WHAT'S IMPORTANT WHEN REPLACING A TURBOCHARGER (2/2010)

A turbocharger is designed and built to match the service life of the engine. However, the high-tech components in the exhaust gas system are exposed to several risk factors which could lead to their failure: for example from foreign bodies that penetrate the turbine, dirty oil, a faulty oil supply or excessively high exhaust gas temperatures. This makes the mechanic’s know-how all that more important. There follow some practical tips and tricks for an effective replacement.

IMPORTANT: ANALYSIS
A precondition for a successful repair is that the cause of the failure has been identified and eliminated – otherwise there is a risk that the new turbocharger will fail again after a short time.

INDISPENSABLE: CLEANLINESS
Even the smallest of foreign bodies acts like a projectile at the enormously high speeds of the turbine and compressor wheels. The air filter therefore has to be replaced in every case. Just as important is the careful cleaning of the entire intake and fresh air system to and from the charge air cooler including the removal of all foreign bodies from the exhaust pipe.

A MUST: LUBRICATION
The rotor shaft bearing needs engine oil.

TIP: pour a little engine oil into the oil inlet bore before installing the new turbocharger, place a clean cloth over this – and then distribute the oil in the bearings with a short blast from a compressed air gun (see illustrations). After installing the turbocharger and before connecting the oil supply, pour some more engine oil into the oil inlet bore. A small syringe with engine oil is enclosed with the MAHLE turbocharger sets of seals to make this work easier.

WHAT GOES IN, MUST COME OUT
The turbocharger needs engine oil for lubrication and cooling and this is supplied by the oil pump in the engine. The oil is returned to the circuit via the oil return pipe. The complete oil return pipe and connection to the crankcase must be laid without any kinks and must be free from deposits so that the oil is transported smoothly. If the return flow is disturbed this leads to blockages and the turbocharger loses oil through the turbine and compressor wheels. Imminent consequences include major engine damage.

TIP: a new oil return pipe is just as sensible an investment as a new charge air cooler.

ANOTHER TIP: there should be no excess pressure in the crankcase. If the crankcase vent is dirty an excess pressure builds up that hinders the return flow of oil in the turbocharger. The turbocharger starts to lose oil through the turbine and compressor wheels as soon as an excess pressure of only a few millibars is measured at the dipstick tube with a manometer.

ALL TIGHT?
All connections on the turbocharger must be tight so that oil, air, exhaust gases and, depending on the type, cooling water can be passed through the turbocharger cleanly, safely and without losses. Each MAHLE turbocharger has a matching mounting kit – consisting of seals as well as exactly fitting builton parts (for example stud bolts, nuts, etc.). Important: only ever use these original sets of gaskets. And never use the old gaskets again – they are often deformed or porous and therefore cannot guarantee the necessary seal. Sealing compound or liquid sealers are also forbidden on the turbocharger. Sealing compound may reduce the cross-section of the bores – and parts of the sealing compound may become loose and block the supply of oil. The consequence: major damage to the turbocharger.

COVER LOST?
The oil supply and return lines often run very close to the turbocharger. Car manufacturers thus protect the oils pipes with shielding. This should also be checked when replacing the turbocharger: is it still in place and undamaged?

NEW TURBOCHARGER – NEW OIL
A new turbocharger should always be given new oil – and a new oil filter. This ensures that any foreign bodies in the oil system are eliminated and cannot damage the new turbocharger. (After all, the old turbocharger is faulty – which may be due to infiltrated foreign bodies or particles.)

TIP: “If a little is good, then more is better” does not hold true for the oil level! Because if the oil level in the engine is too high this leads to disturbances of the compression ratio in the engine. This forces engine oil into the turbine and compressor of the turbocharger where it collects in the charge air cooler. If this oil is sucked in and burnt again by the engine this could lead to major engine damage.

MAN THE PUMPS
Once the turbocharger has been fitted, the oil and possibly water lines connected and the air and exhaust gas line connected to the turbocharger, the oil can be changed and the cooling system vented.

Ignition must be prevented when the engine is started – for example by removing the fuel pump’s fuse or relay. Then turn the engine with the starter until oil pressure has been built up. If the engine is being started for the first time we recommend that you leave the engine idling for 2 minutes before opening the throttle to ensure the supply of oil.

A SCREW LOOSE?
You then have to check whether all screws and connections are tight and that there are no leaks.

TIP: check the tight fit of all screws again after running for approx. 20 hours or 1,000 km.

LONG-DISTANCE RUNNER
Every engine profits from being run for a longer period of time. A turbocharged engine takes poorly to short trips because after a cold start the unburnt fuel and condensation water collects in the engine oil. This worsens the tribological properties of the oil and reduces the carrying capacity of the lube oil film. These unwanted companions only disappear at oil temperatures over 80 °C. If the oil temperature stays below this level for a longer period of time, as is the case with short trips, this constitutes a risk for the turbocharger: the bearings of the crankshaft and camshaft and rotor shaft in the turbocharger become worn. At low outside temperatures the emulsion of engine oil and condensation water in the oil return pipe of the turbocharger and in the crankcase vent can also freeze leading to disturbed engine compression ratios.

BIODIESEL? A HIGHER RISK FOR ENGINE OIL AND TURBOCHARGER
Vegetable oils do not evaporate – and unburnt vegetable oils collect in the engine oil. This becomes viscous as of a certain concentration and the lubrication system collapses soon afterwards. This means much shorter intervals between oil changes if vegetable oils are used as a fuel.

		Pour in a little engine oil before installation to lubricate the rotor shaft bearing.
		Then place a clean cloth over the oil inlet bore …
		… and distribute the oil in the bearings with a shot of compressed air.