83 SC - to add pop off or not to add pop off valve

[mhackney]

I need to replace the air box in my 83 Cabriolet due to old age fatigue. The original air box did not have the pop off valve and in the 8 years I've owned the car, it has never back fired. It really looks like the adhesive that seals the upper box to the lower box has failed due to old age/fatigue. I'll be able to take a closer look and check for any tell-tale backfire signs when I take the air box out. I've read arguments both ways about the benefits - or lack there of - of these pop off valves. I've found several comments from trusted Pelicans that late model SCs like mine don't really need one and that they can introduce issues.

I just wanted to sanity check this before doing anything rash. The car has 160K and I drive it about 15K a year now that it is well sorted.

[theiceman]

my 78 had to pop of valve when I bought it , 5 years in I replaced the airbox and did not put a pop off valve in the new one . been 8 years and absolutely no starting or running issues in that time .
ymmv.

[Flat6pac]

It only takes one backfire to blow it, pop off valve is like an insurance policy, glad you have it when you need it.
Bruce

[stlrj]

I'm not a pop off fan either. They don't need it and why create problems that do not exist?

[GothingNC]

I recently had an issue with contaminated fuel then a bad O2 sensor Relay.

The pop off valve helped prevent the air-box from being blown to bits.

[mhackney]

Although there is a lot posted on the subject over the last decade, this particular post provides some insight as to why a pop-off is not needed for the later CIS cars: Blown airbox - replace, prevent and while I'm in there

or cars with a replacement air box that has the spider assembly.

[Jdub]

Correctly installed the item is totally benign. I would not run my '78 without one.

[mhackney]

I've also searched for a definitive installation procedure and come up short so far. Not an exhaustive search though so I might still find something.

[RSBob]

Originally Posted by Flat6pac

It only takes one backfire to blow it, pop off valve is like an insurance policy, glad you have it when you need it.
Bruce

+1. You wouldn't drive your car without insurance or without a seatbelt. The arguement, if it ain't broke don't fix it is pure BS. In life, the more variables you control, the fewer surprises.

[theiceman]

Originally Posted by RSBob

+1. You wouldn't drive your car without insurance or without a seatbelt. The arguement, if it ain't broke don't fix it is pure BS. In life, the more variables you control, the fewer surprises.

well I hope you are not a surgeon

" you are perfectly healthy but we are going to cut into you and take your appendix out so I can control that "

[mhackney]

Thanks folks, I didn't intend for this to get into a us vs them match. I just want to gather information so I can make an informed decision!

[boyt911sc]

Michael,

POV (pop off valve) is an after market device and not OEM. This is probably the most widely used non-OEM components in CIS engines. It has its weakness too. Over a period of time, it leaks and would need preventive repair. So why would anyone use it? No matter how well you maintain your CIS, your engine will backfire a few times over the years of ownership.

I feel pretty confident about my knowledge in CIS troubleshooting and will not allow a brand new air box installed in my cars or cars I am working without a pop off valve. The spider plumbing that was introduced in SC's have reduced the occurrence of unwanted engine backfire but not totally eliminated it.

A year ago, I was checking the fuel pressures of my '78 SC and did it without running the engine. Just the removal of the FP relay and connecting 87a & 30 terminals. The pressure readings look good and removed the pressure gauge. Tighten the fuel line fittings and put the tools back. Checked the gear in neutral and turned the ignition switch ON. The backfire was so loud and strong that it blew the rubber straps loose, air filter and snorkel lying on the garage floor.

What caused the backfire? A very lean mixture condition because the FP relay was inadvertently not installed. I was very surprised that the air box survived the explosion of that intensity. If I had no POV, that air box had no chance of avoiding breaking or cracking.

And if you do a lot of engine start ups after a rebuild for CIS engines, you would be familiar to the sound of backfires. You might not experience any backfire now but it is only a question when will it happen. And when it does, you better be prepared for it. Good luck.

Tony

[Reiver]

CIS, even when properly maintained, can be finniky. Everything has to work properly all the time.
I had a thermo time valve go bad (enriches a cold engine)...and even tho my AFR is set a bit rich it backfired upon start on a cooler morning. Pop off valve saved my box.
Replaced the thermo time valve and no issue.....this was not due to lack of maint. or negelect...it can just happen due to a worn/tired/no longer functioning part.
I've never had a leaking issue with the pop off but we smoke tested the unit when put in place.

[MARISOL78sc]

+1 on adding it. FAQs on these cars are mostly.....Pop off valve upgrade/tensioner upgrade and head stud replacement........

[911SauCy]

Originally Posted by stlrj

I'm not a pop off fan either. They don't need it and why create problems that do not exist?

This is incorrect and clearly shows you've not experienced a back-fire.

My car back fired once, it sounded like a shot-gun (louder than any other back-fire I'd ever heard) and my pop-off valve "popped" and saved my airbox from exploding.

There's no reason this low dollar/easily installed item should not be used

Some experience a backfire without any damage to the air box, others experience a backfire and have to replace the air box. Some of them had a POV installed others didn't. What I want to say is that a POV is not a 100% insurance - as with any insurance... - but it can help preventing a broken air box after a backfire.

Yesterday, during the top end rebuild of my '78 engine and having the same dilemma for months, at last I decided to install a POV, fixed it with JB Weld, using a dental mirror to check the flow of the glue. Some Molykote 111 on the rubber O-ring should take care of the good seal. Secure installation and a regular check is important because a POV is another weak spot in the CIS chain: if the flap (O-ring) leaks, the engine will not start or run very poor. A POV is a for - say 50% of - the backfires a saver for your air box.

I hope that it will save my air box when I start the engine with a probable backfire after three years in the garage and the partial rebuild.

A well maintained and regular driven car under controlled circumstances will theoretically not backfire. But who drives and garages his CIS-Porsche always conform this ideal situation. Not me!

Otto

[cnwayland]

I've had rough running and back-firing CIS on my 1980 3.0 that has worsened over the past few months. I lit an air filter on fire with a forceful backfire. After reading some of the above posts, I'm beginning to wonder about the integrity of the o-ring on my pop-off valve, as it could be a source of the vacuum leak I've been looking for.

[Vereeken]

I am in between.

I think my PV is in for 6 years. I had to epoxy it twice for small air leaks but only had one relatively minor backfire....

Perhaps if I would install more of them I would get better in the appropriate seating and hole diameter drilling.

cnwayland,

Replace the O-ring and use some Molykote 111 or similar compound. It keeps the O-ring in a smooth condition and closes the vacuum leaks of the flap. Of course you should check it on a regular base. I agree, the POV design is not perfect ...

Otto