The importance of decarbonising before assembly

Decarbonising engine parts before reassembly is vital

Here at ETP, we've seen countless times the sheer volume of carbon build up in the intake manifold and ports of diesel engines that have EGR systems installed. 

Carbon in the crankcase oil is no new thing, it's been a problem for a long time. However with newer common rail diesel engines particularly turbo charged ones that are fitted with EGR valves, the carbon is building up in the inlet tract at a worrying rate. 

Often called 'Blow-By' gasses, the carbon comes from oil that is suspended in the excess crank case pressure. The PCV (Positive Crankcase Ventilation) valve opens and exhaust gas from the EGR (Exhaust Gas Recirculation) valve mix together, mixing soot with suspended oil, in the intake system. 

It's at this point that they combine into a wet sticky mess which sticks onto the insides of the intake system, particularly the manifold and around the ports. 

It's often seen that short trips 'around town' make this worse because of the lower temperatures, the oil does not get hot enough to remain a vapour, thus is sticks to the inlet tracts. Longer highway trips tend not to see this problem as much. 

Whilst the sticky mess in it's own right isn't a huge problem, the problem occurs when the build up hardens over time. It becomes brittle and can break off. When this happens it will go through the combustion chamber. This is the real problem. It will get lodged in the piston ring lands or stuck between the piston crown, valves or cylinder head. All of which is a problem!

When re-assembling a motor, this excess build up must be removed completely from the intake system. When you're refitting a cylinder head this is particularly important as failure to do so will lead to engine damage or at the very least reduced performance. 

Sadly, there aren't really any short cuts when it comes to removing this carbon. It's a fairly manual and labour instensive process. You may use a hot wash only on manifolds that are completely metal. Any plastic components will get damaged in a hot wash, so ensure that your manifold has no plastic in it at all. 

Subaru make a good product for cleaning carbon, it's called SA459 Upper Cylinder Engine Cleaner. It's been used often and seems to be the best product to loosen carbon deposits. To get into the tight spaces, use a bottle brush. 

This should come as no surprise, but to avoid any major issues with replacement cylinder heads, ensure the manifolds are properly cleaned prior to reassembly.