Peugeot/Citroen 1.6 THP – Intake valves with combustion residues buildup

Combustion residues on the intake valves – what to do?

Direct injection engines, such as the EP6-DT and EP6-DTS by Peugeot/Citroen, suffer from combustion residue buildup on the intake valves.  This is a very common problem with all direct injection engines nowadays. Because fuel is injected directly into the combustion
chamber, it does not go through the intake manifold to clean the valve stems – as would happen with an indirect injection engine. As a result, oil recirculating through the PCV system sticks on the valves and so do combustion residues coming from the cylinders. All these oil and carbon particles create a thick film surrounding the valve and altering its geometry.

The oil residues on the valve stem build up and become solid over time. They cause a major loss of power.
The extent of the problem can be easily seen with the valves removed from the engine.

Detecting the problem before opening the engine
It is very common to see 30-40hp losses in the performance of a 150hp engine. The big problem is that through OBD diagnostics, everything looks good and the engine looks healthy. Because the valves are restricting intake airflow and the engine is not getting loaded as should. So the engine looks happy, does not complain about anything, but in reality its performance is very low.


There are two main ways to clean the carbon residues.

  1. By removing and disassembling the cylinder head, which takes time and money but makes it as good as new, without any major drama. This is by far the most expensive method.
  2. By cleaning the valves while on the engine, in the car. This is possible by using chemicals or particle blasting. Cleaning with Seafoam, Soda blasting, Walnut blasting are usual methods to do the job but require a specialized technician. People are even using brushes on them, with different levels of success.

Chemicals act as solvents and remove the residues layer by layer. Combined with brushing, they can bring good results.
Be warned, when the carbon buildup is old it becomes very solid. This makes it almost impervious to chemicals and spraying with Seafoam does not help any more because it can do nothing to that thick layer. This is when blasting the valves becomes a very efficient way of cleaning them and the only way that brings very good results.



  • Tommy Leith Posted December 5, 2018 6:30 pm

    You state that “Direct injection engines, such as the EP6-DT and EP6-DTS by Peugeot/Citroen, suffer from combustion residue buildup on the intake valves”

    Does this also apply to the other engines in the range eg:
    EP6-FDTX……..208 Gti 208 THP

    • Stelios Alexandrakis Posted January 27, 2019 12:28 am

      It applies to the whole EP family, being directly injected. Newer designs however have less gas recirculation and improved PCV systems to avoid this problem. To a degree, it is eliminated compared to earlier engines.

  • Grant Posted January 22, 2019 6:25 am

    Curious to know……. I’ve been told the high-pressure fuel pump on the 308 T then also usually goes. Is this right? Anybody with experience on this I would appreciate a response.

    • Stelios Alexandrakis Posted January 27, 2019 12:25 am

      It usually takes over 80.000km for the fuel pump to require replacement. If you drive the car mostly on highways, you could do well over 120.000km without issues.

  • Andy Posted February 21, 2019 11:45 pm

    Hi, on my sons Peugeot 208 GTI EP6-FDTX the fan stays on when he turns the engine off. Peugeot say it needs an engine de-coke at £850. It’s only 3 years old and has 21000 miles on it. There is no engine warning light on. Does this sound reasonable?

    • Stelios Alexandrakis Posted February 22, 2019 7:32 pm

      It may be an issue with combustion residues on the intake valves, or something more major.
      When the fan stays on the 208Gti, it is because of misfires being detected by the engine computer. It can be a number of causes, the cheapest of which are the intake valves.

  • Polivios Posted April 24, 2019 4:02 pm

    So do you take on cases with carbon build up and clean them or not? When you take the head apart to clean the valves do you do anything for the carbon build up on the pistons too? Then you need new head gaskets etc so more problems to occur if not done accurately.

    • Stelios Alexandrakis Posted April 29, 2019 11:30 am

      Intake valves can be cleaned without removing the cylinder head. That is what makes this process financially possible. Otherwise if the whole head had to come off, it would make more sense to service it before putting it back.

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