1982 911SC Airbox blown after engine out service

[gazzerr]

I replaced mine because it leaked in the classic place along the seal above the 4, 5 & 6 runners. I think after 30 years the adhesive in the airbox just breaks down and they start to leak. There wasn't any real evidence of any previous backfires. The car actually ran fairly well but it was super rich. The PO had jacked up the mixture to the point where the OXS system was not able to compensate. If the leak is big enough the car won't start. I've read about big leaks from the rubber connectors between the air box and runners causing backfires.

[wi54545]

Another question for you guys...


When I initially pulled engine the CIS had two anomalies. The connector on the air sensor plate was disconnected as well as a vacuum line that was capped. Car ran fine before, but the boots were like brittle plastic and the injector sleeves and seals were almost non existent. Why would these things be disconnected? Should I reconnect them or leave them alone?

[OsoMoore]

The connector on the air sensor plate assembly protects the fuel pump from running when the engine isn't sucking in air (ie. running). This is important in the case of an accident - you don't want gas spraying everywhere. I would make sure to put this one back on. Don't accidentally swap it with the cold start injector - they use the same connector and are next door.

My SC is a '79, so I can't help you on the air sensor hose. I had the joy of an EGR (darn California).

[gazzerr]

The other one is the deceleration valve. It holds the revs up a bit longer and bleeds them down slower when you dump the throttle closed after being in it hard. Some guys don't like the way this feels and they prefer the more carb like response so they disconnect and plug the vacuum line that actuates the valve. I actually don't mind the feel it gives the car. If the valve leaks you can get high idle issues.

Like OsoMoore said the sensor plate plug being disconnected is a safety issue. It means your fuel pump will start running when you turn the key on. It should only start running when the engine starts pumping air.

Glad you got the motor out. The CIS refurb will be cake.

[OsoMoore]

Originally Posted by gazzerr

The other one is the deceleration valve. It holds the revs up a bit longer and bleeds them down slower when you dump the throttle closed after being in it hard. Some guys don't like the way this feels and they prefer the more carb like response so they disconnect and plug the vacuum line that actuates the valve. I actually don't mind the feel it gives the car. If the valve leaks you can get high idle issues.

Like OsoMoore said the sensor plate plug being disconnected is a safety issue. It means your fuel pump will start running when you turn the key on. It should only start running when the engine starts pumping air.

Glad you got the motor out. The CIS refurb will be cake.

Ah, my decel valve is in a totally different spot IIRC. I guess they changed a lot when they went to oxygen sensors.

[pmax]

Originally Posted by OsoMoore

The connector on the air sensor plate assembly protects the fuel pump from running when the engine isn't sucking in air (ie. running). This is important in the case of an accident - you don't want gas spraying everywhere. I would make sure to put this one back on. Don't accidentally swap it with the cold start injector - they use the same connector and are next door.

+1

The mechanic who last worked on my 80 left this unconnected on purpose for some reason he didn't tell me when I asked about it. I wasn't too pleased about that.

The other is the decel valve which is not needed really. Better to plug it an eliminate another possible source of air leaks.

[Pazuzu]

I have a cut down BITZ system, which means I can see most of the "vacuum" section of the airbox clearly while the engine is running. It's something you should try to see one day, it's breathtaking how much the airbox internals MOVE when you rev the engine. The whole thing swells and shrinks and pressurizes and sucks in when you sit and rev it. I don't doubt that all of that motion slowly cracks the airboxes over decades.

[gazzerr]

Ha wow that's cool. You just don't appreciate what the components go through! Multiple by 30 years and all those heat cycles and it's amazing it lasts as long as it does really.

OsoMoore - yep the decel valve moved with over to the right with the '80 or '81 maybe?

[Reiver]

Originally Posted by Pazuzu

I have a cut down BITZ system, which means I can see most of the "vacuum" section of the airbox clearly while the engine is running. It's something you should try to see one day, it's breathtaking how much the airbox internals MOVE when you rev the engine. The whole thing swells and shrinks and pressurizes and sucks in when you sit and rev it. I don't doubt that all of that motion slowly cracks the airboxes over decades.

Good point....makes sense too. Hard to believe the old damn things work as well as they do.

[jjeffries]

Mike/Pazuzu: are you still happy with your Bitz system? I have a stock 82 that's working well (touch wood) but I'd likely go that route if (I mean When!) something takes a dump in the CIS.

O.P.: nice job on your rebuild; I bought my car with the engine in boxes, and did similar clean/paint/TLC when building it back up. I only realize now that your fateful error would likely have been mine, too, except for the fact that with my new wires and distributor cap, I used the old parts as my guide to which wires went where. There but for the Grace of Ferry's Ghost would I have gone.

Note connecting my two previous paragraphs: one the Bitz site, there are photos of a lovely custom-fabbed aluminum plenum for the Bitz conversion, better looking than a cut- down airbox. I contacted the fellow who made it but he said that was a long time ago and that he'd tossed or lost the blueprints. Bummer, it looked sweet.

Best, John in CT

[wi54545]

Two engine drops and installs and she is finally back on the road. I was much faster with the drop and install this time. Got my airbox for $230 new from Porsche, I didn't install a pop valve. Tons of suction at the oil cap, and best of all.... No leaks! Counted all the screws from the blown box, don't want any of them finding their way into my engine during disassembly. My virgin valve job was a quiet success. It's great to be driving again!!!

[SCadaddle]

Originally Posted by wi54545

Two engine drops and installs and she is finally back on the road. I was much faster with the drop and install this time. Got my airbox for $230 new from Porsche, I didn't install a pop valve. Tons of suction at the oil cap, and best of all.... No leaks! Counted all the screws from the blown box, don't want any of them finding their way into my engine during disassembly. My virgin valve job was a quiet success. It's great to be driving again!!!

Where did you find a new airbox for $230 as I'd like a spare that I could put a new pop off valve in?

[wi54545]

I bought it from my local Porsche dealer in St. Paul, MN. It was part number 911-110-904-01. Most airboxs are part number 911-110-904-00 which is much more expensive. This one was made in 2014 and has the cold start baffles and 38mm ports. It fit perfectly and car runs excellent.