"Rob" <rwclem (AT) hotmail (DOT) com> wrote


Well,,,let see,,,you failed NOx, both low and high speed. Both CO and
HC are were well within range and about where you would expect them to
be on a good running Porsche. Unfortunately Nox is the hardest of the
gases to control. It is created by high burn temperatures, either from
elevated combustion chamber temperatures or from lean pockets in the
fuel air mixture.

The easiest way to control Nox, the way most manufactors do it, is to
add some exhaust gas into the mix via an EGR valve. (EGR=Exhaust gas
recirculation) But, as I remember, the 944 turbo doesn't have an egr
valve. If by chance I'm incorrect, and it does, make sure it's working
correctly.

Then, make sure the engine is not running hotter then normal. Cooling
fans and thermostat working correctly.

Next thing is to get the spray pattern from the injectors as good as
possible. That cuts down on the lean pockets. Easiest way to do it is
have a "Motorvac" or one of the other professional carbon cleaning
services done on the engine.
The professional versions of this sertvice are much more effective
then the injector cleaners you buy at the auto supply store. But in
some high mileage cars, new injectors might be needed. The nozzles
will get wear grooves with age, and no amount of cleaning will help in
that case. If you have a way of testing the spray pattern off the car,
you want a fine cone shaped mist coming from the injectors, any
distinct streams of fuel are a problem.

Next thing you can try is going to a colder heat range spark plug. But
I've had iffy luck using this trick, it helps sometimes, sometiumes
not.

Last hint, try to have the retest done on a cool and/or humid day, and
the engine temperature as cool as possible. When I have a car that's
giving me trouble with Nox, I always get it to the inspection station
early in the morning, and don't run the engine longer then necessary
to get to operating temperature. You would be surprised at how much
difference this makes.

HTH,
Dave