Turbochargers: the signs before it gets expensive

The turbine wheel sits in the exhaust stream, the compressor wheel in the intake. Both are rigidly joined by a shaft running in the bearing housing between them. Exhaust drives the turbine, the turbine spins the compressor, the compressor pushes more air into the engine. More air means more fuel and therefore more power from the same displacement — that is the entire point.

Shaft speeds are of the order of more than a hundred thousand revolutions per minute, and the turbine side runs as hot as the exhaust manifold. In most turbos the shaft rides in plain bearings, with a bush floating between shaft and housing — the shaft does not touch metal, it rides on a film of oil. Some turbos use ball bearings instead; they depend on oil too.

From which follows the sentence that carries this whole article: a turbo rarely fails because it wears out. It fails because the oil film was briefly absent, too hot, or full of particles. Anyone who repairs a turbo failure without examining the oil side is buying the next one at the same time.

The commonest case is coking. After shutdown no oil flows, but the turbine side is still glowing hot; the residual oil in the bearing housing bakes into hard deposits that gradually narrow the oil feed. Modern cars mitigate this with an electric coolant pump that runs on after shutdown — a short idle after hard driving still helps.

The second case is contamination. Everything floating in the oil passes through the finest point in the circuit, and that is the turbo bearing. An overrun oil change interval, a blocked oil filter, debris from an earlier failure further up the engine — every one of those reaches the shaft. The third case is simply too little pressure: a tired oil pump, a blocked pickup, a level left too low for too long.

So the first question about a turbocharged car is not “how old is the turbo” but “how was the oil looked after”. Invoices naming the specification, with an interval that was not stretched to its limit, are a genuine argument in favour on a turbocharged engine.

Where the shaft passes to compressor and turbine there is no lip seal of the usual kind but a piston ring in a groove. Such a ring does not seal by pressing a lip against a surface; it seals because the pressure on the other side is higher than in the bearing housing. Remove that pressure difference and oil gets through.

That explains an observation that sends many people the wrong way: oil in the charge air system need not come from the turbo. Excessive crankcase pressure — from a failed crankcase ventilation system or from wear at the rings and bore — cancels the pressure difference and pushes oil to exactly that spot. The turbo is then where you see it, not where it comes from.

So before a turbo is replaced, the breather system should be checked and the crankcase pressure measured. That is minutes of work and decides a four-figure difference.

Many diesel engines and some petrol engines carry a turbo with variable turbine geometry. Ahead of the turbine wheel stands a ring of movable vanes; at low engine speed they narrow the cross-section so the exhaust hits the turbine faster, and at high speed they open. They are moved by a vacuum capsule or an electric actuator.

That mechanism sits in the exhaust. Soot and carbon build up on the vane spindles, the adjustment stiffens and eventually seizes. The control unit compares measured boost with requested boost and logs a fault when the deviation grows too large — sometimes as underboost, sometimes as overboost, depending on where the vanes have stuck.

In practice it feels like an engine that pulls to a certain speed and then cuts, or like power that suddenly returns after a restart. Both are reasons to read the fault memory, not to look away.

A turbocharger whistles. A clean, rising note that comes and goes with boost is normal and says nothing about condition. What matters are the departures from it.

The most informative visual check takes ten minutes and needs only a screwdriver and a lamp. Loosen a clamp on the charge pipe between turbo and intercooler and look inside. A fine oily film is usual on older engines; standing oil in the pipe, or a filled low point in a bend, is not.

Second, the smoke. Bluish smoke under load or after the overrun points to oil in the combustion chamber — that can be the turbo, but it can equally be valve stem seals or piston rings. Black smoke is fuel without enough air and fits a boost problem. White, sweet-smelling smoke does not belong to the turbo but to the coolant circuit.

Third, the play. Take the intake hose off the compressor and you can move the wheel by hand. Some radial play is normal on a floating-bearing turbo and is not a finding. A wheel that touches the housing while you do it, or axial play you can feel as a click, is. And any rub mark on the blade tips is one too.

A turbocharged engine gives itself away under load, not in town traffic. The test drive therefore needs a slip road or a hill and an acceleration into the upper rev range in a high gear — that is where boost stands for longest.

Then a look in the mirror while changing down and on the throttle transitions, followed by two minutes at idle before switching off. As you switch off, listen to it spin down. And right at the end, read the fault memory — even if no lamp appeared during the drive, because boost deviations are often stored before they are displayed.

If the turbo really is finished there are three routes: a new unit, an industrially remanufactured exchange unit, or a rebuild at a specialist who renews the core and balances the assembly. The order by price is usually the same as the order by availability of documentation; an exchange unit with a test report is the pragmatic middle.

What belongs to all three is the work before and after: check or renew the oil feed and return lines, change the oil with a new filter, inspect the crankcase ventilation and clear the charge air system of oil. If the oil in the intercooler is not removed, the first full-load run pushes it into the combustion chamber — and a diesel engine drawing in its own lubricating oil cannot be switched off with the key.

For a buyer that means a car with a freshly replaced turbo is exactly as good as the invoice that comes with it. If that invoice names only the part, half the work is missing.