010
  Followup from the Pelican Staff: At the top of the article, we have a note written in somewhat smaller type: [The pictures in this article are of an airbox that was removed from the engine for clarity in illustration. It is not necessary, nor recommended, to remove the airbox for this procedure.] - Wayne at Pelican Parts  
don gilbert Comments: I have put a lot of these in, and have had better results from using urathane windshield sealer instead of epoxy. Ive had a few come loose with epoxy, even with extra attenion paid to prep.
April 20, 2010
  Followup from the Pelican Staff: Thanks for the input on this one. We appreciate the help.
- Nick at Pelican Parts
Louis S Comments: These instuctions were perfect except that no one addressed the fact that the air box on the '80 through '83 911SC is modified from the earlier years. In '80 an internal metal manifold was added to the air box. This manifold routed the fuel mist from the cold start valve to the inputs of each air runner to better distribute this mist to each individual cylinder. Prior to this, the cold start valve sprayed fuel into the open intake manifold part of the airbox. When using the 2" hole saw, care must be taken or the centering bit will penetrate the metal manifold. The manifold resides only about 3/8" below the underside of the air box.

Instructions for this procedure were consulted on this web site, in the "101 projects for your 911" book, in Up-Fixin der Porsche vol IX, and the instructions from the manufacturer, Weltmeister. No one mentioned this problem. Not sure how this will affect engine start, but hopefully won't be significant.
March 3, 2010
  Followup from the Pelican Staff: Thanks for the additional info. We appreciate it.
Let us know how it works out. - Nick at Pelican Parts
kevin v Comments: What is the best epoxy to use brand name to fix the pop off valve in position
April 27, 2009
  Followup from the Pelican Staff: I think a standard hardware store extra strength epoxy would do the trick. I've used that before with excellent results. Loctite is a great brand, I've also used Devcon with good success too. - Wayne at Pelican Parts  

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Page last updated: Fri 1/19/2018 02:19:33 AM