Ah, yes! They do the idle test after the 2500 RPM test. It's been enough years since I had a two-speed no-load test that I forgot the 2500 RPM test is first.

In this case the catalyst temperature is an indirect issue, and my second proposed cause may be the primary reason why idle emissions were closer to the limit.

The injectors have a "chatter valve" that rapidly opens and closes during engine operation, so it's not really a "continuous flow" system, but meters fuel in continuous short bursts that average out to the proper steady flow rate. The "pulsed" delivery at the nozzle helps atomize the fuel for quicker vaporization, but it's a moving part that can wear, and since all will likely not wear at the same rate, the individual cylinder to cylinder and cycle to cycle fuel-air ratio scatter will increase with time, and the greatest scatter will occur at idle when injection pressure is lowest.

When my car was new its first emissions test yielded quite low HC. The second test in '93 at 49K miles showed greatly increased HC. (These were both two speed no-load tests.) This also correlated with the slight idle roughness that developed at about 40K miles, which is typical of many KE equipped engines. Injector flow scatter increases cyclic torque variation , which is what causes the slight idle roughness. Scatter will also increase engine out emissions, which places additional demand on the converter, and the result is higher tailpipe emissions unless the converter is VERY hot.

For this reason it is important to "manage" your emission test to ensure that the catalyst is as hot as possible before they start sampling emissions.

Since that test in '93 I've only accumulated 25K additional miles, so the degradation has not substantially increased, but I've been uncomfortably close to the limit a couple of times. The best numbers were in '01 when I went to a drive through test station on a rainy day (my test comes due in winter every other year) and asked them to do the tire dry test, which they did. I believe the tire dry test heated up the converter enough to significantly improve its performance. In '03 I went on a sunny day and was only one PPM below the HC limit at 15 MPH.

One sure fire way to increase converter temperature is to retard ignition timing as this will increase EGT. Unfortunately, on my 103 engine the initial timing is not adjustable. Through discussions on this forum I found that the rate of ignition advance can be altered by changing a resistor on a pigtail in the battery compartment. By slowing the rate of advance the ignition timing at speed under load will be less, so EGT will be higher and the converter will heat up quicker. As I recall, a shorting plug (zero resistance) will result in the slowest rate of timing increase with engine revs, so I plan to remove the OE resistor and short the plug with a piece of wire for my test next year and see what happens.

If I could change the initial timing I would just retard it 5-10 degrees and this should result in a noticeable reduction in emissions, but it can't be done on the 103 and many other Mercedes engines.

Duke