As part of an effort to reduce exhaust emissions, The most effective method of reducing NOX emissions is to lower the combustion temperature inside the cylinder. The main method employed by every manufacturer is to add some exhaust gases as inert gases inside the cylinder, reducing the amount of oxygen available for combustion during the power stroke, thereby lowering the combustion temperature.
While this method is effective, it also means that – especially for diesel engines – there is a gradual build-up of sooty carbon in the intake manifold and EGR valve. This issue is especially pronounced in vehicles fitted with an EGR cooler.
The main issue is that the crankcase breather hose provides a sticky film of oil inside the manifold, creating the tacky surface for carbon to stick to and build. In nearly every intake manifold, there is an elbow below the EGR valve, before the plenum, that runs into the head. Given that carbon has more weight than air, it also carries more momentum, and is therefore less able to make the turn around the elbow bend. As a result, it is always the cylinders closest to this elbow that receive the most build-up. As an example, the bend in the Hilux 1KD is next to the firewall, and therefore it is cylinders 4 and 3 that will show the most pronounced build-up, and the most pronounced oxygen restriction.
With this carbon material blocking up to 75% of the oxygen supply, and with this blockage significantly worse for some cylinders than for others, this definitely causes issues with engine performance.