To Pop Off or Not to Pop Off
07-29-2005, 12:06 AM
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To Pop Off or Not to Pop Off
I am about to become the proud owner of an '82 911 SC. The car does not currently have a pop off valve and I am considering installing one. BUT, in Peter Zimmerman's The Used 911 Story, on p.125 regarding airbox pop off valves he says, among other things:
"...With the improvements made to the airboxes by Porsche, there is absolutely no need for an add-on device such as this."
The section talks about how pop off valves have been the cause of a number of "emergencies" at his shop.
Everything I've read elsewhere says install one or you'll wind up having a backfire blow up your airbox.
Also, do you have to drop the engine to replace the airbox on an '82 SC?
Thanks in advance for your help.
"...With the improvements made to the airboxes by Porsche, there is absolutely no need for an add-on device such as this."
The section talks about how pop off valves have been the cause of a number of "emergencies" at his shop.
Everything I've read elsewhere says install one or you'll wind up having a backfire blow up your airbox.
Also, do you have to drop the engine to replace the airbox on an '82 SC?
Thanks in advance for your help.
07-29-2005, 12:43 AM
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There is an article on tech.rennlist.com on how to replace a blown airbox using a partial engine drop. While I had one of the later airbox's, it still blew up, so, personal experience guides my recommendation for pop-off valves on CIS-based engines. You do want to put a dab of epoxy on the hinge ends to ensure the rod doesn't work it's way out. Use a good high-heat resistant epoxy on installation, and coat the rubber gasket with silicone grease (not silicone sealer).
07-29-2005, 01:24 AM
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Me personally, I'd use the popoff valve even on an 82, install it right, check it every now and then, and you shouldn't have a problem.
Many who have owned the 80-83s SCs will tell you - the airbox can still blow up.
Many who have owned the 80-83s SCs will tell you - the airbox can still blow up.
07-29-2005, 12:58 PM
#4
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I have mentioned in the past that pop-off valves can be a bandaid for a car that is not cared for in a technically correct manner (regular CO% checks, proper cold control pressure maintenance, visual inspection for air leaks at the injectors, adjustment of the hand throttle on pre-'76 cars, etc.), especially those cars that live far away from a professional service facility. I'm not going to beat a dead horse here, but I have seen a large number of failed (blown) airboxes towed into our facility, that WERE fitted with pop-off valves. My '82 airbox lasted 192K miles, with NO pop-off valve (it did not blow, I found a crack during a routine service), and its replacement does NOT have a pop-off valve. I have owned 5 SCs over the years, none with pop-off valves, and have never gotten even a single cold start issue that might lead to a blown airbox. I use nothing but the best gasoline (Chevron Premium here in CA), and perform all maintenance to factory recommendations. My cars "hunt" for a few seconds on cold start (because the cold control pressure is set toward rich running) and the cars exhibit no starting/running problems.
Our shop has never had a car towed in, that we maintained for at least three years, with a blown airbox. Sure, there were times when we replaced airboxes; (1) while the engine was out for synchro repairs, clutch work, or head stud replacement, or (2) while performing a routine major (15K) maintenance we found the beginning of cracks in the airbox, and sold a replacement to our client. Those cracks are caused by tiny explosions inside the central (plenum) chamber of the airbox. A pop-off valve cannot relieve all the stress from those explosions, therefore the same cracks occur, with or without the valve. On '81-83 cars still fitted with the original box those tiny explosions are all but eliminated with the factory's addition of a fuel diffuser. It's a simple series of metal tubes that extend from the cold start injector into the 6 intake runners, which prevents atomized fuel from gathering inside the box itself. The diffuser has been added to all replacement airboxes, except for the '73.5 T, and if in doubt regarding what airbox you have check its perimeter screws. Boxes with diffusers are assembled with Phillips-head screws, early boxes have slot-head screws. The exception are a handful of the first diffuser cars, which were also put together with slot-head screws.
Some airbox failures occur because of the use of valet service (where the valet uses the foot throttle to "aid" in starting the car), and cars that have more than one driver, often with lack of training or understanding that the gas pedal must NEVER be touched during starting. I have seen SCs driving around with a not-working frequency valve, I've seen CIS cars with the ignition timing off by 10 or more degrees, I've seen fuel mixture so lean the needle wouldn't move on our machine, I've seen cold control pressure so far outside acceptable range that the car would lean-pop on cold start. A pop-off valve might serve a purpose on cars like those, but only as a bandaid. It won't prevent a blow for long, sooner or later the box will fail unless the car's root problem is corrected first.
For me, the problems that a pop-off valve bring to the table (lid stuck open, lean conditions, epoxy failure, etc.) outweigh any of its possible positives, therefore I can't recommend them, and I've never used or sold them. And Porsche never incorporated one into replacement airboxes, which I've always felt leant credibility to my point of view.
About replacement, the airbox can be replaced on hand throttle cars ('73.5-75) without engine removal. Complete removal, due to the organized replacement process required (not to mention cleanliness) of the engine on '76-83 models is highly recommended. As time has passed removal has become even more important because the cars are all more than 20 years old, and additional parts (oil pressure switch, breather hoses, thermostat o-ring, etc.) should all be attended to. Also, new injector seals/sleeves must be installed, along with the 6 sleeves that connect the intake runners to the airbox. Setting the CO% following replacement is mandatory. On pre-'76 cars hand throttle adjustment is also mandatory, because that might have been the reason that airbox blew! Speaking of hand throttle cars, cold and warm starts are not as carefully regulated as in the non-hand throttle cars, so maybe a pop-off valve might help, but I still wouldn't put one on my car.
Pete
Our shop has never had a car towed in, that we maintained for at least three years, with a blown airbox. Sure, there were times when we replaced airboxes; (1) while the engine was out for synchro repairs, clutch work, or head stud replacement, or (2) while performing a routine major (15K) maintenance we found the beginning of cracks in the airbox, and sold a replacement to our client. Those cracks are caused by tiny explosions inside the central (plenum) chamber of the airbox. A pop-off valve cannot relieve all the stress from those explosions, therefore the same cracks occur, with or without the valve. On '81-83 cars still fitted with the original box those tiny explosions are all but eliminated with the factory's addition of a fuel diffuser. It's a simple series of metal tubes that extend from the cold start injector into the 6 intake runners, which prevents atomized fuel from gathering inside the box itself. The diffuser has been added to all replacement airboxes, except for the '73.5 T, and if in doubt regarding what airbox you have check its perimeter screws. Boxes with diffusers are assembled with Phillips-head screws, early boxes have slot-head screws. The exception are a handful of the first diffuser cars, which were also put together with slot-head screws.
Some airbox failures occur because of the use of valet service (where the valet uses the foot throttle to "aid" in starting the car), and cars that have more than one driver, often with lack of training or understanding that the gas pedal must NEVER be touched during starting. I have seen SCs driving around with a not-working frequency valve, I've seen CIS cars with the ignition timing off by 10 or more degrees, I've seen fuel mixture so lean the needle wouldn't move on our machine, I've seen cold control pressure so far outside acceptable range that the car would lean-pop on cold start. A pop-off valve might serve a purpose on cars like those, but only as a bandaid. It won't prevent a blow for long, sooner or later the box will fail unless the car's root problem is corrected first.
For me, the problems that a pop-off valve bring to the table (lid stuck open, lean conditions, epoxy failure, etc.) outweigh any of its possible positives, therefore I can't recommend them, and I've never used or sold them. And Porsche never incorporated one into replacement airboxes, which I've always felt leant credibility to my point of view.
About replacement, the airbox can be replaced on hand throttle cars ('73.5-75) without engine removal. Complete removal, due to the organized replacement process required (not to mention cleanliness) of the engine on '76-83 models is highly recommended. As time has passed removal has become even more important because the cars are all more than 20 years old, and additional parts (oil pressure switch, breather hoses, thermostat o-ring, etc.) should all be attended to. Also, new injector seals/sleeves must be installed, along with the 6 sleeves that connect the intake runners to the airbox. Setting the CO% following replacement is mandatory. On pre-'76 cars hand throttle adjustment is also mandatory, because that might have been the reason that airbox blew! Speaking of hand throttle cars, cold and warm starts are not as carefully regulated as in the non-hand throttle cars, so maybe a pop-off valve might help, but I still wouldn't put one on my car.
Pete
07-29-2005, 01:31 PM
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Failed pop-off valve story. Just about a month ago, as I left home to go to the Porsche Parade in Hershey, PA, my '81 911SC died. First time the car has let me down. I checked everything, including the pop-off valve, but couldn't get it started. I had the car flat-bedded to a very knowledgeable independent, and missed out on the Porsche Parade. Turns out, the epoxy had failed. The valve looked fine to me when I checked it. But it wasn't. After the pop-off was re-sealed, everything was just fine. I'm told the epoxy that is supplied with the pop-off valve isn't the best. Has to be checked and resealed about every two years or so.
Whether the car back fired or not, I don't know. I do know that the pop-off valve blew my chance to see the concours at the Parade.
Whether the car back fired or not, I don't know. I do know that the pop-off valve blew my chance to see the concours at the Parade.
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randomguy (07-31-2023)
07-30-2005, 10:04 AM
#6
Burning Brakes
I had a pop-off valve fitted by a reputable non-franchise specialist and after a week the car failed to start - the pop-off valve had popped out! I re-fitted it myself after roughing up the valve's mating surface - it's just way too smooth.
Been fitted for 5 years now with no problems.
Pete - you must be blessed, I've tried everything to stop my 2.7 from popping on start up, tuning, parts replacement etc. and she still does it occasionally.
PJC
Been fitted for 5 years now with no problems.
Pete - you must be blessed, I've tried everything to stop my 2.7 from popping on start up, tuning, parts replacement etc. and she still does it occasionally.
PJC
08-01-2005, 12:39 PM
#7
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PJC: Greetings! I would place a bet that one of three faults hide under your engine lid. (1) Cold control pressure incorrect, or (2) System pressure is incorrect, or (3) Cold start injector not spraying properly. The pressure issues can actually be tested with the use of a proper pressure gauge, and the procedures are outlined in section 2 of the factory manual. Sometimes it's easier to replace the warm-up (aka control pressure) regulator, but without testing you just never know what you've got. Although regulators "can't be adjusted", they actually can.
The injector issue is a bear to do. I have never seen a cold start injector fail, but anything is possible. It might be interesting to attempt to test yours, or replace it to see what happens.
Contrary to popular belief tuning will not correct your cold start "pop". The warm up regulator creates pressure at the top of the fuel distributor, so regardless of your CO% setting, that pressure will be "controlled." Of course there is a tiny chance that something else could be wrong with your car, but I would not bet on that!
FYI: I would test cold control pressure first, based on your symptom. It's a bit of an inconvenience because the car should be parked for several hours, then the pressure gauge should be installed and bled, then the car should be left to sit overnight. The pressure test and any follow up action may be done the following morning. It wouldn't be a surprise to find that your cold pressure is too high.
Best, Pete
The injector issue is a bear to do. I have never seen a cold start injector fail, but anything is possible. It might be interesting to attempt to test yours, or replace it to see what happens.
Contrary to popular belief tuning will not correct your cold start "pop". The warm up regulator creates pressure at the top of the fuel distributor, so regardless of your CO% setting, that pressure will be "controlled." Of course there is a tiny chance that something else could be wrong with your car, but I would not bet on that!
FYI: I would test cold control pressure first, based on your symptom. It's a bit of an inconvenience because the car should be parked for several hours, then the pressure gauge should be installed and bled, then the car should be left to sit overnight. The pressure test and any follow up action may be done the following morning. It wouldn't be a surprise to find that your cold pressure is too high.
Best, Pete
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08-01-2005, 09:56 PM
#8
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I had the large pop off valve and my 73.5 box still cracked. That said, when I felt the original box in my hands compared to the factory replacement (which was VERY VERY hard to find), the brittleness of the old one was sooo obvious. I don't think a pop off valve the size of a shoe would have saved it. Also, I too had the pop off valve in the new one come lose and make it impossible to drive the car. But, it's been fine for many years since that was sourced and fixed (from the above mentioned slippery sides that the glue did not stick to).
That said, I did have some popping problems after I got a batch of bad gas. I don't know if the pop off valve saved me from that unexpected problem.
That said, I did have some popping problems after I got a batch of bad gas. I don't know if the pop off valve saved me from that unexpected problem.
08-02-2005, 04:32 AM
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hello,
2 points of no need pop off:
1. to have a good engine settings (CO & head start)
2. no accelerate during the start engine ( the mix air/essence is more rich at cold due to the seventh injector)
the air box blow because there is too much gasoline in... fire return
A+
hope my english is understandable....
2 points of no need pop off:
1. to have a good engine settings (CO & head start)
2. no accelerate during the start engine ( the mix air/essence is more rich at cold due to the seventh injector)
the air box blow because there is too much gasoline in... fire return
A+
hope my english is understandable....
