Thoughts and advice on the care and feeding of fine automobiles from Machine Aficionado and bestselling author John Elder Robison, owner of JE Robison Service in Springfield, Massachusetts

We are independent restoration, repair, sales and service for Audi, BMW, Bentley, Jaguar, Land Rover, Mercedes-Benz, Porsche and Rolls-Royce automobiles.

Bentley archives

Land Rovers

Back in the 1970s, Porsche went from carburetors to fuel injection in their 911 series cars.  The system they ended up using for most of the cars was called CIS, and it relied on a mechanical system of sensing air flow and metering fuel in response.

The incoming air was pulled through a box whose outlets were the six pipes to the intake and whose inlet was the metering flap.  There was just one problem with this arrangement - the metering flap went down(in) but did not go up (out.)  When the engine backfired through the intake, there was nowhere for the pressure to go, and the plastic airbox blew open.

This started happening as soon as the cars went on sale, and the market responded by developing a blowoff valve - a spring loaded valve that pops open and releases backfire pressure harmlessly while holding tight in normal operation.

This system was fitted to every Porsche 911 right through the appearance of Motronic engine management in the 1984 model year.  Amazingly, we still see cars that have never had the blowoff valve installed.  And they still get towed in, blown up!

Airbox with blowoff valve installed

Blown air box - the cost of not having a valve
The valve is under $100, and can be installed in less than two hours.  Fixing a blown air box is much more involved, requiring engine removal and $700 of additional parts.  

The moral of the story - have your car serviced by people who know the vehicles, and have them inspect the car from time to time.  An inspection by a qualified specialist would have revealed the missing valve and saved this most recent owner a $2,000 repair.


Robison Service is an independent Porsche service center in Springfield, Massachusetts.  They are a four-star Bosch Car Service facility with 30 years experience on Porsche automobiles.  Visit them online at

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