A generic 2.0 litre, four-cylinder common-rail diesel of the kind found in many family cars and vans: 81 mm bore, 95.5 mm stroke, 16.2 : 1 compression, 110 kW at 4000 rpm and 340 N·m from 1750 rpm. Its variable-geometry turbocharger hangs on the exhaust side, facing you; the charge-air cooler sits on the far (intake) side. Sizes follow common designs rather than one maker’s engine. The cutaway is on: it opens the near half of the block and head, and cuts the turbo, air box and filter through their own axes.
Parts
Block and bottom end
Cylinder head and valve train
Fuel injection
Turbocharger
Air and exhaust path
Step 0 of 29
Fully assembled
Step through the teardown in the order a repairer would take it apart.
Drive up a hill · crank drawn 50× slower, turbo 2000× slower
Starting…
Engine speed–
Boost–
Turbo speed–
Torque–
Fresh air
Hot compressed air
Cooled charge air
Fuel at up to 2000 bar
Exhaust gas
Filtered exhaust
Oil
Coolant
Heat
Fault · months of driving compressed into 20 s; shaft movement drawn 10× larger
Turbo bearing failure and runaway
Starts in the turbo’s bearing housing after hot shutdowns · spreads to the wheels, seals and the engine itself
0.0 s
Oil at turbo–
Bearing oil–
Shaft play–
Engine speed–
1A hard drive, then straight off
After a long climb at full power the turbine housing is at about 700 °C and the turbo spins at 180 000 rpm. The driver stops and switches off at once. The oil pump stops with the engine, so no fresh oil flows through the bearings, but the turbo coasts on for a few seconds and the hot turbine side keeps pouring heat into the bearing housing.
2Heat soak cooks the oil
With nothing flowing, the oil left in the bearing housing heats to around 300 °C. Engine oil breaks down when it is held that hot and leaves a hard, black carbon deposit called coke. Each hot shutdown leaves a little more, lining the bearing housing and the hot end of the oil feed line.
3Months later: the bearings starve
The coke has narrowed the oil passages, so less oil gets through: at the turbo the pressure falls from about 3 bar to under 1. The oil films can no longer hold the shaft off the bronze bushes, so metal rubs on metal and the bushes wear. The shaft’s play grows and it starts to whirl, orbiting round its centre, which the driver hears as a rising whistle.
4The wheels hit the housings
With nearly a millimetre of play the compressor blades strike their housing at over 400 metres per second: their tips are shaved off and boost collapses. The worn shaft no longer runs true in its ring seals, so oil is pushed past them, into the compressor, where it is blown into the charge-air pipe and cooler, and into the turbine, where it burns as blue smoke.
5The engine runs on its own oil
Oil pooled in the charge-air pipe and cooler is drawn into the cylinders, and engine oil burns in hot compressed air just as diesel does. The driver lifts off and the controller cuts the injectors, but the engine keeps speeding up on oil that nothing meters. This is a diesel runaway: left alone it ends with a burst turbo or a wrecked engine.
6How it is caught and prevented
Here the controller shuts the throttle flap: with no air the oil can’t burn, and the engine stops within a second.
After hard driving or towing, let the engine idle for 30 to 60 seconds so the oil carries the heat away before the flow stops. Modern turbos have water-cooled bearing housings, and some engines keep the coolant pump running after shutdown.
Change the oil and filter on time, with the specified low-ash oil: old oil cokes sooner. Replace the oil feed line with every new turbo, and fill the bearings with oil before the first start.
Don’t rev a cold engine: the thick oil takes a few seconds to reach the turbo.
Warning signs: a whistle or siren noise, blue smoke, oil inside the charge-air pipes, and the controller logging “boost too low” and cutting power (limp mode).
Without a throttle flap, a runaway can be stopped by stalling the engine: top gear, brakes hard on, clutch out.