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.

      • Giannis Skiadas Posted August 10, 2019 12:27 am

        Exactly this, included all line of VW/Audi TFSI turbo direct injected engines and other new engine generation with this kind of latest technology.

  • 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.

      • adrian Posted July 29, 2019 2:47 pm

        My intake valves have been cleaned but misfires still detected.
        what next coils? happens after cool start, warning light on.
        208 thp 156.
        thanks for any help.

        • Stelios Alexandrakis Posted July 31, 2019 11:55 am

          I really do hope you have replaced your sparkplugs after the valves were cleaned. They usually get fouled during the process.

  • 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.

  • Martin Posted July 12, 2019 2:03 am

    Having just bought a 208 GTi with 57K on, I need some step by step instructions on how to remove the inlet manifold so I can clean the intake valves. Do you know where I could find this information? Can’t find anything on YouTube about how to actually take off the manifold.

  • Daniel Garay Posted August 20, 2019 11:43 pm

    Does anyone have information about the fault code u1208, p1161, p2101, p3010, and faulty anti-pollution system message? the car loses a lot of power after the message only does it once it reaches its working temperature, please velo

  • Ralf Moritz Posted January 31, 2021 1:25 pm


    Whats about Water/Methanol Injection. Will this not work? I ride it on my Nissan 300ZX Twin turbo and on my Mitsubishi L200 Arctic Truck. Intake valves on this both are looking Brandnew.

    • Stelios Alexandrakis Posted February 7, 2021 9:58 pm

      Ralf, neither of your cars is of direct injection technology. This problem only affects engines that have injectors inside the combustion chamber, because fuel is not sprayed onto the valves. In older, indirect injection engines, the injectors are constantly spraying the valves with fuel so there is no combustion residues on them. Always clean like brand new.

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