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Old 05-05-2003, 03:44 PM
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Join Date: Mar 2003
Location: Southern California
Posts: 2,007
103 emissions and KE system diagnosis

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 ( ).

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.

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Old 05-05-2003, 04:03 PM
it leaks, its german
Join Date: Mar 2002
Location: raleigh nc
Posts: 1,111
Sounds to me like the cat is "less than efficient".

EGR only has real effect on NOX emmisions.

Project Smoker, '87 603 powered wagon
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Old 05-05-2003, 07:45 PM
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Join Date: Mar 2003
Location: Southern California
Posts: 2,007
Like I said, since the exhaust O2 content was only 0.1% the catalyst can't be condemned. Once the available O2 is consumed the catalyst can do no more.

Understand that EGR only affects native NOx out the cylinder, but EGR as installed on '89 models would reduce the amount of NOx in the native exhaust, making less O2 available from disassociation in the catalyst.

I had a theory that the '88 valve timing may have had excess overlap to create a greater exhaust residual as this is a strategy that has been used for NOx reduction, but the manifold vacuum is high enough to indicate normal overlap and a check of part numbers yielded the same camshaft for '88 and '89.

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