Airbox Questions '81 SC

[MichaelN]

I actually have only a simple question but I feel compelled to offer some background, I'm afraid. Sorry.

1981 SC, stock, regular (and fairly recent) tune-ups and valve adj. Car has difficulty maintaining an idle immediately after startup:

Cold Start - Engine fires immediately (w/o throttle) but idle drops and runs rough with popping and occasional mild backfire. Applying mild throttle only increases the popping but engine will slowly gain rpm. After 30-60 seconds engine runs smoothly enough for driving. When warmed up engine is smooth with good power and no symptoms other than a more pronounced "back-rap" than previously from the exhaust on deceleration. One anomaly- occasionally when it's very cold, engine will start immediately and idle well. But this is only occasionally.

Hot Start - After sitting 1/2 hour to an hour after shutdown engine is very reluctant to start without some throttle. Finally gets going with the same symptoms - rough running, popping, occasional backfire. After a minute to "clear its throat", runs great.

Took the car in to my mechanic for analysis and this were his findings:

"Measured fuel pressures, fuel pressures are within spec Smoke tested the intake system to check for vacuum leaks , found that the air box is leaking from the seam. Replaced air box half screw (?) in an attempt to seal air box. Tested car, still popping and back firing. Smoke tested again, and found that the intake runner boots are leaking. Suspicious that the air distribution pipe under the air box may be cracked and leaking as well."

He recommended replacing the air box, intake runner sleeves, and any other problems revealed during the job. But in our discussion afterward he indicated that he could not honestly guarantee that this would solve the problem, but that it was a "good place to start".

So, here's my question (finally?). For those of you that have had damaged/leaking air boxes, did your cars have symptoms that matched my own? Or maybe some others? As you know, replacing the air box is going to be a very expensive job, and I'd like some assurance (or not) that this will be the happy ending to the story, not just a new chapter

Of further information, within the last 5K miles or so the car has gotten a new fuel pump and check valve, new accumulator, rebuilt CDI, and a rebuilt distributor, and I replaced an old rusting Bursh exhaust with the original muffler that came with the car.

[DAP231]

I’ll let others weigh in on potential sources of your issues (Tony?), but in my opinion before you start replacing expensive parts you need to take your car to someone who has a deeper understanding of the mysteries of CIS. I’ve replaced a ton of CIS parts on my 79 SC, including the airbox, intake rubber seals, fuel distributor lines, and fuel injector seals. My car runs better than it did, but I’m still experiencing similar cold start issues to the ones you are describing.

[targa72e]

intake leaks will cause issues you mention (as will other things). Cracks at the seams of air box can be fixed with epoxy, much less expensive than new air box.

john

[boyt911sc]

Originally Posted by MichaelN

Took the car in to my mechanic for analysis and this were his findings:

"Measured fuel pressures, fuel pressures are within spec Smoke tested the intake system to check for vacuum leaks , found that the air box is leaking from the seam. Replaced air box half screw (?) in an attempt to seal air box. Tested car, still popping and back firing. Smoke tested again, and found that the intake runner boots are leaking. Suspicious that the air distribution pipe under the air box may be cracked and leaking as well."

He recommended replacing the air box, intake runner sleeves, and any other problems revealed during the job. But in our discussion afterward he indicated that he could not honestly guarantee that this would solve the problem, but that it was a "good place to start".

So, here's my question (finally?). For those of you that have had damaged/leaking air boxes, did your cars have symptoms that matched my own? Or maybe some others? As you know, replacing the air box is going to be a very expensive job, and I'd like some assurance (or not) that this will be the happy ending to the story, not just a new chapter

Of further information, within the last 5K miles or so the car has gotten a new fuel pump and check valve, new accumulator, rebuilt CDI, and a rebuilt distributor, and I replaced an old rusting Bursh exhaust with the original muffler that came with the car.

Michael

Whatever you decide to do, that airbox has to be fixed, repaired, or replaced with a good one. No amount of troubleshooting could fix your running problem with a cracked air box. Is your mechanic very familiar with CIS troubleshooting? What is this “distribution air pipe”? Were you referring to the intake runners?

Lastly, were the fuel pump, FP check valve, CDI, and distributor tested and confirmed to be defective? Why were they replaced/rebuilt? Remember not all good and honest mechanics know and understand CIS troubleshooting. So if you find a good one, stick with him.

Tony

[targa80]

Here is my thread on repairing blown air box.

https://forums.pelicanparts.com/porsche-911-technical-forum/1081238-blown-air-box-repair-upgrade-spider.html

[Robey5]

If I were to listen to anyone on this forum about what to do next, I’d most def listen to Dr. Tony. He coached me through various things CIS, and my car is running pretty nice when called upon to do so.

One thing that I would do is grab some starting fluid and - while it is running - hit the culprit area with some starting fluid and see if the RPMs jump when you spray. Hit the boots, runners, injector sleeves with small sprits to see if you have leak on the intake side; rule it out one component at a time. CIS airboxs ain’t cheap; but if you keep a very close eye on the classifies you can pick up a decent used one that is repair worthy.

Enjoy!

[Coastr]

You’ve had good advice - I know the feeling of wanting to be certain that the cost of a repair will fix the problem - but nobody can give you this certainty. You have to roll with it. Fix the airbox and runners -then retest. Maybe it solves the problem, maybe it doesn’t. But you’ve found you have air leaks and they do need to be fixed to get it to run properly.

[pmax]

Do the leak test yourself.

Smoke test for vacuum leak novice question

This isn't a modern car with 1001 sensors which can go wrong.