I ran a series of tests over the weekend looking at converter inlet temperature versus different timing maps, and it should give you some insight into why these engines are usually at the ragged edge of emission limits (if not above), especially HC, in California's Acceleration Simulation Mode (ASM) emission test and why they have been tagged by the Bureau of Automotive Repair (the agency that supervises California emission testing) as "High Emitter Profile".

The tests were run using an IR gun aimed at the converter inlet, just ahead of the converter heat shield and just above the seam in the pipe. It's important to shoot the same exact spot to achieve consistent and comparable readings.

The first temperature reading was taken following a cold start and about five miles of driving in normal suburban traffic. At this point the inlet temperature measurement was about 400F. Note that this is the surface temperature of the catalytic converter inlet. The exhaust and catalyst bed are hotter and cannot be measured directly with an IR gun, but, obviously, the hotter the surface temperature, the hotter the exhaust and catalyst bed.

AFTER FIVE MINUTES OF IDLING THE TEMPERATURE DROPPED TO ONLY 250F!!!!!!

Returning to my driveway, timing readings were taken using a dial back timing light for the OE and modified ignition advance maps using information previously posted on this forum to modify the rate of advance with engine revs. This consists of changing the R16/1 resistor, and tests were done with each different resistance value with the vacuum advance active and disabled. After about a minute at 2000 revs with each ignition map, the catalyst inlet surface temperature was measured.

1. OE config. vac. adv. active, 750 ohm resistor...............35@2000, 400F

2. Vac adv. disabled, 750 ohm resistor............................21@2000, 410F

3. Vac. adv. active, zero ohm (plug shorted) resistor........ 21@2000, 410F

4. Vac. adv. disabled, zero ohm (plug shorted) resistor......13@2000, 450F


I also did some reading on catalyst aging including a recently published PhD thesis. There are many factors that lead to catalyst degradation (or "aging") that are a function of basic catalyst technology, operating time and conditions, and maintenance. It's a very complicated subject, but, in general, as the catalyst ages for whatever reason, the "light off temperature" (the temperature at which 50 percent of potential reactions are achieved) increases as does the temperature at which 80 percent of potential reactions are achieved. O2 sensors also age and their response time can slow at a given temperature relative to a new sensor. O2 sensors can also go out of calibration due to clogging of their outside air reference source, so be sure your O2 sensor is externally clean or dirt, mud, or oil residue.

From the above testing, you can see that the catalyst cools signficantly after several minutes of idling, and this combined with catalyst aging will increase measured emissions in the CA ASM test until engine load and increased exhaust flow increase catalyst temperature. I believe this is the reason M103 engines have relatively high emissions, especially HC, during the 15 MPH ASM test, which is the first test of the ASM procedure. Measured emissions on the second, 25 MPH, test are usually lower because the catalyst is hotter.

You can gain some insight into how well your catalyst is working by looking at the O2 content in the test report. If it is 0.0 percent the catalyst is doing everything it can in terms of oxidation reactions by using all the available O2. If there is a tenth or a few tenths percent, your emissions will be higher than if the catalyst was hot enough to use all the oxygen. As a rule of thumb I figure 0.1 percent O2 increases HC emissions about 20-30 PPM relative to zero percent.

For any engine operating condition, retarding timing will increase EGT, which will heat up the converter more quickly and yield a higher steady state operating temperature under a given load, so reducing ignition advance will aid in this goal. Reduced ignition advance will also reduce peak flame front temperature, which will reduce NOx formation during combustion. No changes can be made to idle ignition timing, and this is why it is important to go to a drive through test station with a short line and keep the engine at 2000 revs - A/C on, windows open - anything you can do to put load on the engine. Another strategy is to go on a rainy day and ask them specifically to run the "tire dry procedure". This is about 30-60 seconds at 25 MPH, no load on the rollers, and it will definitely heat up the converter. The time I did this four years ago my 15 MPH HC dropped from 121 to 87 and O2 from 0.1 to 0.0 percent. Last year I went on a dry day and HC increased to 113 - barely below the 116 limit - and the O2 content increased from 0.0 to 0.1 percent.

Never take your car to a shop and just leave it there and pick it up later. Always go to a drive through, most of which are test only (and if you have a M103 engine and live in California you will likely be DIRECTED to a test only station). It's quick and convenient, and you can be reasonably assured that your car is properly "conditioned" - i.e. the converter is as hot as possible prior to beginning the actual test.

The above, of course, only applies if your fuel system, ignition system, and all other emission control hardware and software, including the O2 sensor are in reasonably good working order, but even if everything is in spec, the effects of catalyst aging and catalyst cool down at idle (and similar effects for the O2 sensor being too cold) can combine to increase emissions, especially 15 MPH HC to near or even over the limit. You can have a perfectly functioning M103 (except for normal catalyst aging) and still bust the test! You can even bust the test with a new catalyst and O2 sensor, because the layout allows excess cooling of the catalyst and O2 sensor at idle.

Note that most modern cars have "closely coupled catalysts" - the main catalyst is attached directly to the exhaust manifold, which encourages quick light-off and maintenance of temperature. Modern catalysts will survive high temperature better than older designs, and since excess temperature can degrade catalyst performance, most older cars have them mounted under the floor board to prevent overheating.

This general discussion also applies to contemporaneous four-cylinder and V8 Mercedes engines since they all share the same basic emission control and catalyst technology, but the V8s don't seem to have as many problems when everything is functioning properly.

As a side note I also tested the ignition curve with no resistor (open circuit), which is supposed to yield the most advance, but the results were no different than the OE 750 ohm resistor, however, I did not rev beyond 2000, so there may be additional advance at higher revs. Later this week I will retest at higher revs to find the total ignition advance @ revs allowed by zero, 750, and infinite resistance.

Duke

PS - March '05 emission test results and discussion at:

http://www.peachparts.com/shopforum/tech-help/117048-successful-ca-asm-emission-test-ke-fuel-system.html#post833484