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Old 11-17-2004, 01:46 PM
Duke2.6 Duke2.6 is offline
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Join Date: Mar 2003
Location: Southern California
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Your test results are different than the typical M103 readings in CA. M103s tend to be high on HC - close to or a little above the limit - on the 15 MPH test, but much lower relative to the limit on the 25 MPH test. I attribute this to the converter being relatively cold at the start of the 15 MPH test, which is why "conditioning" is important i.e., go to a drivethrough test station with a short line with the car thoroughly warmed up (at least 20 minute since the cold start) and keep the revs at 2000 with A/C or defroster on, so the compressor is placing load on the engine.

The 15 MPH test has a relatively higher load than the 25 MPH test and heats up the converter. The O2 content indicates how efficiently the converter is operating. If O2 content is zero, then the converter is operating at max oxidation efficiency - using all the available O2. Typically the O2 reading is a little higher at 15 MPH than 25, indicating that converter efficiency is higher on the second test, which is the 25 MPH test. O2 content is typically zero to 0.3 percent and anything above zero means that HC can be reduced if the converter is hotter.

The higher NOx in the 25 MPH test is inexplicable to me given that this test is done at lower relative load, and you can see that the stardard is a little lower than at 15 MPH. Given that your 15 MPH NOx is well below standard I cannot explain the high number at 25 MPH, but I would not discount test or measurment error. I've seen this before.

For example, my '91 MR2 was tested last year and the O2 content was 2 percent with passing scores on the pollutants that were well below standard. Such a high O2 reading on a O2 sensor car is virtually impossible unless there is a malfunction and none iwas indicated either by the OBD or test results. Prior test showed zero to 0.1 percent O2 with very low emissions.

Keep you test report and if you have previous test reports keep them all together or handy so you can look at trends, which are useful in diagnosing emission problems.

P.S. Ignition timing has a signficant impact on emissions. The M103 ignition system controls advance based on a combination of engine speed and load. Load is represented by manifold vacuum - higher vacuum is less load, and usually more advance via the vacuum advance system. Retarding the timing from normal will reduce peak flame front temperature, which reduces NOx generation and increases EGT, which will get the converter hotter, faster and reduce HC and CO.

One "trick" you can use if your M103 is on the ragged edge or marginally fails is to disable the vacuum advance by plugging the vacuum line to the EZL module. Also, there is a resistor module plugged into a pigtail and the value affects the rate of advance with engine revs. There have been a number of discussions on this and IIRC it you short the plug (zero resistance), the rate of ignition advance will be slower, which will retard the timing for any speed load condition relative to the OE timing map.

Since the 25 MPH test is done at lower relative load, there would probably be more ignition advance due to the vacuum advance.

The fact that engine revs are the same indicates that the 15 MPH test was second gear and the 25 MPH test was in thrid. Some ignition advance systems also use transmission gear to determine advance, sometimes by locking out vacuum advance in the lower gears. I don't know if this is the case with the M103.


Last edited by Duke2.6; 11-17-2004 at 02:01 PM.
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