I want to add right away – for my cars (for the last ten-plus years, I have driven BMW DI petrol engines), I have not noticed any problems with the contamination of the inlet manifold.
It is possible that the driving style is “to blame” – I drive aggressively enough. Accordingly – the airflow in the inlet channels “cleans” the valve stems and walls of the inlet manifold.
Yes, I have seen images of the contaminated inlet manifolds on the internet. It is possible that these engines had a different driving profile.

In this entry, I draw your attention not to the cleaning methods (the most popular – with the walnut shells) or technological nuances (I leave them to the work performers) but – to some myths.
The main thing to understand is that a clogged inlet manifold worsens the air intake to the cylinders in the max load mode. So – in the range of max required torque and max RPM (both parameters should be max simultaneously).
If at least one parameter (required torque or RPM) is not on the max range, the clogged inlet manifold will have NO impact!
In the max power mode, a contaminated inlet manifold will be one of the causes of the too-rich fuel mixture. The contaminated inlet manifold CAN NOT be the cause of the lean fuel mixture! In the case of a lean fuel mixture, the fuel is “missing” not the air!
Instead, in all other driving conditions (low .. average torque; low .. average RPM; high torque and low .. average RPM; high RPM and low .. average torque), the clogged inlet manifold will not cause any problems.
Clogged inlet manifold will NOT BE to blame for:
a. uneven performance of the engine; incorrect fuel mixture at those modes;
b. misfires, turned-off cylinders;
c. incorrect individual fuel mixture of cylinders.
I know several cases in which such cleaning has helped to prevent misfires and shivering of the engine. At least, that is what the owners of the cars claim. What is the truth? What were the problems before the “repair”? Error messages regarding lean fuel mixture, Offset type LTFT – on max. What did the owner of the car do? Cleaned the inlet manifold, replaced all gaskets, and cracked the CCV pipe. Prevented unregistered air intake. Exactly, the prevention of the excess air intake solved the problem, not the cleaning of the manifold.
Of course, no one who wants to clean the inlet manifold/receive service is refused. Even if the servicing company is aware that there are other problems, this is the source of profit. Even more- typically, the customer does not ask to perform the diagnostic (and the manifold cleaners are not the diagnostics specialists). The customer asks to clean the inlet manifold. It is sadder if such a “solution” is recommended by the diagnostics specialist. My suggestion – if the diagnostics specialist recommends such a “solution” in case of uneven engine performance – is to change the diagnostics specialist!
And finally. On the internet, you can find numerous videos in which the owner of the car, after such cleaning, turns on the engine and starts to drive aggressively. In my opinion, this is the most stupid thing to do! In the driving conditions in which the cleaning has an impact (max power), now/after cleaning, the cylinders have more air. If previously DME has adapted a correct fuel mixture, now the fuel mixture can become lean. Lean fuel mixture in the mode of the max load means knocking. Accordingly – there is a large risk of damaging the piston group!
After such cleaning (even more – if the whole inlet manifold was dismantled, many mechanical and preventive works were done):
a. the re-adaptation of the engine should be performed;
b. new fuel adaptations and fuel mixture (Lambda and STFT) should be evaluated; the engine’s power should be increased gradually.