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Old 07-27-2005, 03:18 PM
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Join Date: Apr 2005
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87 300E Fails Ca Smog

My 87 300 E failed CA smog. Everything was OK except HCs at idle which were double allowable limit. It does idle a little rough.

Car has 240K plus miles but runs well other than at idle. What should I look at first?
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Old 07-27-2005, 03:45 PM
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Location: Southern California
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Idle? Was this a two speed no load test or a ASM loaded test?

More information would be helpful - like all the gas concentrations in the report including the limits and averages. Scan or photograph the report and post it if you can.

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Old 07-27-2005, 04:00 PM
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Will Do

I'll post the numbers later this afternoon.

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Old 07-27-2005, 04:37 PM
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Test Results

Idle Emissions Test
MPH =0 RPM=677 %CO2=12.6 %O2=3.4 HC(PPM)=237 CO(%)=.07 Fail
MPH =0 RPM=2356 %CO2=14.3 %O2=1.2 HC(PPM)=63 CO(%)=.22 Pass
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Old 07-27-2005, 04:42 PM
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Location: Ewa Beach, Hawaii
Posts: 333
Start with the cheap stuff....
Clean out the ICV
Use Cheveron techron before taking it in for testing
Use 93 octane gas before taking it in for testing
change spark plugs
clean/change rotor & cap
change O2 sensor
replace all filters
change OVP
If that doesn't work you can change your MAS potentiometer (fixed my idling problem)
1986 Gold 300E 4Dr automatic
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Old 07-27-2005, 08:36 PM
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Location: Southern California
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Originally Posted by shoutiwawa
Idle Emissions Test
MPH =0 RPM=677 %CO2=12.6 %O2=3.4 HC(PPM)=237 CO(%)=.07 Fail
MPH =0 RPM=2356 %CO2=14.3 %O2=1.2 HC(PPM)=63 CO(%)=.22 Pass
Analysis: The CO is okay, which indicates the mixture is probably okay, which means the lambda system, including the O2 sensor, is probably functioning normally; however the HC is high, and the O2 is VERY high. If the ignition and lambda systems are in proper working order and the car was properly warmed up and properly conditioned prior to the test, idle HC should be not more than about 100 ppm with O2 no more than 0.1 percent at both idle and 2500. The O2 reading is a key diagnostic tool.

The combination of high HC and high O2 is indicative of misfires, so I would concentrate your effort on ignition system diagnosis.

The first thing to check is the spark plugs. They should be Bosch H9DC, which are non-resistor type. If you have any kind of plug with an "R" in the designation, it is a resistor plug and it DOES NOT BELONG on a M103 engine.

The ignition system was designed specifically for NON-RESISTOR PLUGS. Resistor plugs will reduce spark energy and alter the secondary voltage wave form, which can cause misfires, especially at idle. You can buy the correct Bosch H9DC from Fastlane or other Mercedes parts specialists. You will likely NOT find them or even be able to order them from local parts stores. DO NOT BUY the HR9DC. This is a RESISTOR plug. Same applies to ALL commonly available Bosch Platinum plugs. THEY ARE ALL RESISTOR TYPE! DO NOT BUY/INSTALL THEM!

These ignition systems achieve RFI suppression with a resistor built into the spark plug boots, cap terminals, and rotor and are specifically designed for non-resistor spark plugs. The spec for each these resistances is 700-1300 ohms

I suggest you do thorough visual inspection and functional test of the secondary ignition components. Remove the cap with the plug wire harness attached. Carefully inspect the cap terminals for erosion - also the rotor tip. You can probably work on the rotor without removing it. Clean them up with a steel dental pick or Xacto knife. Measure resistance from each cap terminal to the plug end. The value should be about 3K, but the important thing is that they should all be about the same. Repeat the measurement with a proper Bosch H9DC plug attached to each boot. The resistance should be the same. If not, there is a problem with the plug connection. Twist and bend the wires as you measure and look for a steady reading.

Separate the wires from the cap (make a diagram and number the wires, if they don't already have numbers, so you know how to put them back properly) and measure each cap terminal and the rotor. Also measure the wires, which includes the spark plug end terminal resistor. A few years ago, when I measured my wires they were all 2K regardless of length, so assuming the 1K resistor in each plug terminal was within the 700-1300 ohm spec, the rest was in the wire. Also inspect the coil wire and measure its resistance. The resistor connection in the plug boots can be damaged by yanking the boots off the plugs without the proper tool, which will show up as high resistance. With 240K miles on your car I would be willing to bet it has been subjected to this kind of abuse.

The cap, rotor, and plug/coil wire harness are expensive, so you might want to try to salvage what you have as best you can.

For more background on emission testing M103 engines read the following thread and the thread it refers to. There are other diagnostics you can run and tricks you can use without spending several hundred dollars replacing parts willy nilly.

Pay particular attention to the discussion of properly managing your emission test which includes conditioning prior to the test

I would also suggest measuring the duty cycle, which is a check of the adjustment and health of the lambda system, including the O2 sensor. You can search this site under "duty cycle" to learn what it is, and how to measure it.

Familiarize yourself with these two sites.

Both have portions of the Mercedes Benz service CDs that you can view on line or download the pdfs. They can be slow and cumbersome sometimes, but the information is invaluable if you're going to work on your car. The ignition system is Group 15.


Last edited by Duke2.6; 07-28-2005 at 01:12 AM.
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Old 12-18-2006, 03:50 PM
Old School Mechanic
Join Date: Dec 2006
Location: Auburn California
Posts: 127
Duke, When I go to skinnerbox, it says I don't have permission. How do I get it. Gary I'm trying to run down a high idle on a 90 560sel that passes smog great.

And,, must have a new location.

Thanks, Gary
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