Citroen C5 1.6 THP – Before and after cleaning the intake valves


We can never stress the importance of cleaning the intake valves frequently on THP engines. Even more so, when the car is to be remapped. The engine has to be completely healthy to produce more power, or the problems will come back to hunt you and cause big failures.

While the owner had suspected his C5 was underperforming, we got a better idea of what was going on when we tested it on dyno. In theory these engines produce around 150+hp at the flywheel when everything is working properly. As we found out, the engine was producing only 135hp with 220Nm of torque and behaving badly during acceleration.

Citroen C5 1.6 THP - clogged intake valves

Citroen C5 1.6 THP – clogged intake valves

As combustion residues and oil starts piling on the intake valves, performance drops more and more, day by day.
This is the comparison between clogged intake valves and having them cleaned. The difference is crazy!

Black lines – power and torque with clogged intake valves (stock map)
Blue lines – power and torque with cleaned intake valves (stock map)

Citroen C5 1.6THP - Clogged intake valves vs cleaned intake valves

Citroen C5 1.6THP – Clogged intake valves vs cleaned intake valves

The low end hesitations that were a major problem are completely gone. Maximum horsepower after cleaning was 152hp and torque 245Nm, as should be.


  • Antonis Marinos Posted March 1, 2017 2:06 pm

    Some comments on this issue, since this is my car and I’ve been researching the whole “direct injection carbon build-up” issue for some time now.

    This is common to almost all DI engines, and of course THP/Prince engines, since this is an inherent problem to DI engines due to the way the injection works. Newer versions of these engines have tried to combat or remedy the problem using various methods, with varying degrees of success. However, for most older DI engines, this is an unpleasant reality, and needs to be addressed by the owners.

    Most authorised dealers will either ignore the problem, or wait until a CEL and an error code pops up. Most car companies have acknowledged the issue and have issued several Technical Service Bulletins. For instance, Citroen has issued TSB B1AW0011JQ0, which directs the dealer to perform a cleaning process using a specific cleaning spray. Problem is, by the time the car has met with power-loss issues and CELs, the carbon build-up is beyond any cleaning agent’s capacity for decarbonising and cleaning the intake and valves. In this case, only blasting (for medium cases) or dissassembly and manual cleaning (for serious cases) will work.

    In my case, since indeed I was puzzled by my C5’s performance, I had already performed the cleaning spray process before, obviously with little or no effect. After seeing the initial dyno (the black lines on the chart), I decided to perform the blasting method, and more specifically, soda-blasting. The results and improved performance were noticeable almost right away, but it was only when the car was dyno-ed, that we saw the difference that the cleaning process made on my THP!

    I’ve also already install a catch-can (OCC) on my C5, in order to reduce the engine blow-by that gets re-directed into the intake, and is one of the few things that every DI-engine owner can do to battle this problem.

  • gidea75 Posted June 12, 2017 5:39 pm

    Very interesting! Which was the mileage before you made the cleaning? My car is about 50k kilometers run in 8 years and fortunately, it has none of the common problems known on the thp engine. Since i’d like to keep this car as long as possible and look for a remap, i’d like to have an idea on “how urgent” is the valve cleaning to me.

    • Antonis Marinos Posted August 30, 2017 3:50 pm

      I had noticed the reduced power output and overall performance already even at 50K km, and it became really noticeable at 60K km, even though I had already performed the procedure suggested by Citroen’s TSB. The blasting process was performed at around 65K km, and coupled with the Stage1+ remap, it offered a h-u-g-e boost of power and performance. Like I say nowadays, this is how the car should have been released from Citroen.

  • Graeme Sears Posted September 14, 2017 5:54 pm

    How do you go about cleaning the intake and valves? Is there some chemical that needs to be sprayed in? The dealers want to charge me a fortune for this.

    • Stelios Alexandrakis Posted January 27, 2019 12:29 am

      Yes, that is perfectly normal.

  • NicktheSick Posted October 13, 2018 5:08 pm

    Would it be normal for an already mildly remapped car with a hybrid turbo to face a decrease of boost at around 0.2 bars after it had its valves blasted?

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