Naturally aspirated engines suck air directly from the atmosphere by means of vacuum. This vacuum exists because the pistons in the engine pull air in when they move down, just like a very large syringe. This is effective, but in order to gain greater power and efficiency from a small engine, a turbocharger is used to force air into the engine, so instead of the engine pulling in air by vacuum, the engine is receiving pressurized air from what could be considered a powerful turbine.

As the engine runs normally, the exhaust from the engine does not go directly to the exhaust pipe as normal, instead, it is forced through the hot side of the turbocharger. On this side of the turbocharger, there is an impeller that is blocking the passage of the exhaust gas. Once pressure build, the impeller is forced to spin, allowing the gas to escape into the exhaust system, and out of the tail pipe. This hot side of the turbocharger is sealed from the cold side (compression side), but the impeller is connected by a shaft to the compression wheel on the cold side. As the exhaust gas forces the impeller to turn the shaft on the hot side, the compressor wheel begins spinning extremely quickly. When the compressor wheel begins spinning, air is sucked in from atmosphere, through the air filter, and crammed into the intake manifold at high pressure. This creates a situation in the engine that allows far more air to enter the cylinders, which in turn allows far more fuel to enter the engine on each combustion cycle. This allows more power and engine speed to be made with less application of the throttle, and actually has been used extensively to either make large power, increase fuel economy, or both.

Turbochargers need oil from the engine to continue running at the extremely high speeds they are designed for, so when the oil seal breaks, they spit blue-black smoke from the exhaust pipe at an alarming rate. Also, a rattling and scraping noise may be heard from a turbocharger if the bearings fail as the turbines (impeller and compressor wheels) are able to rub against the inner housing of the turbocharger. Finally, turbochargers use a wastegate to vent excess exhaust gas, preventing overpressure to the engine. When the wastegate fails, it is notorious for rattling at a certain RPM range. Other than these symptoms, power loss and oil consumption are by far the most common symptoms. A turbocharged vehicle with a completely failed turbo may not be able to start or idle, and the check engine light will certainly be displayed if the engine runs at all. Lastly, in the early stages of failure, the fuel economy of the vehicle will fall, and the catalytic converter could become ruined from excess fuel in the exhaust.

Turbocharger failure must be addressed immediately. Particles from the turbocharger may find their way into the engine or catalytic converter, and can destroy an engine and clog a catalytic convert. Also, oil consumption can become so high that the engine may run out of oil in an extremely short amount of time, which may lead to engine failure.

Factory grade turbochargers will not last the lifetime of most turbocharged vehicles without compromised performance and fuel mileage. Many turbocharged vehicles limp the last half of their service life with failed catalytic converters, oil consumption issues, lack of power, and light to moderate smoke from the exhaust. The average turbocharger will run between 100,000 and 125,000 miles, but harsh driving conditions, aftermarket components, and low quality factory parts will certainly cause the turbocharger to fail ahead of schedule. Many luxury brands focus on the use of turbochargers due to the high altitude of their home manufacturing and engineering facilities, and these manufacturers tend to use higher quality products that should last at least 150,000 miles under normal use and proper maintenance.

Turbocharger diagnosis is a matter of pressure testing, visual inspection, and oil or coolant consumption data. Many times, a turbocharger may be failing, but several other components must be ruled out because there are no additional symptoms besides oil consumption. In the event that a turbocharger is making noise, the turbocharger will be removed and inspected with a dial indicator for the amount of play in the shaft. The impeller and compressor wheels will be inspected for degradation or damage from debris, and the turbocharger piping will be pressure tested. In the most difficult cases of diagnosis, the compressor side of the turbocharger assembly may be pressure tested, but this provides very limited data, and magnaflow testing is often a better option to identify if there are any cracks in the turbocharger housing. Finally, the presence of oil in the turbocharger downpipe is a giveaway that the turbocharger has lost its oil seal, and wear patterns on the impeller, compressor wheel, inside the turbo housing, and around the wastegate are all signs of a failed turbocharger. In the most difficult cases, the engine may be compression tested, the PCV valve replaced, and the oil changed and monitored for consumption over a short period of time. In many cases where the turbocharger is suspected of failure, the turbocharger or turbocharger piping is the culprit.

Before removal of the turbocharger or turbocharger assembly can commence, the engine cooling system and oil pan must be completely drained. Afterwards, the cooling and oil lines for the turbocharger can be removed, and the turbocharger assembly, which often includes the exhaust manifold, can be separated from the cylinder head, mounting brackets, downpipe, intake air tube, and finally removed from the vehicle. In some applications, the engine mounts may need to be removed to rock the engine and transmission assembly to a position which allows space for removal. In the most extreme cases, the engine may need to be removed. Installation is the reverse of removal, but the turbocharger must be run for a short time, the engine oil filter changed once more, and all fittings for cooling and oil lines must be retorqued with fresh seals. The exhaust manifold gasket must be replaced, as well as the flange gaskets mounting the turbocharger to the exhaust.

Purchasing a new turbo versus rebuilding a turbo can be a cost effective way of replacing a failed turbocharger. However, the turbocharger housing should be inspected thoroughly for damage or wear before proceeding, as the turbocharger may have significant wear which precludes it from being rebuilt. Most manufacturer repair facilities will not rebuild a turbocharger, and the only option may be to purchase new or choose another repair location. In any case, a new turbocharger is likely the best choice.

When pressure testing a turbocharger system, the PCV lines should be completely blocked. If new oil is found in the exhaust or compression side of the turbocharger after the pressure test is complete, this indicates a failed turbocharger center housing rotating assembly oil seal. If the PCV lines are left installed, the PCV system can pour oil into the turbocharger, which will likely ruin the catalytic converter as it blows blue-gray smoke from the tail pipe for the next day or so. Turbochargers are, by far, the hottest thing underneath an engine. They are hotter than the exhaust manifold, and will scorch cloth and flesh instantly. If handling a turbocharger for any reason, wait until it is cool before proceeding, and do not attempt to cool it with water as this will lead to cracking of the turbocharger housing.

Turbocharger diagnostic testing and replacement is not recommended for the home mechanic. This is a complex system that requires a high degree of understanding to properly diagnose, and a good amount of skill to replace. Incorrect diagnostic procedures can quickly lead to damage to multiple components, and lack of experience in assembly and disassembly may cause damage and reduce reliability.