Free running & lubrication issues
In an effort to reduce frictional energy losses, GDI engines incorporate low-tension piston rings (pictured) that exert up to 60% less force on the cylinder walls than older port-injection types. These rings are placed under higher pressures, quite literally, by higher GDI compression ratios, which, typically, have increased from approximately 9:1 to 14:1. While these rings function as designed when new, compression can leak past them at a surprisingly young age. BG Products told us that its tests in the US saw a Ford 2.0-litre GDI lose 8% compression after 11,000 miles and a Mazda3 shed 14% of its specified compression in 36,000 miles.
Oil thickening is one reason given for the oil behind the piston rings hardening, which restricts ring function, promoting further compression loss and continuing the downward spiral.
Degradation of the engine oil is another issue. As temperatures within a turbocharged GDI engine are relatively high, the risk of oil vaporisation increases. Sludge-like deposits can be left in cooler regions of the engine, such as within the top engine cover (look for black sludge under the oil filler cap), or within the intake tracts, as mentioned earlier. As with all engines, combustion gases that leak into the crankcase reduce the oil’s effectiveness further. The resultant blow-by increases the quality of oil mix entering the intake via the PCV, hastening deposit build within the inlet.
Modern synthetic oil blends are designed to resist vaporisation to an extent, but low sump capacities, extended service/drain intervals and oil with incompatible specifications make matters worse. When buying and maintaining a car, therefore, pay attention to blow-by, performing compression tests if necessary. Check the condition of the PCV valve. Use a good quality engine flush that will help remove any hardened oil from behind the piston rings, which can restore much of the lost compression; some flushes promote GDI compression restoration on their packaging. Additionally, reduce the manufacturer service intervals and conduct frequent oil changes.