Porsche Repair: carbon (soot ) build up, rear wheel wells, fuel injection specialist

Question
I have a 87 Carrera Cabriolet.  I have had major work done lately. (valve guides, clutch, tie rods, shocks, etc)..  I notice even after a short drive of maybe 50 miles a large build up of soot (carbon) all over the rear of the car.  This condition existed before the major work was done at 102,000 miles.  The build up is not concentrated in the area of the exhaust pipe but is pretty evenly distributed evenly over the entire rear of the car above and behind the rear wheel wells.  Interestingly, the car just passed emissions testing here in Arizona.  

Any help would be appreciated.  Thank You !

Answer
Hi Bruce.  I'm surprised the car passed the emissions testing with the soot problem you're describing.   But, anytime a car can get by the bureaucratic fumblers I'm pleased.  Of course I'm from the Bay Area, California, so I have even more special words for the smog police here.

Anyway, back to the real issue.  Soot appears when unburnt fuel exits the exhaust pipe.  This could be a problem with the catalytic converters (if equipped)...but the real story is that fuel entering the combustion chamber is not completely combusted.  OK, so that's probably not a word...you get what I mean.  As a reuslt you are losing power, losing gas mileage, and losing the view of your car's lovely rear end.

Perhaps the fuel injection system is tiring.  If the car is running rich there would be too much fuel available for the amount of fresh air available.  This would cause soot.  If the spark isn't hot enough, this would also cause unburnt fuel.  So the ignition system could be a likely culrpit.

If it were me, I'd do a bit of research to see if there is a fuel injection specialist who could hook your car up to some diagnostic equipment and measure if the fuel injection system is performing as new.  And if it's not, determine where the real culprit is.

There are quite a few possibilities...and I'd hate to have you rack up a lot of labor costs for the average mechanic to "search" till he finds something.  If there is any expertise in this area...paying for a couple hours evaluation may be worth it this time.

Dave