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Old 05-31-2015, 05:31 PM
Join Date: Feb 2006
Location: Ventura County, CA
Posts: 383
w126 300sel spark plug gap

Own a 1988 300sel (w126) and would like to know the recommended spark plug gap.

Currently using NGK no resistor plugs and Bosch wires.

Spark plugs are currently gapped at .032 in.

I do not have the smoothest idle.

Recommendations greatly appreciated

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Old 06-01-2015, 09:50 AM
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Join Date: Mar 2003
Location: Southern California
Posts: 2,281
If the car still has the "emission label" on the top radiator support the gap is listed at .032".

A little idle uneveness is typical of the M103 I-6 likely due to valve overlap and uneven fuel distribution to the cylinders, which increases with age due to wear of the injector chatter valves, and this is often seen as increasing HC emissions with age.

...somewhat the nature of the beast, and probably a reason why Mercedes eventually abandoned the CIS-E system and went with a more industry standard electric solenoid injection system.

Idle roughness and HC emissions can be a lot worse if resistor plugs are installed.

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Old 06-01-2015, 11:14 AM
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Join Date: Dec 2009
Location: Shelbyville, Tn
Posts: 1,907
I set my M103 plugs at .032 and was not happy at all. Went to .035 and the idle was much smoother.

1985 300D 198K sold
1982 300D 202K
1989 300E 125K
1992 940T

"If you dont have time to do it safely, you dont have time to do it"

"The democracy will cease to exist when you take away from those who are willing to work and give to those who would not."
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