Sump oil level rising on the Mazda 5 and 6 diesel engine is a very common issue, and can be a good indication of the condition of your DPF (Diesel Particulate Filter). But first lets explain what a DPF is and how it can affect your oil level.
A DPF is a filter fitted to the exhaust system of the car, as exhaust gasses pass through the DPF filter soot particulates are trapped inside the honeycomb structure. As exhaust gas temperatures increase the captured soot is combusted leaving behind ash as a waste product.
If exhaust gas temperatures don’t reach a high enough level due to frequent short journeys at low speed the ECU can place the engine into regeneration mode. During regen mode the ECU will conduct post injection, this is the process of injecting extra diesel into the combustion chamber after the main injection cycle. Post injection helps to increase exhaust gas temperatures however much of the injected diesel is left unburnt and filters its way down to the oil sump resulting in oil level rise.
Oil level rising on a regular basis could indicate that the DPF is coming towards the end of its life and therefore the engine is entering regen mode on a regular basis in order to reduce the DPF saturation level.
What can you do to prevent it?
Long motorway journeys on a regular basis will help to combust the soot inside the DPF without the need for the ECU to initiate regen mode. However this isn’t always practical for those who live in the city or countryside and therefore DPF removal is the only guaranteed solution to prevent future regen and the associated oil level issues.
Can oil level rising cause problems?
Yes, engine oil diluted with diesel is a poor lubricant, premature engine wear will occur and engine life will be reduced. Furthermore, if the oil level was to rise too high oil seals can be damaged and in the worst case scenario oil could be drawn into the intake system which would lead to total engine loss if this wasn’t caught early.