Loss of boost or limp mode
When boost pressure strays from what the engine control unit asks for, the engine logs a fault code, commonly P0299 for underboost or P0234 for overboost, and may drop into limp mode: a reduced-power mode that protects the engine and turbo. The turbo should not be the first suspect. Holset’s list for an engine that lacks power starts with a dirty air cleaner, restricted intake or exhaust, a cracked flange or missing gasket, and fuel pump, injector or valve timing faults, and reaches a damaged turbo last. Split hoses, deformed sealing rings and leaking intercoolers all lose boost, and so does an exhaust leak ahead of the turbine. A turbo starved of oil can also fail to reach full speed. The guide to a turbo losing boost goes further.
Blue smoke or oil leaks
A turbo has no rubbing lip seals. Garrett explains that it relies on piston rings and on pressure differences: the compressor and turbine housings normally sit at higher pressure than the centre housing, and oil drains out of the centre by gravity. Anything that slows the drain or raises crankcase pressure can push oil past the rings, such as a kinked, blocked or carbonised drain pipe, a blocked crankcase breather, heavy blow-by or an overfilled sump. At the compressor end, a restricted air filter or intake can draw oil past them too. Garrett cautions that a new turbo may not stop a leak, because the leak is so often a symptom of a fault elsewhere. Worn turbo bearings let oil past as well, and blue smoke can also come from worn engine pistons and rings, which MAHLE advises checking. Oil pooling in the intercooler needs investigating: MAHLE warns it can be drawn into the engine and burned, making the engine overspeed. More in turbo leaking oil.
Whistles, whines and grinding
A healthy turbo makes some noise; a change in the noise is what matters. MAHLE and Holset both point first to leaks: a whistle that arrives with a loss of boost is often charge air escaping from a hose, joint or damaged intercooler, or exhaust gas escaping from a gasket before the turbine. A whine or siren that rises with engine speed is more serious. Garrett describes bearing wear from dirty oil producing a sharp rise in noise, followed by oil leaks and smoke, by which point the turbo may have little life left. Grinding or rubbing suggests a wheel touching its housing or damaged blades, often from debris. Holset’s advice is not to keep running an engine whose turbo is noisy or under suspicion. See turbo noises.
Shaft play
Some movement in the shaft is designed in. Garrett and Holset both note that the running clearances of a healthy turbo are often mistaken for worn bearings. The useful test is whether either wheel has touched its housing. If one has, look for the cause in the oil supply before fitting anything.
Sticking vanes
Many diesel turbos control boost with a ring of moving vanes around the turbine. Carbon and coked oil can jam them, causing boost faults in either direction, and MAHLE notes that vanes stuck in their low-speed position can let the turbo overspeed as revs rise. A new actuator on its own leaves the jammed vanes in place, and Garrett warns against moving its electronic actuators by hand, which can break their gears. See sticking turbo vanes.
