This bulletin applies to 2014-2018 Mazda3 and Mazda6 vehicles equipped with GDI (gasoline direct injection). If the vehicle is driven on extremely short trips most of the time (where the engine never reaches normal operating temperature), the engine oil may become diluted with fuel, and engine oil level will increase.
Engine oil dilution with fuel occurring under these specific driving conditions is normal with GDI engines. If the vehicle is driven on a longer trip after “a lot” of short trips, the check engine light may turn on, with DT P0172:00 (fuel system too rich).
Newer GDI engines are designed so that high-pressurized fuel is injected directly into the combustion chamber, which may cause the fuel to adhere to the cylinder wall. The fuel is then scraped by the piston ring and flows down into the crankcase where it mixes with the engine oil.
If the vehicle is driven in a fully warmed-up engine condition with an increased engine oil level diluted with fuel, a larger than normal amount of fuel evaporates and burns off in the combustion chamber. This causes the air-fuel ratio to become rich. If the amount of fuel coming from the PCV system increases further and exceeds the air-fuel ratio correction system limit, the system turns on the check engine light and sets DTC P0172:00.
Customers who routinely make very short trips should be advised to have the engine oil and filter changed on a more frequent basis.