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Jonny H Jonny H is online now
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Join Date: Jul 2013
Location: South East England
Posts: 1,065
The 'toilet seat' is a win-win for the marketing men. If it ever 'saves' your airbox, then I'll wager your airbox is already split. You can easily check by attempting to tighten the screws round the edge or inside the box (particularly the ones near the ridges). If any screw is loose and fails to tighten, your box is split. End of story.

Here's my take on it:

The original airboxes are made from ABS plastic which is notoriously difficult to glue and if you've seen the bond made between top and bottom, you'll see a melted bond, probably done with acetone or similar solvent in the factory. The screws also indicate that the bond is not sufficient on its own.

After years of service, the seam starts to fail due to heat cycles and vibration and minor leaks cause the occasional backfire. Of course, the owner then thinks the car needs a tune and sets the mixture richer but he/she is never convinced it is running exactly right. CIS gets branded as 'temperamental' and difficult to setup. The owner 'drives around' the issue. Sound familiar?

The car still back fires occasionally so owner fits a pop off valve 'just in case'. One day, a backfire occurs and the pop off valve saves the day!! Wow, wasn't that amazing! Er, no. The airbox is still split.

Secondary to this, when a toilet seat is fitted, the installer is required to bond the device to the airbox with epoxy. This is not the correct glue for ABS and will eventually fail, causing an air leak. Boom! Wow, the pop off valve saved the airbox! Oh, wait, what caused the backfire? Er, the pop off valve.

Here's my own story. My car was on the original '82 airbox and hasn't got a pop off valve. It started to get a bit hesitant and I was having to tune the mixture a fair bit. This was about a year ago. It still started and ran ok and then one day, boom! Prior to this it had never backfired in my 7 years of ownership.

I ordered a new airbox from Porsche and the first thing I noticed is that it is made from PA6, not ABS. This is a glass fibre filled plastic which is far stiffer. It uses epoxy to bind the two sections together and the screws have been changed to self tappers with deep threads. This leads me to believe that the real problem is to do with poor bonding and the do longevity of the original ABS part.

Having fitted the new airbox, the car is a joy to drive, the idle is smooth and can be coaxed along on the throttle at walking speed with no jerkiness.

No toilet seat for me!
Old 11-06-2017, 12:38 PM
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