A turbocharger isn't a single part. It's a system, and when one part of it fails, it usually takes the turbo with it. Knowing what each component does helps you diagnose problems correctly and avoid paying for the same turbo twice.
The easiest way to understand the system is to follow the air through it.
New to turbos? Start with How a Turbocharger Works and Why It Fails.
The air path
1. Air filter and intake
Clean air is the turbo's first line of defence. A torn filter, a missing clamp or a cracked intake hose lets dust and debris straight into the compressor wheel, which spins at well over 100,000 rpm. Even a small particle chips the blades.
2. Compressor housing and compressor wheel (cold side)
The compressor wheel draws in fresh air and pressurises it. The housing is shaped to turn that high-speed airflow into boost pressure. More air going in lets the ECU add more fuel, and that's where the extra power comes from.
3. Charge pipes and blow-off/diverter valve
Pressurised air travels through charge pipes to the intercooler and engine. On many petrol engines, a blow-off or diverter valve releases boost when you lift off the throttle, so pressure doesn't surge back against the compressor. A split pipe or a loose clamp causes boost leaks: you'll get power loss, a whistle and, on diesels, black smoke.
4. Intercooler
Compressing air heats it, and hot air carries less oxygen. The intercooler cools the charge before it enters the engine. It's fitted to virtually every modern turbo engine, not only performance cars. If you find oil inside the intercooler or its pipes, the turbo's seals or the engine breather need investigating.
5. Exhaust manifold
After combustion, the exhaust manifold collects the hot gas and feeds it to the turbine. Cracked manifolds and blown manifold gaskets are common, especially on engines that tow. They leak exhaust energy, reduce boost and can cause a ticking noise.
6. Turbine housing and turbine wheel (hot side)
Exhaust gas spins the turbine wheel, which drives the compressor through a shared shaft. The turbine side runs at several hundred degrees Celsius, and on petrol engines it can go well over 900°C. It's built from high-temperature alloys for that reason.
The core: the CHRA (cartridge)
The centre housing rotating assembly is the heart of the turbo. It contains:
- The shaft, joining the turbine and compressor wheels
- The bearings, which are journal bearings or, on some turbos, ball bearings. The shaft floats on a film of engine oil.
- The seals, piston-ring-type seals that keep oil in and boost and exhaust gas out
- The centre housing, with oil passages and, on some turbos, water jackets
The CHRA wears out first. When a turbo has failed but the housings are undamaged, replacing only the CHRA is often the most cost-effective repair. The cartridge must be correctly balanced and matched to the application.
Boost control
Wastegate
A valve that sends some of the exhaust gas around the turbine once the target boost is reached, to prevent overboost. It's common on petrol engines and older or simpler diesels. A wastegate stuck open causes low boost. One stuck closed causes overboost and limp mode.
Variable geometry turbo (VGT/VNT)
Most modern diesels don't use a wastegate. They use adjustable vanes inside the turbine housing. Closed vanes speed up the exhaust flow for quick boost at low revs. Open vanes control boost at high revs. The weak point is carbon build-up from short trips and EGR soot, which makes the vanes stick and leads to limp mode, overboost and underboost fault codes.
Actuator
The actuator moves the wastegate or the VGT vanes.
- Pneumatic actuators use vacuum or boost pressure on a diaphragm. They fail through split diaphragms, perished vacuum lines or a faulty control solenoid.
- Electronic actuators are motor-driven with a position sensor, controlled directly by the ECU. They're precise, but they fail electronically, and many need calibrating or setting when they're fitted to a turbo. Skip that step and a perfectly good turbo will perform badly or throw fault codes.
Sensors and ECU
The ECU sets the target boost from the boost/MAP sensor, the airflow (MAF) sensor, engine speed and load. It then drives the actuator to hit that target. A faulty sensor can make a healthy turbo look like it has failed. Always read the fault codes and check live data before condemning the turbo.
Support systems
Oil feed line
Delivers pressurised engine oil to the bearings. A blocked, kinked or carbon-restricted feed line will destroy a new turbo within minutes. Always replace the oil feed line when fitting a turbo.
Oil drain (return) line
Returns oil to the sump by gravity. If it's blocked or kinked, or if crankcase pressure is too high (from a worn engine or a blocked breather), oil backs up and gets pushed past the seals. The result is blue smoke and oil in the intercooler. It's very often blamed on the turbo when the turbo isn't the cause.
Coolant lines (water-cooled turbos only)
Not every turbo is water-cooled. Where it is, mostly on petrol engines and some diesels, coolant carries heat away from the centre housing. It keeps circulating after shutdown to stop oil coking in the bearing housing.
Gaskets and heat shields
These are small, cheap parts that matter. A reused or wrong gasket leaks oil or exhaust gas. A missing heat shield cooks nearby hoses, wiring and the actuator.
Quick reference: failure signs by component
| Component | Common failure signs |
| Air intake / filter | Damaged compressor blades, whistling, power loss |
| Charge pipes / BOV | Boost leak, whistle, black smoke (diesel), limp mode |
| Intercooler | Oil inside, power loss, high intake temperatures |
| Exhaust manifold / gasket | Ticking noise, low boost, exhaust smell in the engine bay |
| CHRA (bearings / seals) | Shaft play, siren-type whine, blue smoke, oil leaks |
| VGT vanes | Limp mode, overboost/underboost codes, lag |
| Actuator | Limp mode, boost fault codes, no response to commands |
| Oil feed line | Sudden bearing failure, often on a newly fitted turbo |
| Oil drain line / breather | Blue smoke, oil in the intercooler, oil past the seals |
The bottom line
Most turbo failures start somewhere else in the system. Before replacing a turbo, find out why it failed, and replace the oil lines, gaskets and filters with it. Otherwise the new turbo will go the same way.
Turbo parts from TDC
Turbo Diesel Components supplies complete turbochargers, CHRAs (cartridges), actuators, gasket kits and turbo components for passenger, commercial and industrial applications. Search our catalogue or contact our team for help identifying the right part.