Turbocharger oil control and turbocharger oil leakage are commonly misunderstood topics. Primary oil control in a turbocharger is accomplished via oil slingers machined into the compressor and/or turbine end of the rotating assembly that sling oil off of the assembly as it rotates. The oil that is slung from the rotating assembly is collected by the center housing and funneled to the turbo oil drain. Therefore, the turbocharger center housing must be oriented such that the turbo oil feed and turbo oil drain passages are completely vertical or within 15° of a vertical position with the turbo oil feed at the top. If the center housing is improperly positioned, the oil will not be able to exit the center housing properly causing the oil level in the housing to rise to the level of the seal areas and the turbocharger will "leak oil."

Similarly, the turbocharger oil drain system, if improperly designed, will also cause the turbocharger to “leak oil”. To prevent this, the turbo oil drain system must be designed according to the following guidelines.

1. The turbo oil drain system must be constructed with a reasonable inside diameter, typically no smaller than the turbo oil drain hole in the bottom of the center housing. For example, a turbo oil drain system designed for a Garrett T3/T4 turbocharger should maintain a 7/16” minimum inside diameter at all points in the system to facilitate proper turbocharger oil control.

2. The turbo oil drain system must maintain a continual downward slope free of kinks and sharp bends from the turbocharger to the engine penetration (i.e., engine block or oil pan). No “traps” are allowed in the turbo oil drain system.

3. The turbocharger oil drain system must return into the engine above the engine oil level. The turbo oil drain may not be connected to the oil pan drain connection.

The turbocharger “seals” play a secondary role in turbocharger oil control since the oil slingers are designed to prevent oil from reaching the seal areas. The primary role of the turbocharger seals is to prevent external gases (i.e., boost pressure and/or exhaust gas) from entering the center housing. Under most operating conditions in a blow-thru configuration, the gas pressures in the compressor and turbine housings are equal to or greater than the gas pressure in the turbocharger center housing (i.e., engine breather pressure). In this configuration, the pressure on the turbocharger seals is from the outside toward the inside and is actually trying to push any oil near the seal areas into the turbo.

If the engine breather system has been modified so that the engine crankcase cannot breathe and is causing excess crankcase pressure, this pressure is transmitted to the turbocharger center housing via the oil drain tube. In this situation, the crankcase pressure in the turbo center housing may exceed the compressor and/or turbine housing pressure; therefore, the pressure on the turbocharger seals is from the inside toward the outside and the crankcase pressure will push oil out of the center housing into the compressor and turbine housings. Excess crankcase pressure may be caused by engine breather modifications such as improperly installed performance breathers/catch cans or by worn or damaged engine components (i.e., pistons, rings, or cylinder walls) that result in severe combustion gas blow-by that pressurizes the engine crankcase and turbocharger center housing.