Causes for carbon buildup on valves. How you can remove them

If your engine has carbon buildup on valves, this will lead to poor engine power, its unstable operation when driving and in some rare cases even damaging the whole fuel distribution system.

Petrol engines, especially multi-point injection engines(MPI) and gasoline direct injection systems (GDI) are more likely to develop carbon deposits with the highest chances being for gasoline direct injection and less on the MPI engines.

What do engine valves do?

Before going in-depth with why carbon deposits form on the valves, we will make a short summary of the valves to better understand what their role is and the impact later on.

There are two types of valves, the intake valves, and the exhaust valves. The needed air for the fuel-air mixture enters the engine through the intake valves and after the mixture, exhaust gases exit through the exhaust valves.

So valves have the role to close the combustion chamber at some point so that the fuel-air mixture does not come out. The valves are the ones pressing the pistons which, in turn, spin the crankshaft ensuring the normal operation of the engine.

Why is carbon building up on valves?

First of all, carbon buildup on valves is natural because of how engines work. This process can be much faster depending on the age of the engine, how much you drive the car, under what conditions, and of course how you perform its maintenance.

Secondly, carbon can build up as a result of a malfunction, especially a malfunction in the timing system. Carbon deposits are formed if oil flows through the valve seals. Then it will coke, which leads to the formation of carbon buildups on valves.

In MPI engines, for example, the fuel is taken in the intake manifold where it is mixed with air, after that, the resulting mixture goes in the engine cylinders where it’s burned. In this case, the valves are “rinsing” and that is why no carbon deposits are formed because it simply “pushed away” in the air-fuel mixture.

On the other hand, direct injection systems are more prone to carbon formation on the valves. Even if these engines are much newer than MPI engines, their big disadvantage is the formation of carbon on their valves because GDI engine is designed to supply fuel directly to the cylinders because the injector is located directly in the cylinder block, and clean air is supplied through the intake manifold, thus air-fuel mixture occurs in the engine cylinder itself.

Fuel is supplied to the cylinders, bypassing the valves, and only air passes through them, which is not able to burn the fuel deposit, which eventually forms on the valve body.

What are the reasons for carbon buildup on valves?

  • Poor quality fuel. Quality fuel is responsible for many problems the engine can have, even carbon buildup on valves. Because fuel has harmful substances, in the air-fuel mixture, these substances change and will deposit on the valves.
  • Engines with EGR system. Above I have talked more about petrol engines, but diesel engines are not an exception in this case, because most of them have an EGR system. Due to its role, the EGR returns in the intake manifold a certain amount of exhaust gases, which include carbon deposits, soot, debris, and so on. All these buildups can deposit on the valve walls and the timing system valve. So, cars with EGR are more likely to form buildups on valves.
  • Oil on valves. Oil-removing caps are bad or failing. Because of this, the engine oil falls, and then settles and burns on the valve body.

Additional to the above reasons, we should mention that age and the passing of time are also the reasons why carbon builds up on valves because it’s natural that an old engine and the higher mileage it has, the more likely it is that such carbon deposits will form.

What are the symptoms of carbon buildups on the valves?

Engine power loss. You will feel a power loss especially if the car is full of luggage and you’re up on a hill. There you will notice the engine power loss possibly due to this problem.
Floating speed on idle. You will notice that at idle because an insufficient amount of air needed for the air-fuel mixture enters the engine, the RPM will be very low, and it will float leading even to an engine stop.
The car jerks when pressing the gas pedal. This is also due to a smaller amount of air in the engine than usual because of deposits on the valves
A jammed valve. Because of high buildups, no matter where they are, the valve could jam leading the engine to operate with one cylinder less. There is also a possibility that the valve will hit the piston, which will lead to very expensive repairs.
Decreased cooling. Because of these deposits, the combustion chamber cooling will suffer. This will lead to high engine temperature which results in a significant load on the cooling system, and the engine can get into an even bigger problem because coolant can mix with oil in the engine.
A burned valve. Due to engine power loss, its compression value also drops. Driving the car this way, in a depressurized chamber at the moment of ignition, the burning air-fuel mixture bends the valves, which gradually leads to their burnout and complete failure.

How carbon buildups on valves are removed?

On average the buildups form on every 80.000 – 90.000 km (50.000 – 60.000 miles) even less on GDI engines.

The valves should be cleaned from carbon deposits periodically, especially on GDI engines. You should do this without waiting for the amount of buildup to be critical. There are two ways to clean the deposits from the valves, with or without removing the intake manifold.

In the case the intake is not removed, you could use special products for this case. And, if the intake is removed, you should do this in a specialized workshop that uses the proper equipment.

You should know that using special carbon cleaners helps clean only a small amount of deposits, or you can use these agents as preventive measures. If the amount of buildup is greater, then there is no point in cleaning the carbon deposits with those products, and you need to go to a specialized service.

Recommended products for cleaning the carbon buildups on valves

Perfect for petrol cars, Liqui Moly Valve Clean can be used as an additive with any kind of gasoline. The main role of this additive is to remove buildups formed on valves, injectors, carburetors, and other elements of the intake manifold, and to protect these parts from corrosion.

It can be used even on turbocharged engines, and it’s one of the best in removing soot, deposits, and carbon buildups on valves.

Another good cleaner for the valves, especially because it is used for GDI engines which are more likely to buildup carbon deposits on the valves. It cleans very well not only valves but also other elements of the power system. Can be used with any GDI engine, including those equipped with turbochargers.

Also a good additive for cleaning not only the valves but the entire fuel system. Designed especially for GDI engines Red Line has fast results.

How to prevent carbon buildup on valves?

The best way to prevent carbon buildups on valves is to check from time to time if deposits are formed to act until the soot layer forms and becomes hard to deal with.

  • Check the valves using an electronic endoscope for a smartphone. A great way to find out if you have carbon buildups is to see them using a tiny camera with a diameter of about 4 – 10 mm. It allows you to see the inner surface of various components of the engine.
  • Change the fuel filter and oil filter regularly. An easy and preventive task for reducing buildups on valves and not only, because they help to retain chemical elements
  • Use high-quality fuel for your engine. If you add low-quality fuel, this will do more harm than good because it will help deposits form.
  • Use high-quality fuel. The same as above, using low-quality fuel does a lot of bad things for your engine, being careful with this aspect will have a benefit in the long run, even if it costs a little more.


Carbon buildups on valves are very harmful to the engine, if this problem is not fixed promptly, it could lead even to a total engine failure or very expensive repairs.

You should periodically check the presence of carbon deposits, and remove them if possible. It also makes sense to use special additives, because their main task is to prevent the appearance of carbon deposits.

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