It is clear, that the engine with direct injection has problems with carbon residues in the intake manifold because the fuel (as for “usual” engines) does not clean’s it.
Various sources of information (basically companies, which business is cleaning the inlet manifolds) on the internet are promoting the information, that these residues can decrease the power of the engine, cause uneven performance and also misfires.
Can the carbon residues decrease the max power of the engine? Yes, because they decrease the cross-sectional area of the intake manifold and make the surface of it uneven – the air resistance increases.
Can the carbon residues cause uneven running and misfires? Only in case, if/yet the residues are causing problems with valve tightness. If the valves are closing good, I don’t see any reason, why uneven running and misfires could appear.
The carbon residues are “growing” slowly, MSD will modify the position of the throttle (mostly – offset adaptation), if necessary, change the fuel map (if needed) for high RPM and BMAP to reach the perfect proportion of the fuel mixture in the cylinders.
Moreover, as the inspection of many N43/N53 shows (even with mileage above 200’000 km) – the carbon residues are not the huge problem. The much bigger problem is residues of burned oil!
The image from the internet:
This is oil coating! And, looking to the color and texture, it look’s that wet oil has been sucked in the inlet system – for that, we can say “thanks” to crankcase ventilation system, not EGR.
Of course, when the cleaning of the engine is performed (when, usually, also the inlet manifold is taken off and opened, very often – the engine “head” and pistons are also cleaned), also the real defects are unintentionally repaired.
For example – preventive replacement of the crankcase ventilation system or cleaning the VANOS valves. If after these procedures the error messages and old adaptations are deleted – no wonder, that the performance of the engine becomes more normal. But not the carbon residues are the cause of uneven running – it’s crankcase ventilation, VANOS valves or some other defect.
I don’t deny, that, if significant residues are detected, the cleaning of the inlet manifold and pistons has to be performed. But we have to understand, what kind of residues are present – is it carbon or oil, and also we have to understand, that there could be also other causes of uneven running!
As mentioned here, uneven running can have around 20 different causes, and you have to have significant experience and tools to detect them.