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Old 08-10-2003, 08:57 PM
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Join Date: Mar 2001
Location: Accokeek, MD
Posts: 683
Need sugestions for reducing Nitrogen Oxide in 1987 300E

I followed previous advice and looked for vacume leaks but the 1987 300E has only a few small vacume lines and all were OK.

If the emission reading was a point high because of lean fuel mixture... then what else can I do to bring the Nitrotren Oxide down a wee bit? Would increasing the octane or running the car for about 15 miles just prior to the emission test help in any way?


1993 190E 2.3
2000 Toyota 4x4 Tundra
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Old 08-10-2003, 10:11 PM
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smoke gets in your eyes
Join Date: Jun 2000
Location: SF Bay Area
Posts: 20,119
Have you tried a bottle of that guarranteed-to-pass-smog stuff?

95 S420
91 300SE
87 300SDL
83 300SD
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Old 08-10-2003, 10:11 PM
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Join Date: Aug 2001
Location: Evansville, Indiana
Posts: 8,150
You need to check the fuel mixture -- see Steve Brotherton's excellent description in the "DIY" section on this site.

Don't forget the idle control valve hoses, and make sure that all the things hooked up to the vacuum also hold vac, not just that the hoses are good!

1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!
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Old 08-10-2003, 10:27 PM
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Join Date: Jan 2002
Posts: 18,353
In Denver, because of our air pollution problems, a cleaner burning fuel is sold in the wintertime. I always made a point of making sure I had that fuel in the tank when I went for an emissions test. An experienced tech or engineer could tell you whether such a fuel would have any effect on NO readings.
I also religiously change the oil and air filters before an emissions test but I think the oil change only affected the hydrocarbon readings.
Your engine should be hot befoe taking the test. In the UK, they measure the temperature of the oil before the test.
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13
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Old 08-10-2003, 11:01 PM
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Join Date: Mar 2003
Location: Southern California
Posts: 2,055
Diagnosis would be easier if you posted the entire test results and the type of test, but if they tested NOx it was probably a loaded test on a dyno.

NOx is produced by peak combustion temps, but should be fairly low if the 02 sensor and KE system are maintaining the proper stoichiometric mixture.

Retarding the timing reduces peak combution temperature, and there have been many discussions on replacing the resistor on these cars to alter the timing. I recall that removing the resistor and shorting the pins will result in the lowest spark advance.

Prior to having the car tested you should always be certain that it is fully warmed up and the converter is hot as possible. If the converter cools off too much is is not as effective, and this appears to be a problem with the 103 engines, but it usually results in excess HC.

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Old 08-10-2003, 11:42 PM
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Join Date: Mar 2001
Location: Accokeek, MD
Posts: 683
Thanks for the replies. I had asked my wife to take the car in for the test and I now realize that she took the car in after she had parked it for a doctors appointment and then drove about three quarters of a mile to the emmision control station.

My guess is they tested a relatively cold engine and cat converter.

Looks like I better take it through again... this time myself and hot!


1993 190E 2.3
2000 Toyota 4x4 Tundra
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Old 08-11-2003, 08:42 AM
Posts: n/a
Gas additives are worthless for 'improving' emissions test results, even if a vehicle is in fuel control; they may increase HC readings.

if when you retest the NOx is still high, check the temp sensors for accurate function, timing, and cat.

If HC and/or CO readings are marginal passes, check O2 for slow switching.
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Old 08-11-2003, 10:05 AM
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Join Date: Jul 2001
Location: SoCal
Posts: 1,303
Reducing NOx emissions pre-cat is the job of EGR. A plugged EGR tube at the intake end is common with these engines, I hear. I would disconnect the tube and examine the port.

'91 MB 190E 2.3
'08 RAV4 Ltd 3.5
'83 Lazy Daze m'home 5.7
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Old 08-11-2003, 10:09 AM
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Most 87 300E's do not have an EGR. mine does'nt, I believe only California emission compliance vehicles did.
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