Congratulations and thanks for posting the results and analysis. I thought the new measurements would be a bit lower with the R16/1 and vacuum advance "adjustments".

It's tough to get these M103 engines below about 50 ppm HC, and I think it has somewhat to do with the relatively high surface area to volume ratio of the "semi-hemi with quench zone" combustion chamber.

Something I figured out at least 20 years ago is that resistor plugs can cause enough misfires to increase HC and you can feel them at idle. Bosch no longer offers non-resistor plugs, but I recall that either NGK or Denso offers a non resistor plug equivalent to the H9DC. Their websites should have cross reference data.

The fact that there is still 1.5% O2 could be an indication of catalyst aging. How many miles on the car? Zero percent O2 indicates that the catalyst is accomplishing near 100 percent of all available oxidation and reduction reactions.

Here is the most recent test of my '88 190E 2.6 five-speed with 85K miles. It has the original catalyst and O2 sensor. I checked the duty cycle about ten years ago and it was within spec... no need to adjust. The 15 MPH test was done in 2nd gear and the 25 MPH test in third.


15 mph results 1661 rpm (2nd gear)
%CO2 16.2
%O2 0.0

HC, ppm
max 116, ave. 27, meas. 54

%CO
max 0.74, ave. 0.09, meas. 0.20

NO, ppm
max. 791. ave. 255, meas. 127


25 mph results 1627 rpm (third gear)
%CO2 15.6
%O2 0.0

HC, ppm
max. 91, ave. 18, meas. 28

%CO
max. 0.62, ave. 0.07, meas. 0.17

NO, ppm
max. 791, ave. 223 meas, 42

Your cutpoints are lower because your 300E is heavier. There is a chart on the BAR Web site that explains how the cutpoints are calculated.

The "average" is the average of passing vehicles in our year group that is something like 1986-1991. As you can see we are both above average in HC and CO, but below average on NO. I have no explanation for this.

It looks like you have enough margin to pass the next test or two with good conditioning, like you did. Let us know the mileage and whether the cat and O2 sensor are original and the type of installed spark plugs.

Up until my last two or three emission tests I was directed to a STAR (test only station) due to my car being tagged as "high emitter profile", but this is no longer the case, but I don't now the reason.

Duke