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1974 911 w/ 83 SC engine
Join Date: Aug 2009
Location: Austin, Texas
Posts: 602
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I popped off my pop-off valve!
Started my car today on one of the coldest mornings in Austin in a long time. Drove to the gas station and filled up, started the car back up..and within 30 seconds the car backfired twice and died. I pushed it through 2 stop lights and got it to a parking lot. I opened up the airbox and the saw the pop-off valve had been blown out of its seating. So many question is...what caused such a massive backfire as to blow the pop-off valve off of the airbox, and what is the recommended adhesive to re-attach the PO valve to the airbox? Thanks!
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10-27-2012, 05:18 AM
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I hope you were able to push the pop off back in it's hole and that the car started ? Others may chime in but I have been told that 2 part epoxy after cleaning the area works well.
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15 year PCA member  1972 911 E Coupe - gone now 1987 Mazda RX-7 2+2 - still cooking Swift DB-1 FF & Swift DB-3 F2000 -not forgotten 1979 911SC 3.0 & 2000 Camaro |
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10-27-2012, 05:42 AM
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1974 911 w/ 83 SC engine
Join Date: Aug 2009
Location: Austin, Texas
Posts: 602
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Quote:
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10-27-2012, 05:44 AM
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ROW '78 911 Targa
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JB weld will work better than most clear epoxies.
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10-27-2012, 07:59 AM
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Registered
Join Date: Aug 2005
Location: Maryland
Posts: 122
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I had the same thing happen on my 73.5. I never did figure out what caused the explosion that would blow it out of the box. I re-glued it, wrote it off to a poor install the first time and never had the problem again. Ken
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10-27-2012, 09:45 AM
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In any event, it saved your airbox. Now just epoxy it back in. Check the mechanism to make sure it opens and your golden.
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jtp911 |
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10-27-2012, 11:17 AM
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Functionista
Join Date: Jul 2011
Location: CO
Posts: 7,717
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Another measure I like to take is to put a thin layer of grease on rubber boot that goes from throttle body to CIS airflow meter. Lightly snug clamps. Hopefully a giant backfire will lift off one end of boot if pop-off valve is not enough relief
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10-27-2012, 12:12 PM
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1974 911 w/ 83 SC engine
Join Date: Aug 2009
Location: Austin, Texas
Posts: 602
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I installed it back in, the epoxy is still setting but it looks to be fine. So my last question--I've never heard my car backfire like this before. It was colder this morning than is has been in a long time, but I'm not sure how that would affect it. Any thoughts as to why it backfired and if it's something I can prevent in the future?
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10-27-2012, 12:28 PM
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Functionista
Join Date: Jul 2011
Location: CO
Posts: 7,717
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Regarding your CIS injection, what components are from 74 and 83?
Fuel pressures and fuel mixture would be the first place I'd look. |
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10-27-2012, 12:37 PM
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A few weeks ago my SC backfired and with no pop-off it blew the airbox.
 So I've been doing a lot of reading on this issue and one thing the experts (and I am definitely NOT one of them) recommend after a backfire incident like yours is to check the air metering plate as it may have been "knocked" or "bent" out of place. Failure to do so may result in further backfires due to incorrect air/fuel mixture. Glad you don't have to replace your airbox like me!
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10-27-2012, 02:36 PM
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Functionista
Join Date: Jul 2011
Location: CO
Posts: 7,717
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^^ Excellent advice.
I'd forgotten to mention checking for elusive vacuum leaks as well. |
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10-27-2012, 03:33 PM
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ROW '78 911 Targa
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It's that time of year where low temperatures and out of adjustment CIS fuel pressures, ignition components and or small vacuum leaks will begin to show up.
Time to get everything in spec again after a spring and summer of fun! 
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Dennis Euro 1978 SC Targa, SSI's, Dansk 2/1, PMO ITBs, Electric A/C Need a New Wiring Harness? PM or e-mail me. Search for "harnesses" in the classifieds. |
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10-27-2012, 07:28 PM
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When fitting the pop off valve get some 80 grit or similar just to take the shine off the glued area.
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10-27-2012, 11:47 PM
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Registered
Join Date: May 2002
Location: Portland, Oregon
Posts: 12,651
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I had this happen to me and I reglued my pop off in using silicone glue. I figured it will seal and if the back fire is massive enough, the glue would fail prior to the box failing. Hopefully the remnants of the glue will be pliable enough to allow me to reinsert the valve and drive off.
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Harry 1970 VW Sunroof Bus - "The Magic Bus" 1971 Jaguar XKE 2+2 V12 Coupe - {insert name here} 1973.5 911T Targa - "Smokey" 2020 MB E350 4Matic |
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10-27-2012, 11:58 PM
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grateful user
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black urathane people, it will never come out again .
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fully disassembled, blasted, customized and restored 75 targa with factory hard top, 993 style turbo ft fenders, steel flares, C2 bumpers and rockers, 82 3.0 sc 9.5/1 engine with PMS flywheel, 964 cams, flowed heads, ssi's short geared 915 w/lsd, polybronze, bilstein,working lambda, modified and highly tuned cis, tensioners, pop valve, backdated exhaust and heater, 2300 lbs. no bolt left untouched. 1970 911E. Nice car but needs a re-do. |
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10-28-2012, 06:22 AM
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Registered
Join Date: May 2002
Location: Portland, Oregon
Posts: 12,651
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Quote:
To my way of thinking, this is not good, I want the valve to fail prior to the box failing.
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Harry 1970 VW Sunroof Bus - "The Magic Bus" 1971 Jaguar XKE 2+2 V12 Coupe - {insert name here} 1973.5 911T Targa - "Smokey" 2020 MB E350 4Matic |
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10-28-2012, 09:05 AM
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Chain fence eating turbo
Join Date: Dec 2008
Location: Austin, TX
Posts: 9,126
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Yellow and brittle sounds like Super Weatherstripping adhesive.
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10-28-2012, 09:29 AM
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grateful user
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If the valve is working, the box will not fail.
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fully disassembled, blasted, customized and restored 75 targa with factory hard top, 993 style turbo ft fenders, steel flares, C2 bumpers and rockers, 82 3.0 sc 9.5/1 engine with PMS flywheel, 964 cams, flowed heads, ssi's short geared 915 w/lsd, polybronze, bilstein,working lambda, modified and highly tuned cis, tensioners, pop valve, backdated exhaust and heater, 2300 lbs. no bolt left untouched. 1970 911E. Nice car but needs a re-do. |
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10-28-2012, 04:57 PM
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Molly
Heinz
911 Cabriolet
Grüne Hölle
75 Porsche 911
bike
1970 porsche E
