About six weeks ago I joined this forum searching for answers why my '88 190E 2.6 is on the ragged edge of California's ASM emission test (HC at 15 MPH), and though I didn't get much response to that initial post, reading the various threads on 103 issues has given me a real education along with Dan Landis' procedure to check the O2 sensor duty cycle ( www.landiss.com/mixture.html
Yesterday along with a friend who is an accomplished gasoline and HD diesel engine mechanic, we ran a compete series of tests on my engine. Along with my input of new knowledge from this forum we studied every bit of Bosch and DB documentation on the KE system in our combined libraries.
The idle duty cycle was in the range of 50-55% and 40-45% at 2000, which is within the accepatable range. The closed circuit O2 sensor output was close to a square wave with the voltage jumping between the range of 0.7 to 0.2 with a frequency of about 0.5 to 1.0 Hz, so the O2 sensor behavior appears perfectly normal.
Sean did an "informal" test he learned back when he worked on spark ignition engines. He "wired" himself in series between the positive battery terminal and the O2 sensor signal line. This provides a constant voltage bias of close to 1.0V, and if the control system is working properly, it should bias the mixture lean until it hits the limit of control range at which point the engine should be exhibiting a rough idle. Sure enough from a smooth 700 RPM @ 16-16.5" the vacuum slowly dropped to 13-14" with obvious roughness from lean misfire. Speed stayed the same due to the idle control system, and it took about 10 seconds to achieve the final steady state vacuum and roughness.
From 2500 @ 20" no load, the engine became rough as the speed and vacuum dropped to 1600 @ 18.5", again in about 10 seconds.
Bottom line is: Nothing appears to is amiss on my KE, but I still haven't figured out why I have relativelly high HC and CO with only 0.1% O2. If the O2 was higher I might be willing to believe that the converters are degraded, but once all the O2 that comes from the native exhaust or is produced from NOx disassociation is consumed, the converter can't do anything more. (The NOx numbers are about average for my year group and well below the limit.)
We figured that an air pump would make it squeaky clean, but in '88 there was no air pump or EGR. I do recall that DB added EGR to the '89 2.6, but did they ever add an air pump to the 103s?
I'm beinging to think that the '88 engines were certified with less margin relative to the standards, so they tend to show high numbers in field testing, and the later adding of EGR and (?) air pump lowered the certification emissions and provides more margin in field testing.
Thanks to everyone who contributed to our education, but Sean and I are still scratching our heads about how the air flow plate position sensor affects idle quality. I don't disbelieve it. I just can't understand why nothing in the Bosch or DB documentation we have says anything about the function of the air flow valve position indicator other than cold acceleration enrichment.