I was just inspecting some plugs and noticed that on each plug, one side was relatively cleanish, while the other side was dirtier and had built up deposits.

Why would one side be significantly dirtier than the other?

  • What type of engine is it? Do you have injectors in the intake manifold or directly in the cylinder head?
    – Elias
    Oct 2, 2015 at 15:45
  • Could also be that someone only changed the plugs on one side and not the other.
    – barbecue
    Oct 2, 2015 at 20:20

2 Answers 2


If you are running rich (like I believe you said you are in another thread), you'll experience carbon build up. There could be two reasons I can think of which might attribute to the carbon build-up just being on one side:

  • Plug shrouding - If the carbon build-up seems to be located on the same side as the ground electrode, the electrode itself may be helping the carbon to buildup due to shrouding the flame front when the spark plug ignites the air/fuel mixture.

  • Direct fuel buildup - (Note: I don't have a better name, so just go with it.) The carbon buildup area could be due to where the spark plug sits in the cylinder in relationship to the air intake path. If the spark plug sits in the path of the air as it enters the cylinder, it could leave a residue on the spark plug. If there is an excess amount of fuel which is not being burnt, this is the area which would be left with the carbon build-up.

  • 1
    I thought of another possible culprit - I homebrew seafoamed it recently and I figure that might have cleaned one side of the plugs. Oct 3, 2015 at 16:46
  • 1
    Very good conclusion. Oct 3, 2015 at 18:09

This situation is common on certain Honda 3.5L engines with VCM, because one bank is deactivated regularly for fuel economy reasons.

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