Carbon Deposits – Description – Causes – Affect – Prevention – Removal

Carbon Deposits
Carbon Deposits

The black soot that collects and hardens in your engine is better known as carbon buildup or carbon deposits.

Every engine produces carbon to some level but some are worse than others.

The reason all engine have carbon is simple. Carbon is a by-product of the combustion process.

Any time you have heat, pressure, and oxygen around fuel, you’re going to get the formation of carbon deposits.

Whether its gas, diesel or even firewood in your fireplace, carbon deposits are going to form.

Carbon deposits create cold performance and fuel economy concerns long before they show up as severe drivability issues.

A buildup of carbon deposits inside the combustion chamber also increases the risk of hot spots causing engine damaging preignition.

Consequently, the problem is that the carbon buildup restricts the airflow to the engine and causes obvious problems.

That means the internal combustion engine is a perfect place for the formation of carbon deposits on:

  • Intake valves
  • Combustion chambers
  • Injector nozzles
  • Pistons
  • (EGR) valves

All can rob the system of power and performance.

Problems From Carbon Deposits

Carbon buildup in engines can cause all kinds of problems in both port injected and direct injection engines. If the carbon buildup is in the combustion chamber, it disrupts normal airflow causing turbulence. That turbulence causes the air and fuel to mix unevenly which means you’ll have areas of rich and lean mixtures. As a result, creating hotspots in the combustion chamber, forming even more carbon buildup.

When carbon builds up in the car’s engine it can cause a couple different problems.

Sticking Valves From Carbon Deposits
Sticking Valves From Carbon Deposits

First of all, the engine will lose power. One of the main problems carbon buildup does is change the ratio of fuel to air in the engine. Consequently, this can happen if sensors stop functioning properly because of carbon buildup. Also, if buildup keeps the intake valve from closing properly. This can make the engine sluggish and cause it to stall. Carbon buildup can also significantly degrade fuel efficiency, causing the car to get fewer miles to the gallon.

So, there are some things that will cause carbon to buildup faster. One is filling the tank with low grade fuel, which is less purified and usually has more contaminants. If the ratio of fuel to air is off, as mentioned above, carbon will accumulate faster. Using the car primarily for short trips can also increase problems with carbon buildup.

Consequently, the engine doesn’t run:

  • Long enough
  • Hard enough
  • Hot enough

To burn off any of the carbon.

Where Do Carbon Deposits Form

Depending on the areas where they form, these deposits can have different effects on the operation and performance. Deposits in the combustion chamber are almost inevitable. It can take just a few hundred hours of operation to see deposits in the combustion chamber.

Carbon Deposits On Injector
Carbon Deposits On Injector

Deposits on injectors happen for mostly the same reasons. Sometimes small amounts of fuel remain in the injector tip. After the engine is shut down the heat is still there.

Basically, the fuel will get “slow cooked”, polymerizing and reacting with oxygen to finally form carbon deposits. Deposits that form on the intake valves can restrict airflow through the intake ports.

Which causes a loss of high speed power. The deposits also can act like a sponge and momentarily soak up fuel spray from the injectors. Deposits can also cause valves to stick or even burn.

This disrupts the mixing of air and fuel:

  • Causing a lean fuel condition
  • Hesitation and reduced performance

Common Symptoms Of Carbon Deposits

  • Hard engine starts
  • Rough cold idle
  • Decreased acceleration
  • Engine misfires
  • Black exhaust clouds under hard acceleration
  • Check engine light comes on

Direct Injection Engines Are Prone To Carbon Buildup Problems

On gasoline direct injection engines with carbon buildup on the valves, you need an entirely different approach. Fuel injector cleaners will not work on that kind of carbon buildup because the fuel never touches the valves.

In a direct injection engine, the fuel is sprayed directly into the combustion chamber. So, the back of the intake valves never get cleaned.

How To Slow Down The Accumulation Of Carbon Deposits

Fuel injector cleaners can also help injectors maintain the correct spray pattern. This can ensure that the droplets have the correct size and distribution during ignition. In addition, extra detergents can help to break down deposits.

One of the most effective methods for preventing a carbon buildup problem is updating the engine management software. New software can reduce carbon deposits by the proper adjustment of valve and spark timing.



Though carbon is going to form, you can limit its creation by:

  • Using name brand fuel that contains a fuel system cleaner
  • Limiting idling time and cold starts
  • Employing high quality oil
  • Keeping the carburetor/fuel injection system tuned properly

Another contributing factor is constantly evolving technology. As engines become more efficient and able to squeeze more power from smaller displacements, things heat up even more. So, by playing with the air/fuel mixture, timing and combustion pressure, carbon build up is not going away soon.

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