Why does a turbocharger lose oil?

It could be so simple: if oil seeps from the turbocharger, it is leaky, gets replaced, and everything is fine. Yet, the fact is: if only the turbocharger is replaced, further repairs will usually be needed within a short time. Sounds outrageous? Unfortunately, it is true all too often! If you don’t have a closer look, further repairs are inevitable. Here are some examples that will help to determine cause and effect.

IMPORTANT: A TURBOCHARGER WILL NOT LOSE OIL UNDER NORMAL CIRCUMSTANCES! IT ONLY BEGINS TO LEAK IN CASE OF A MALFUNCTION IN ITS PROXIMITY. THERE CAN BE SEVERAL REASONS FOR THIS:

1. PROBLEMS WITH THE RETURN FLOW

If oil leaks immediately after installation from both sides of the turbocharger, this suggests the following possible causes:

  • Clogged or bent oil return lines (Figure 1(B) and Figure 2)
  • Too much back pressure in the crankcase due to an excess of blow-by gases (Figure 1(E)), defective crankcase ventilation (Figure 1(F))
  •  Oil level is too high (Figure 1 (C+D))
  • Fig. 1(A): Optimum level Fig. 1(B): Oil leakage
  • Fig. 1(C+D): Oil leakage
  • Fig. 1(E+F): Oil leakage
Fig. 2: Bent oil return line
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All of these cases have one common denominator: the oil is being prevented from returning to the oil sump/engine. But this is crucial for a turbocharger! As soon as the oil encounters any sort of blockage on its return, it will seek out the path of least resistance—and this is usually via the compressor or the turbine side of the turbocharger.

RETURN LINES SHOULD THUS BE REPLACED, CRANKCASE PRESSURE CHECKED, AND OIL LEVEL INSPECTED

2. RESIDUAL ISSUES

The new turbocharger is pressing out oil on the exhaust side immediately after start-up. There is blue smoke blowing from the exhaust and the turbine side (Figure 4) is completely covered with oil. However, the new turbocharger is not to blame—its predecessor is the real culprit! The old turbocharger had already been leaking and had pushed plenty of oil into the charge air line (Figure 3). Then the day came when that turbocharger finally failed and was replaced with a new one. No further inspection of the surrounding area was made though. Consequently, the new turbocharger is conveying the old oil that had accumulated in the charge air line and charge air cooler into the engine. From there it reaches the exhaust side of the new turbocharger, where it burns off and produces blue smoke.

THE CHARGE AIR LINE SHOULD THUS BE CLEANED AND THE CHARGE AIR COOLER REPLACED ( SEE ALSO TECHNICAL MESSENGER 05/2016)

3. DISTURBED RELATIONSHIP

The turbocharger is part of a complex system. Together with the combustion engine, intake system, charge air line, exhaust system, lubrication and cooling circuit, and engine control, it forms a harmoniously functioning team. However, if deviations arise on the intake side, for example, this harmony is quickly disturbed. A clogged air filter, for example, can become evident through the loss of oil on the compressor side (Figure 6). Increased intake resistance can go so far as causing oil to be pulled from the turbocharger behind the impeller and infiltrate the charge air line.

THE AIR FILTER SHOULD THUS BE CHECKED AND REPLACED (FIGURE 5)

  • Fig. 3: Accumulated oil is pressed out of the charge air cooler
  • Fig. 4: Oily exhaust side
  • Fig. 5: Increased resistance: contaminated air filter element
Fig. 6: Very oily compressor

4. WEAR

Badly worn groove with shaft ring
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Two types of sealing are used to seal the shaft in a turbocharger. First, the actual rotation of the shaft is exploited. At rotational speeds of up to 300,000 rpm, the oil is simply spun from the shaft by centrifugal force. Second, several shaft rings form a sort of labyrinth seal. These methods are reliable in preventing oil from leaking out of the turbocharger ... but only under normal circumstances.

After all, what would a rule be without an exception? In the case of major wear on the bearings, a broken shaft, or another serious defect, a sufficient seal can no longer be ensured, resulting in oil leaking from the turbocharger. It is therefore worth examining the issue from both sides—the compressor side and the turbine side—and establishing the reason for the increased wear.

If a pressure equilibrium prevails, no oil is lost from the bearing housing (yellow), either in the turbine (red) or in the compressor (blue).

THE MAHLE TURBOCHARGER SPECIAL COMMISSION RECOMMENDS

  • FIND THE CULPRIT.

    Never suspect only the turbocharger. Instead, always try to find the root cause and fix the issue.

  • LOOK FOR MORE VICTIMS.

    Replace all parts that were affected by the damage: clean the charge air line, exchange the charge air cooler.

  • PREVENT CONSEQUENTIAL DAMAGE.

    Change the oil (including filter replacement), exchange the air filter, replace all oil lines.

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