Turbochargers on Marine Diesels: Lag, Smoke, Oil Leaks and What's Normal

Almost every marine diesel over about forty horsepower built in the last thirty years is turbocharged, and the turbo is the component owners understand least and worry about most. It spins at well over a hundred thousand revolutions a minute on a film of engine oil, it sits in the exhaust at several hundred degrees, and when it fails it does so expensively. Most of what owners report as turbo trouble is not the turbo, and most turbo failures are caused by something upstream of it. This guide sorts the two.

What it is doing

Exhaust gas leaving the engine spins a turbine wheel; a shaft carries that spin to a compressor wheel on the other end, which packs more air into the cylinders than the engine could draw in on its own. More air means more fuel can be burned cleanly, which is where the horsepower comes from. On most marine diesels the compressed air then passes through a charge air cooler, an aftercooler or intercooler cooled by seawater, which makes the air denser still and brings its temperature down before it enters the engine. The whole assembly is lubricated and cooled by engine oil fed under pressure from the main gallery and drained back to the sump by gravity, and that oil supply is what the turbo lives or dies by.

Lag and what is normal

A turbo diesel does not come on boost at idle. Push the throttle up from a low speed and there is a pause while the exhaust flow builds and the wheels spin up, then the boat picks up as the boost arrives. That lag is normal and on a heavy displacement hull it can be a second or two. What is not normal is boost that never seems to arrive, so the engine will not reach its rated full-throttle revs and makes black smoke trying. That is a restriction somewhere in the air path: a dirty air filter, a collapsed intake hose, a charge air cooler blocked with salt on the water side or oil on the air side, an exhaust restriction after the turbo, or a wastegate stuck open. An engine that overrevs with a light load but still smokes under load has a different problem again, usually propeller or injection, and the exhaust smoke guide works through the colours.

Smoke and oil

Blue or grey smoke with an oily smell at the exhaust, particularly on start-up or after a long idle, points at oil getting past the turbo's shaft seals into the exhaust or the intake. Before condemning the turbo, check the oil drain. The return line to the sump relies on gravity, and if it is kinked, partly blocked with coke, or the sump is overfilled so the drain outlet sits below the oil level, oil backs up in the bearing housing and is pushed past seals that are otherwise fine. A blocked crankcase breather does the same thing by pressurising the sump. Oil in the intake hose between compressor and engine is normal in small quantities, since the breather often vents there; a pool of it is not. Pull the intake hose off the compressor and look at the wheel: a light film is fine, oil dripping off the blades is a seal.

Noise and shaft play

A healthy turbo whistles as it spools, a smooth rising note that follows the throttle. A siren-like scream, a rattle, or a note that changes suddenly means a bearing or a wheel touching the housing. With the intake hose off and the engine cold and stopped, take hold of the compressor wheel nut and feel for play. A little side to side movement, radial play, is designed in because the shaft floats on oil; you should feel it but not hear the wheel touch the housing when you push. In and out movement, axial play, should be almost nothing. Any axial play you can feel means the thrust bearing is worn and the turbo is on borrowed time. Spin the wheel by hand: it should turn freely and silently for a couple of turns. Chipped or bent blade tips mean something has been through it, which on a marine engine is usually a piece of the air filter or a nut off the intake, and the engine is at risk as well.

What kills turbos

Oil, or the lack of it. Starting a turbo diesel and immediately loading it up before oil pressure has reached the bearing, or shutting it down hot straight after a long run so the oil in the bearing housing cooks into carbon with no flow to carry the heat away, are the two habits that take years off one. Let it idle for a minute after starting and for three or four minutes after a run before you turn the key off, which on a boat you are doing anyway while you tie up. Dirty oil, stretched oil change intervals and the wrong grade all wear the bearings, and a blocked oil filter on bypass sends unfiltered oil straight to the tightest clearances on the engine. The Volvo Penta filter range has the genuine oil and air filters, and the service kit by engine is in the service kit guide. Salt is the other killer: a charge air cooler that is not serviced corrodes through on the water side and puts seawater into the cylinders, and a leaking exhaust elbow lets water back toward the turbine.

Repair, exchange or new

A turbo with worn bearings but sound wheels can be rebuilt with a core kit, but the balance of the rotating assembly matters at these speeds and it is a specialist job. For most owners the sensible path is an exchange unit, which is a factory-reconditioned turbo supplied against the return of your old core. Volvo runs exactly that programme, and the exchange turbochargers and cores are in the Volvo Penta exchange and reman range, with new units and the gaskets, oil lines and clamps that go with them in the Volvo Penta turbocharger range. Whatever you fit, prime the oil feed before the first start, fit new gaskets on the oil lines, and find and fix the reason the old one failed, or the new one follows it. For an exchange unit against your engine's serial number, ring 0412 532 984.