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Blown airbox - replace, prevent and while I'm in there
When I attempted to start the car on Tuesday, fully packed and ready to go on a three day vacation/roadtrip with my girlfriend, it threw a huge backfire (no backfires at all before this one) that shot out the pop-off valve (leaving a dent in the mesh on the air filter) and cracking the air box
  Well, nothing could be done right then as we had a ferry to catch and we set off in another car. During the last days I've spent some  time reading up on replacing the air box and tomorrow I will attempt to get the whole CIS out without dropping the engine (I currently don't have access to a garage so everything will be done while parked on the street).While it's out I also want to accomplish two things; prevent future backfires and do some while-I'm-in-there-things. Prevention
I checked the fuel pressures a couple of months ago and they were OK but lately I think the fuel pump has started to whirr a bit more (it's only 4 months old) so might be worthwhile to check them again. While I'm in there
Any ideas for more things to add to either list or general advice are more than welcome! I'm in Sweden so I'm off to bed now but will read/respond to any posts when I'm back up again  /b
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1980, 911 SC. Ex-US car in Sweden since June 2010. Insta: @911scowner |
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07-19-2012, 02:50 PM
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When I still had my shop (pre - '99), we used to replace blown airboxes at a fairly consistent rate. One of my long-time mechanics, who was small physically, but very strong, asked if he could replace a Lambda-equipped SC airbox with the engine in the car. I gave him the OK. Keep in mind, my mechanic had done at least a hundred airbox replacements, and knew intimately the size of every nut, the correct placement of every hose clamp, the size of every screw driver, and could take all of the tools needed from his toolbox, put them on a cart next to the subject car, before he started. He managed to get the job done, including installation of the special bracket at cylinder #6, and the job took him between 11 and 12 hours. The car was perfect when it was finished, and he didn't damage the engine bay sound/insulation pad.
The condition, and amount of cuts and bruises, on his arms, caused him to declare something like, "Holy crap, never again." And this was the statement of an expert. That said, your car is fitted with the old style box. Your replacement will have an internal fuel diffuser, which prevents the collection of fuel vapor in the plenum chamber, and sends cold-start fuel directly into the six intake runners. Because of this improvement, you will not need a pop-off valve. Let me know if you need me to print an article that I wrote, a few years ago, about pop-off valves. You will need: 6x intake manifold gaskets 6x intake runner/airbox sleeves 6x injector o-rings 6x injector sleeves 6x injector sleeve o-rings 1x thermostat o-ring 1x oil pressure switch (for the idiot light) 2x throttle lever (bell crank) bushings for the throttle linkage console on top of the engine 2x rubber bushings that secure the airbox brackets to the airbox (at cyl #6 & at the throttle lever console) 1x cold start injector o-ring 1x cold start injector spacer o-ring 1x throttle body o-ring 1x sensor plate housing gasket You might need breather hoses, can't tell until the old box is removed. You might need a new air "dome," the rubber boot that connects the sensor plate housing to the throttle body. When the unit is apart pay close attention to the sensor plate, be sure it's adjusted properly and is not bent. After assembly is complete, CO% adjustment is mandatory, as is checking/setting cold control pressure. Good luck, I certainly don't envy your decision to; (1) work at the street-side, (2) try the job with the engine in place, and (3) work without the rear of the car elevated.
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Keep the Shiny Side UP! Pete Z. Last edited by Peter Zimmermann; 07-19-2012 at 04:49 PM.. |
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07-19-2012, 04:22 PM
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makes you wonder if fuel was leaking in and filling the intake runners to explode that much
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82 SC , 72 914 |
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07-19-2012, 04:44 PM
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'81 924 , '85 944 , '78 911SC , '82 928 5.0L "They run best being run close to the ‘limit’ and done so regularly" - Grady |
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07-19-2012, 05:36 PM
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I managed to remove and replace mine while the engine was in the car. Very time consuming as Peter stated. But it is doable. The list Peter gave you is great and you will have a car that runs better than you ever thought possible. I found that the number 4 intake runner nuts were the hardest. A partial engine drop will help
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07-19-2012, 07:01 PM
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Location: Southeast Wisconsin
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I had the same thing happen.
Sorry to hear about your trouble.
I too had the same thing happen. My guess is a crossfire resulting in a backfire up through the box. At any rate, In looking at the equipment, I decided to go the 3.2 intake with a mega squirt option. Now, keep in mind that mine is a 1978/9 3.0 grey market targa, so originality will probably not maintain the value of something like you see in Excellence... You have to decide what you want to do with your car...,keep it forever, use it until you can move on, run it into the ground, etc... My thoughts, for what they are worth, (maybe 2 cents?), for sticking with CIS are you are not just looking at a new box. The box is just the first part to fail, most likely due to something else, (bad timing, poor spark control, stuck valve, you pick...), the point is, a blown box is a symptom. If you replace the box, you should look at the hoses, injectors, connectors, fuel distributor, regulator, keep going... and replace any component looking worn... Stuff gets old. For me, it was more cost effective to replace the whole intake, and change,(notice I didn't say "upgrade"), to a 3.2 intake with an aftermarket mega squirt system. Good luck with your path.
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'79 SC Targa '93 Audi S4 |
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07-19-2012, 07:31 PM
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By the way...
Again, for what it's worth, a few connections and 4 bolts drops the whole engine/tranny. Put it on a motorcycle lift, and you can work on the whole thing without slipping a disk. Plus you get a better feel for the wear and tear going on with the power plant/transmission unit...
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'79 SC Targa '93 Audi S4 |
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07-19-2012, 07:38 PM
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Despite all the potential problems with this approach I feel that it is what will have me back in a reasonably ok running car in the least amount of time (an important factor as the running season here in Sweden is shorter than what one might wish for). Again, thank you for your advice! I will post my progress later today 
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1980, 911 SC. Ex-US car in Sweden since June 2010. Insta: @911scowner |
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07-19-2012, 11:39 PM
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Replace your spark plug wires and distributor cap.
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07-20-2012, 01:20 AM
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The pop-off valve article is too long, I think, for a single post, so I will split it into two parts. Enjoy!
The Pop-Off Valve - PART 1: I watched the flatbed, with a Petrol Blue SC aboard, back into our parking lot until it was aligned with our service door. As the driver unloaded the car he said, “The owner told me that she heard a bang from the back, and then the car wouldn’t start.” It took me about the same amount of time as an F1 car takes to change gears to know what was wrong, a blown airbox, of course. After the tow truck driver was gone I raised the SC’s deck lid, reached down and wiggled the air filter housing. I was right, the airbox was so thoroughly blown that it was in two pieces. Only later, when we started to repair the car, did we make an interesting discovery – the car was fitted with an anti-backfire, aka pop-off, valve. Obviously, it didn’t work. Much has been said and written about the pop-off valve, with dissenters in the clear minority. I’m in the minority. Porsche forums, chat rooms, DIYers, even some mechanics, have joined hands and pledged to honor said pop-off valve. Why?, is what I wondered. This is a part designed by someone in his back yard, not the R&D center at Porsche. I don’t like, with a few exceptions, aftermarket parts. I don’t like the pop-off valve. Why has it had success is the compelling question. CIS (Continuous Injection System) is a wonderful fuel injection system used on Porsche 911s from the 1973.5 T thru the end of SC production in 1983. The central component of the CIS system is a large, plastic part called an airbox. The airbox provides mounting points for other CIS components, such as the throttle body, the sensor plate housing, the fuel distributor, the cold start injector, six intake runners, the air filter with cover, and other parts that do specific jobs. When Porsche introduced the CIS system it wasn’t crude, like some designs, but it did prove to be basic. 1973 – 1975 911s with the system required the use of a hand throttle for starting the car. Instructions regarding its use, in the rarely read owner’s manual that comes as standard equipment with every new car, were clear. Pull the handle up to its stop and crank the engine. After the engine started ease the throttle down to a point where the car would idle smoothly. The items in italics were often ignored, or miss-understood. In addition to owners not operating the car correctly, the hand throttle required that its adjustment be checked, and corrected if necessary, at scheduled maintenance intervals. Adjustment, one more strike against the hand throttle system. Many mechanics don’t read that handbook either. During cold starts, as an engine cranks, a very specific amount of extra fuel and air is needed for the car to start quickly and run smoothly. As the pollution control noose tightened around car manufacturers necks, cold start emissions were also measured, so intake systems like carburetors could no longer dump large amounts, at least more than needed, of fuel into the engine for that delicious, overly-rich cold start mixture. Into the fray enters the cold start injector, a part with its own electrical connector; that reacts to a signal received when the engine is cold enough to need a squirt of fuel to start properly. Air is the other component, and is supplied by an intake air system, with a butterfly valve similar to that in the throat of a carburetor, called a throttle body. The butterfly is operated by the driver pushing down on the throttle pedal, which (bear with me) in turn produces intake manifold vacuum that causes a sensor plate, mounted next to the throttle body, to move upward. That action, in turn, activates a counter-weighted arm that pushes up on a plunger inside the fuel distributor, which can then distribute fuel to the six injectors. So, the throttle body butterfly opens a measured amount, the sensor plate moves up a measured amount, the plunger moves up a measured amount, the fuel distributor distributes a measured amount of fuel – which is a perfect mixture for how far the butterfly opened. Hypothetically, let’s say that the perfect mixture is 14.8 parts air to 1 part fuel. Let’s say that the perfect cold start mixture, for a typical spring ambient temperature, is 14.8 parts air to 1.2 parts fuel. How can one regulate that? Porsche engineers determined that by using a hand throttle, which would pull on the mechanical linkage at the throttle body, the driver could open the butterfly seven degrees, which was the perfect amount of air when combined with the amount of gasoline squirted into the airbox by the cold start injector. To adjust that seven degrees, a simple, mechanical device was used, which was accessed through a hole in the tunnel between the front seats. Adjustment was done by loosening a set screw on a small metal cylinder, which allowed the cylinder to be moved along the throttle rod inside the tunnel. When the warm engine would reach about 3800 rpm, with the hand throttle all the way up, the hand throttle was adjusted correctly. If the hand throttle opened the throttle body butterfly too far, the fuel air mixture was disturbed and the cold start mixture became too lean. A lean cold start mixture can cause backfires inside the airbox, eventually it will blow. Porsche took care of the hand throttle issue fairly quickly, and in 1976 introduced an updated system that only required the driver to turn the key to start the car. Newly developed CIS components took care of all the fuel/air mixing, insuring that the engine received the perfect mixture with which to start at all ambient and engine temperatures. All of this discussion, to this point, has involved only the few moments that it takes for the car to actually start. Another important factor, and part, in the starting process is the Warm up Regulator (WUR). The WUR is an electrical device that controls something called Cold Control Pressure. Cold control pressure is crucial, and the factory manual shows a temperature chart for each CIS model year, and instructions regarding how to measure that pressure. For some reason those pages in the manual are probably the cleanest ones in the book. Anyway, the cold control pressure is the amount of pressure in the system, at the top of the fuel distributor, and it regulates how easy, or hard, it will be for the sensor plate arm to lift the distribution plunger up into the fuel distributor. Less pressure, richer mixture; more pressure, leaner mixture. Once again, a lean mixture is not the airbox’s friend.
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Keep the Shiny Side UP! Pete Z. Last edited by Peter Zimmermann; 07-20-2012 at 09:55 AM.. |
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07-20-2012, 09:41 AM
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Pop-Off Valve - PART 2:
One more component of airbox life to consider is the spark plug wires. If you are looking at a properly maintained, CIS 911, you will immediately see that the plug wires appear to have a metal sheath covering each wire. Enter the aftermarket. Manufacturers of spark plug wires have claimed that those wires covered by a metal braid are dangerous and can shock an owner or mechanic. Really? Well, I’ve been repairing/maintaining CIS 911s since they were new, and I’ve never been shocked. Those braids, when correctly installed, are connected to ground terminals near the ignition coil. Aftermarket plug wire manufacturers also claim that original equipment plug wires are ridiculously expensive, and have short life expectancies. Well, so that you know the rest of the story, those braided wires, including their connectors, have a typical life expectancy that exceeds 100,000 miles. The braiding also serves a purpose that somehow escapes those manufacturers. The wire braiding (sheathing) controls any stray sparks that might result from Porsche’s high energy ignition system, and diverts those sparks to ground. That spark control, according to Porsche, protects the CIS airbox by keeping sparks away from the intake manifold area where fuel and air combine to form a combustible mixture, and braided wires actually can reduce the chance of a mechanic or owner of being shocked. So, instead of verifying cold control pressure, correctly adjusting the hand throttle, and making certain that the state of tune of the car, including the plug wires, is correct, the “fix” has become to throw a pop-off valve at the car. Instead of being a viable improvement, it’s nothing more than a bandage. Installed (glued into a hole drilled through the floor of the airbox) it sits downstream from all of the special apparatus, that Porsche and Bosch conspired to design, that could provide the perfect mixture to that wonderful six-cylinder engine. Downstream means that when the pop-off valve’s seal leaks (they often do), or when the epoxy that holds the valve into the airbox doesn’t provide a total seal (which can easily happen), the engine will run too lean. A lean mixture in an internal combustion engine can cause everything from detonation, to burned pistons and valves. Not a pretty picture. The theory behind the pop-off valve is simple. In the event of an explosion inside the airbox, severe enough to crack, or blow, the airbox, the pop-off valve is supposed to pop open, preventing the airbox from absorbing most or all of the internal force. After the event is over the spring-loaded valve is supposed to close, whereby the system will work as though nothing had happened. That, of course, is a scenario from a perfect world, maybe even Hollywood. The reality is that the reaction time of the pop-off valve seems to be too slow, and if the box doesn’t blow the first time that a heavy backfire occurs, it probably will blow somewhere down the road, and not too far down that road. Every explosion impacts an airbox, and over time fine cracks appear where the upper and lower sections of the box are joined together. Introduce a lean mixture, and those explosions occur more frequently, and with more violence. Add a stray spark from an unshielded spark plug wire, and blammo! A blown airbox, in other words, is far too complex an event to be cured by a bandage. Analogize it as a bullet wound severe enough to kill its victim without surgery, a bandage won’t fix the damage or prevent loss of life. A pop-off valve will not prevent a blown airbox, delay it maybe, but not prevent it. An additional strike against the use of a pop-off valve is a remarkable improvement made to the airbox sometime during the 1980 model year. No, Porsche didn’t invent its own pop-off valve; instead, it came up with a real fix. They installed an internal diffuser which mounted so that the cold start injector’s tip fit inside its end. The diffuser is a long, thin, metal section from which six skinny tubes protrude, each pointing directly into one intake runner. As the cold start injector squirts atomized fuel into the diffuser, at the moment that the engine starts to crank over, manifold vacuum pulls the fuel air mixture directly into each intake runner. This prevents fuel and air from collecting in the airbox’s central chamber, the deadly mix that just hangs around waiting for a stray ignition spark, or a spark from the carbon that’s formed on the backside of an intake valve. For purposes of recognition, every airbox, when manufactured, is made of an upper and lower section. Those sections are joined with a layer of epoxy, and then receive additional strength from screws installed around the joint. Some of those screw heads are visible by simply peering into the engine compartment and looking at the left side of the airbox below the intake air snout of the air filter cover. If those screws have a slot for a flat-bladed screwdriver the airbox is the early, non-diffuser version (except for an unknown amount, maybe a handful, of boxes made when the update was first done), while late, diffuser equipped boxes are held together with Phillips head screws. All replacement airboxes, except the box for the ’73.5 T, have been fitted with internal diffusers for many years. To replace a blown airbox, and then add a pop-off valve to it, is nothing short of heresy. The Petrol Blue SC discussed at the beginning of this chapter is not the only tow-in that I’ve seen with an airbox/pop-off valve related problem. Actually, many CIS 911s with blown airboxes, fitted with pop-off valves, have been towed in to my shop. They’ve also been towed in with a pop-off valve that had popped open, but refused to re-close, producing such a lean mixture that the engine had no chance to start. They’ve been towed in after the epoxy failed and the valve had come loose from the box, which is another no-start condition. They’ve been brought in by the owner with a whistling noise at higher rpm, caused by a leaking pop-off valve lid. They’ve been brought in with a complaint of poor top-end performance, a lean condition that caused detonation. This article covers a subject ripe for heated debate. Here’s a thought to hold on to, Porsche never designed and then superseded the airbox with their version of a pop-off valve. I’ve said recently that maybe a pop-off valve would be OK for a ’73.5 - ’75 911 in an area of the country where little or no competent service is available, reluctantly, maybe. That said; I don’t like the pop-off valve.
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Keep the Shiny Side UP! Pete Z. Last edited by Peter Zimmermann; 07-20-2012 at 09:57 AM.. |
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07-20-2012, 09:52 AM
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Well, it took about 6-7 hours to get it out and apart but now it's done. Right now the parts are in my hallway waiting to be cleaned and inspected but that will have to wait until tomorrow as it was, not unexpected, a tiring job.
One thing I noticed was that the AAV has a nipple on it (similar to the de-acc valve) that was not connected (although there was a plugged vacuum line on the right side of the engine bay) and is not there in the Bentley book. The S/N is 0 280 160 404 which I found is for the 78-79 turbo...    A times today I felt as if I was in over my head but now that I have had a better look at it I think I will be able to get it back together  I'll post more pictures of the various parts as they are cleaned /b PS Peter, thanks for posting your article, I'll be sure to read it before ordering any new parts!
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1980, 911 SC. Ex-US car in Sweden since June 2010. Insta: @911scowner Last edited by bsimonson; 07-20-2012 at 10:21 AM.. |
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07-20-2012, 10:18 AM
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As a pop-off valve 911 has come my way, I'm leaving the large rubber boot above airbox greased where it connects and not gorilla tight. Hopefully engine vacuum will keep it sealed tight but a backfire will blow it off, thereby protecting airbox.
@bsimonson: I'd apply 12 volts to your CSV and backflush it, then connecting it to fuel pressure and checking spray pattern before re-installing it.
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Jeff 74 911, #3 I do not disbelieve in anything. I start from the premise that everything is true until proved false. Everything is possible. |
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07-20-2012, 10:50 AM
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Good job!!!!
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bsimonson, You have done the easy part (removal in situ). If you could do the installation under 12 hours (no cheating), you are A+ DIY'er in my book. You are a good example to others for initiative and effort. I've removed my CIS before this way and I'll never do it again because it's a back breaking task. Tony |
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07-20-2012, 11:24 AM
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@Peter: Thanks for the article! It made for a very interesting read, as well as providing an easy-to-understand explanation as to how the CIS system works. I found the part about the original type HT leads especially interesting.
@Björn: Good job, you are a man with great courage and skill! Just make sure to get it back together before the Tidö meet... 
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/Felix '82 Porsche 911 SC Coupe '77 Porsche 924 Targa Instagram: @orebro.classics | @924.resurrection Last edited by felix83; 07-20-2012 at 11:39 AM.. |
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07-20-2012, 11:34 AM
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'78 - '79 airbox....
While we're on the subject of airboxes, maybe Mr. Zimmermann can comment....
Does anyone know if Porsche modified the early SC (44mm port) CIS airboxes with the "fix" that Mr. Zimmermann described to prevent backfires throught the airbox? I plan to "backdate" the CIS on my '82 to the early SC 44mm port version (large port intake head swap) on my '82 for better performance. I might spring for a new airbox when I swap the CIS components (since the old airbox has the pop-off valve) if new versions have been modified to prevent backfires. Thanks |
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07-20-2012, 11:39 AM
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When cold control pressure is ideal, the car will cold idle "surge" gently for a few seconds (sometimes as many as 30 seconds) after being cold started. This is because the cold mixture is set on the rich side of the range that can be found in the factory repair manual.
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Keep the Shiny Side UP! Pete Z. |
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07-20-2012, 11:53 AM
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bsimonson; Be sure to cap off those fuel lines! A tiny piece of crud in the gasoline/fuel distributor/injector can make your life miserable on start-up!
BTW; the unused hose nipple is that way because the vacuum hose nearby used to be connected to that valve. If you re-connect the hose the car will probably have fast idle issues. To make it tidy, most of the time the hose will have a BB, or similar, shoved into it, and then it gets connected to it's nipple.
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Keep the Shiny Side UP! Pete Z. Last edited by Peter Zimmermann; 07-20-2012 at 12:07 PM.. |
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07-20-2012, 11:57 AM
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Thanks for letting us pick your brains!
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/Felix '82 Porsche 911 SC Coupe '77 Porsche 924 Targa Instagram: @orebro.classics | @924.resurrection |
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07-20-2012, 12:17 PM
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Keep the Shiny Side UP! Pete Z. |
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07-20-2012, 12:42 PM
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The Varmint
Porsche
1972 porsche 914
Le Frog
Red Riding Hood
