A turbo is not meant to be a service item. MAHLE's turbocharger damage brochure says it is "designed to last the service life of the engine", and Garrett states that "Less than 1% of turbos fail because of manufacturing defects." So when a turbo fails early, the useful question is why. MAHLE warns that unless the cause is found and removed, the new turbocharger "might fail again after a short period of time."
Turbo makers divide the blame slightly differently. Garrett puts more than 90% of failures down to oil, through starvation or contamination, and calls oil starvation, oil contamination and foreign object damage the three "turbo killers". BorgWarner says 90% of failures come from four causes: foreign bodies entering the turbine or compressor, dirt in the oil, an inadequate oil supply and high exhaust gas temperatures.
Why oil matters
In a small turbo the shaft can reach up to 300,000 rpm, according to MAHLE. Garrett's training notes say that at full load it runs on an oil film that may be only 7 microns thick. After the engine stops, heat soaking back from the turbine takes the bearing area to its highest temperature two or three minutes later, and oil left inside can burn into hard carbon, known as coking. Garrett also warns that "It takes just a few seconds for a lack of oil to cause catastrophic damage to a turbocharger."
Oil starvation
Common causes, from MAHLE, Garrett and Holset guidance:
- an engine oil level that is too low, or low oil pressure from a faulty lubrication system
- a feed pipe that is blocked, leaking or carbonised by hot shutdowns
- sealant, or the wrong flange gasket, narrowing the oil feed
- a turbo or oil filter fitted without being primed with oil, or an engine left unused for long periods
- hard acceleration from cold, before the oil has circulated
- oil that is too thick or too thin, or engine oil thickened by biodiesel or vegetable oil
A starved turbo, MAHLE says, shows a shaft discoloured by heat, bearing metal smeared onto it and wheels that have struck their housings. On the road the rotor cannot reach full speed, so boost falls and the exhaust can smoke black.
Dirty oil
Garrett calls contaminated oil "the biggest single cause of turbo failure". Its training notes name fine carbon from combustion as the most common cause of bearing damage, acting as "a very effective abrasive" until the oil can no longer control the shaft. The bearing is much softer than the steel shaft, so a scored shaft generally points to hard particles in the oil. MAHLE adds water from a leaking head gasket or oil cooler, and fuel, both of which dilute the oil and reduce the load it can carry.
Foreign object damage
On the intake side, MAHLE lists dust, sand, screws and parts left loose after work, which can get in through a leak in the intake or a clogged or defective air filter; in winter, ice from condensation in the intake can break a single compressor blade. On the exhaust side, pieces of a broken valve or piston ring strike the turbine wheel. The result is usually total failure, and Holset warns never to keep running a turbo with damaged blades because the rotor balance will be affected.
Heat and overspeed
MAHLE says a turbo pushed past its design temperature "can fail after just a few seconds". Cracked turbine housings and coked oil lines are the signs, and the causes MAHLE lists are tuning, combustion faults and switching the engine off hot. Overspeed has its own guide: turbo overspeed.
Warning signs
Lost power, new noises, smoke and rising oil use are the classic signs, but Garrett points out that each can also come from faulty fuel injection, a blocked air filter, a damaged exhaust or a lubrication problem. Our guides to a turbo leaking oil, turbo noises and a turbo losing boost take each symptom in turn, and turbo diagnostics explains how a fault is traced.
Prevention
- Use the oil the vehicle maker specifies, with a good-quality filter, and change both on time.
- Let the engine warm up before working it hard.
- After hard driving, give the turbo a short cool-down before switching off, but avoid long idling (see the questions below).
- Avoid only ever driving short distances, as MAHLE advises.
- Keep the air filter, intake and crankcase breather in good order.
Rebuild or replace
Garrett's full remanufacture process is a useful benchmark: the turbo is completely dismantled and cleaned, worn or damaged parts are replaced, the rotating assembly is precision-balanced and every unit is tested. Rebuilt or new, a turbo only lasts if the original cause has been fixed. Our turbo rebuilds page covers rebuilding and reconditioning at GP Turbos.