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Re: Emissions
Posted by John D. Fox - http://saabnet.com/tsn/forms/e.php3? (more from John D. Fox - http://saabnet.com/tsn/forms/e.php3?) on Wed, 16 Apr 2003 00:16:17
In Reply to: , Kok Chen, Sat, 17 Dec 1988 12:00:00 

You have a smog test question with measurements that are
high on HC at '15 m.p.h.', but OK at '25 m.p.h.', and way high NOx ( he is
missing the x in the oxides of nitrogen, it is not NO). From the two
speeds, I am surmising this is a rolling dyno test.

            Hydrocarbons(ppm)       CO(%)                NO(ppm)
                 max    mine              max  mine            max
mine

15mph      116    162                .74    .67             791     2466
25mph        91      56              .62  .48             730       868


High HC is, with reasonable CO ( his is legal but a little high) is most
likely from lousy spark or misfiring, either from fouled/old plugs, lousy
HV wires and components ( distributor cap/rotor), or possibly lean
misfire, if one or more cylinders is very lean, so that some of the fuel
is going out the engine as raw fuel.

The high NOx is from combustion temperature - it takes high temps to make
lots of NOx, and it is also a function of thae catalyst to reduce the NOx.
ignition timing too advanced will do this, as will a mixture that is off
the proper stochiometry, as the catalyst is only effective over a narrow
range of fuel mixture. lugging the engine at very low RPM will also do
this - what gear was the car in for the two test speeds?

So, I'd start by verifying that the closed loop O2 sensor voltage was
dithering around 550 - 600 mV - if it is wandering around, or pegged at
one extreme or the other ( e.g. very small 200 mV or 800 mV + numbers) the
feedback control isn't working. Test this at idle, and also while cruising
around at the 15 and 25 mph speeds. You can test the O2 sensor easily by
opening the feedback loop, measuring the O2 sensor voltage while driving
the computer sense lead at ground ( signals a lean mixture, O2 voltage
should then go up to 800 mV as the feedback tries to richen things) and
use a 1.5 V AA or C cell to drive the computer sense lead at 1.5 V ( this
signals rich, and the O2 sensor voltage should plummet down to 200 mV or
less as the feedback tries to lean things).

If the sensor is good, but the closed-loop mixture is not right, check for
gross air leaks, or incorrect fuel pressure, which put the operating point
too far for the feedback to correct.;

If the spark is good, the mixture is controlled, the HC and CO should look
right. If then you have the high NOx still, check the ignition timing, EGR
( if equipped on your car - it is there to lower combustion temperatures),
and try running the test at higher RPM. I doubt the catalyst is bad based
on the CO numbers, unless it has been poisoned or overheated I think they
last a very long time.

Check these basic things first.

John

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