What are the effects of dirty ( allot of carbon build-up ) piston heads and intake valves?

I just used an endoscope to take a look at my piston heads while changing my spark plugs and the heads had quite significant carbon build up.

So I'm curious, in general, what the common effects of this carbon build up can be.


Here's a screen capture of one of my piston crowns, it's the only one which had a bit of a clear patch:

enter image description here

2 Answers 2


Carbon Build up on the valves and the exhaust is very common for any old engine no matter how you maintain it , its the by product of the combustion of gasoline.

Issues due to carbon build-up on the valves:

  • Clogs the valves thus allowing less fuel/air to get inside the cylinder thus reducing power and efficiency of the vehicle.

  • The above point can cause an uneven power stroke causing engine knock.

  • Since fuel is not properly burned it will increase the emissions and might fail pollution test.

  • The EGR or Exhaust Gas Recirculation will now be sending the bad exhaust into the engine. increasing the carbon build up further.

    However carbon buildup is not something which affects only one parameter of a car, it can affect the air/fuel intake.

Abstracts from here shows it affects additional parameters

The carbon contributes to abnormal combustion in several ways, but the most dramatic effect in a modern fuel injected engine is the "sponge effect". As the fuel mixture in the cylinder is compressed, the carbon has a tendency to absorb both oxygen and fuel. Once the ignition spark fires, the flame front normally spreads through the chamber, consuming the fuel and air, however, the carbon has a tendency to extinguish the flame front and stifle combustion.

This, combined with the fuel and air that was absorbed, results in poor efficiency. As the chamber decompresses during the exhaust portion of the stroke, the unburned fuel and air is released, resulting in both excessive fuel and air (containing oxygen) entering the exhaust system. The O2 sensor detects the excessive oxygen and the vehicle computer (ECM, ECU, PCM) compensates for this by enriching the mixture.


In general:

  • carbon build-up (CBU) impairs the ability of the valves to properly seat, resulting in lack of compression, which leads to loss of engine torque.
  • if the vehicle is equipped with AIR (secondary-air pump), CBU in secondary air tracts prevents the emissions-control from functioning properly during cold starts. If bad enough, this can lead to the clogging up of catalytic converters.
  • exhaust tracts can get clogged over time from CBU, resulting in cold spots in the combustion chamber.
  • so carbon buildup on the valves more of a problem than on the piston heads? Aug 12, 2015 at 12:55
  • @RobertS.Barnes : In general, yes. Valves not seating properly will have a more direct consequence on performance than carbon build-up on the piston crown.
    – Zaid
    Aug 12, 2015 at 13:01
  • I've also seen carbon buildup on the back of the valve head. This would cause air flow issues going into the cylinder. Aug 12, 2015 at 13:58
  • @Paulster2 Can I imply from pistons that the valves probably look just as bad? Aug 12, 2015 at 15:35
  • @RobertS.Barnes - Unfortunately "dirty" is a subjective term. There could be a fine coat of carbon buildup which could be very normal buildup and very easily cleaned. It just may look far worse on your endoscope than it actually is. Having not seen the buildup personally, it's hard to say what the buildup might be behind the valves. You can possible use your scope to go down the intake past the throttle body and see. Usually the injectors push fuel on the back side of the intake valve and keeps it clean. The exhaust valve stays clean through the combustion process ... Both are "usually". Aug 12, 2015 at 15:59

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