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Old 03-30-2004, 02:39 PM
Duke2.6 Duke2.6 is offline
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Join Date: Mar 2003
Location: Southern California
Posts: 2,272
I'm not sure if your car has KE, but it is common for KE equipped engines to see higher HC as they age, and many are at the ragged edge or fail, marginally, on the 15 MPH portion of CA's ASM test.

I have two theories. The first is catalyst performance degradation, which requires higher catalyst temperature to achieve equal efficiency compared to when the catalyst was new. Since the coolest catalyst tempearture is at idle, the period the engine idles prior to the loaded test reduces catalyst efficiency. Once the loaded test starts the catalyst heats up, but the emissions from the first part of the test when oxidation efficiency is low may be high enough to exceed the standard, even if the test is run the full time duration. If low emissions are detected early in the test, the test terminates before the full time period, but if emissions are high the test runs the full (minute I think) duration, and the final report number is an average over the duration of the test.

Another possibility is uneven injector flow rate due to deposit buildup or wear of the nozzle chatter valves. The O2 sensr(s) maintain an average stoichiometric fuel-air ratio, but if injector flow rate is unbalanced some cylinders will be rich and some lean, which will increase engine out emissions. Uneven injector flow can also cause the slight idle roughness that many of these engines develop as the miles and time add up. Flow balance at idle is a fundamental problem inherent to continuous flow injection systems, which is why Mercedes ultimately replaced it with a full electronic system with electric solenoid injectors.

I don't think there is anything specifically wrong with your car, but make sure you use non-resisitor plugs, which is the original type used by Mercedes. This will likely limit your choice to those recommended replacement plugs listed in the manual. Most replacment plugs other than Bosch or Beru at resistor type.

From you idle speed readings the nominal is probably 650, and the variation is nothing to be concerned about.

Make sure the car is as hot as possible for your next test. I recommend going to a "drive-through" test station when the line is short and preferably after some highway driving. Keep the engine at 2000 revs in neutral until the tech is ready to drive the car onto the rollers. The above will tend to increase catalyst bed temperature.

Another "trick" is to go on a rainy day (not too many this year) and ask them to run the "tire dry test". They will run the car on the rollers with no load at about 2000 RPM for 30 seconds to a minute, and this will heat up the converter enough to reduce the 15 MPH HC by 10 to 20 percent.

The 25 MPH HC is usually not a problem because the converter heats up during the 15 MPH test.

I'd appreciate it if you would post all the test result numbers/limits at both 15 and 25 MPH including the 02 and CO2 readings. With the numbers I can give you a better analysis, and it will help me in the informal research on how to keep these cars from busting the emission standards.


P.S. Reading your response to Mike's post it sounds like the problem was definitely too cold catalyst temperature, especially if the engine was shut off during the rain dance about traction control. Even if it was left idling, more than five minutes of idling is going to allow the catalyst to cool off, and this will increase emissions. I'd still like to see all the exhaust gas results/standards even it it was only a two-speed no load test.

Last edited by Duke2.6; 03-30-2004 at 02:49 PM.
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