Reducing NOx in DigifantII for etest

I’m posting this message in the hope of helping anyone who has
problems in getting a DigifantII engine through etest due to high NOx
levels.

I have a ‘92 Golf 1.8l gas with DigifantII engine control. The car
has been in the family since new. In smog tests over the years in
both Vancouver, B.C. and in Ontario the NOx emissions have varied by
more than a factor of 10, in an apparently random fashion. For
example, in one test with just 21,000km on the car the NOx emission
was 790 ppm (a fail in Ontario, but a pass in B.C.). At 36,000km
without any tuning or other changes the NOx was 36ppm. I suspect the
random fluctuation in NOx output results from VW design issues, not a
problem with the engine. In fairness to VW, the car was designed to
pass the EPA test, not regional smog tests. For example, the EPA test
almost certainly allows airflow through the radiator to duplicate that
expected in actual driving, while smog tests often allow cars to idle
for extended periods, raising the engine temperature above normal
operating conditions.

In May 2010 the car failed the Ontario test with the following
results:

idle CO 0.06% idle HC 36ppm load CO 0.01% load HC 9ppm load
NOx 990ppm

The timing was set precisely as specified in the Bentley manual. A
check with an oscilloscope showed the oxygen sensor was operating
correctly. The catalytic converter was one year old.

Before retesting the car in November I did the following:

1. Fuel with a combination of E10 and E85 to give an ethanol content
of about 20%

2. Wire the radiator fan to run continuously while the engine is on
(done by removing the fan overrun relay and rear wiper relay at the
fusebox and shorting between appropriate pins)

3. Disconnect the knock sensor, then raise RPM to over 2500 RPM three
times. Confirming the comments in Bentley, the timing light shows
this reduces the advance by about 10degrees at 2500 RPM

4. Disconnect and plug the vacuum line controlling intake air
temperature (i.e. only allow cold air to the intake)

5. Change from platinum to copper spark plugs

6. Raise front tire pressure to 35psi to reduce rolling resistance

The results we

idle CO 0.08% idle HC 20ppm load CO 0.09% load HC 11 ppm load
NOx 373 ppm


Unfortunately this shotgun approach doesn’t identify which changes
were primarily responsible for lowering the NOx. However, all the
changes are easy to implement. Of course the modifications should be
removed immediately after the test. The car seems to run fine on E20
but it might be risky to use this for extended periods.