MAHLE's damage brochure says that when an engine starts using more oil and blowing blue smoke, the turbo "should definitely be included in the root cause analysis". Included, not convicted. Garrett goes further: "Changing the turbo may not cure the problem, as the oil leak is very often just a symptom of some other problem on the vehicle."
A turbo has no rubbing oil seals
Garrett gives three reasons a turbo cannot use the lip seals found on a crankshaft: the shaft speed, the heat, and the generous shaft clearances a turbo needs to run properly. It seals with pressure instead. Under most running conditions the pressure in both the turbine housing and the compressor housing is higher than in the centre housing, so gas tends to push inwards rather than oil outwards; Garrett calls this a dynamic, differential pressure sealing system, because the spinning shaft plays its part too. Piston rings sit at each end, and Garrett notes that at the turbine end they are compressed into place during assembly and never change position. Oil leaving the bearings is flung outwards by oil throwers and drains back to the sump by gravity.
Two details follow. Garrett's training notes point out that the oil inlet is much smaller than the drain, so the turbo depends on the drain staying clear. And anything that pressurises the centre housing, or pulls a vacuum at either end, pushes oil the wrong way past the rings.
Common causes
A restricted oil drain. MAHLE lists a return line clogged with carbon or kinked; the carbon can come from missing heat shields, poor routing, hot soak, poor oil or liquid sealant. Garrett adds drains that are too small, do not run continuously downhill, or enter the sump where oil flung off the crankshaft backs up the pipe.
Crankcase pressure. Heavy blow-by past worn pistons and rings, or blocked crankcase ventilation, pressurises the sump and so the drain.
Too much oil. MAHLE notes that with the oil overfilled the drain cannot empty, and the crankshaft whips the oil into foam.
A restricted intake. Garrett links compressor-end leaks to an air filter that is too small or clogged, and to intake tubing that is too narrow or has too many bends.
Other causes. Garrett also lists excessively high oil pressure, a turbine piston ring collapsed by excessive exhaust temperature, and a turbo tilted too far on its axis. It warns that prolonged idling can create a vacuum in the turbine.
A genuinely worn turbo. Contaminated oil wears the bearings until the shaft is no longer controlled, which Garrett's training notes say usually brings a sharp increase in noise and oil leaking past the seals.
Symptoms
MAHLE lists blue exhaust smoke, oil collecting in the intake and charge air cooler, loss of power and, on variable-geometry turbos, coked guide vanes. It also flags a safety risk: oil pooled in the charge air cooler can be drawn into the engine and burnt, causing what MAHLE calls "uncontrolled overspeeds" of the engine.