Low boost, black smoke and a worn turbocharger
A 2016 four-cylinder common-rail diesel van (2.2 L, 180,000 km) is brought in with reduced power, black smoke under load and a stored P0299 boost-deviation code. Road test shows no power above 2,500 rpm and a derate lamp. Data: idle fuel-rail pressure 280 bar (spec 250–320), commanded vs actual boost at 2,500 rpm 1.15 vs 0.55 bar, MAF 38 g/s (expected 55–70), intake vacuum after air filter 18 kPa (spec <8), EGR commanded 0% at full load, DPF differential pressure 12 kPa at idle (spec <5), turbo shaft has 0.9 mm axial play (limit 0.5), and a manual boost gauge confirms low boost. The owner has already replaced the MAF sensor and cleaned the EGR valve, with no change. The workshop must decide whether to replace the turbo, investigate an intake restriction/DPF, or test the boost control circuit first, and must justify the order of work to a cost-conscious fleet customer.
1. Interpret the data. Which findings are primary faults and which are consequences? Explain the apparent contradiction between low boost, low MAF and high intake vacuum.
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The high intake vacuum of 18 kPa after the air filter is a primary restriction upstream of the turbo. The turbo is being starved of air, so low MAF (38 g/s) and low boost (0.55 bar against 1.15 bar commanded) are largely consequences. Black smoke follows from over-fuelling relative to the air available, and P0299 sets because actual boost cannot meet commanded boost.
The DPF differential pressure of 12 kPa at idle is also abnormal against a <5 kPa limit and suggests a possible exhaust restriction, but it must be confirmed at higher flow because idle readings can be noisy. The EGR is commanded to 0% at full load, so it is not the cause of the black smoke. Rail pressure is normal, so a low-pressure fuel supply fault is not supported.
The turbo's 0.9 mm axial play exceeds the 0.5 mm limit and is a genuine mechanical fault, but it does not explain the high intake vacuum before the turbo. Both can be true: a restricted intake/DPF can mask or aggravate a worn turbo. The MAF replacement was a guess; the data do not support another MAF.
2. Give a cost-conscious diagnostic sequence with expected results and stop/go decisions. State what you would tell the customer about turbo replacement.
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Intake check: inspect the air filter, housing and turbo inlet hose. A collapsed hose or blocked filter should bring intake vacuum below 8 kPa and raise MAF and boost on retest. If the restriction is found, repair it first and clear the code before condemning the turbo.
Exhaust/DPF check: measure backpressure with a manual gauge at idle and raised speed. If it remains well above specification, diagnose the DPF or collapsed exhaust before replacing the turbo; a restricted exhaust can cause low boost and overheating.
Boost control check: use a vacuum pump/gauge on the VNT actuator and command the solenoid. The vane lever should move through full travel and hold vacuum. A stuck lever or failed solenoid can explain low boost, but it does not explain the turbo shaft play.
Turbo decision: confirm axial/radial play with a dial gauge and check oil feed and intercooler for debris. With 0.9 mm axial play against a 0.5 mm limit, the turbo is worn and should be planned for replacement, but only after the intake/DPF restriction is corrected and the oil supply is verified.
Tell the customer: the turbo is worn, but replacing it alone risks repeat failure and will not correct a starved intake or blocked exhaust. Authorise a short diagnostic sequence first; then repair the restriction and replace the turbo if play is confirmed. This is cheaper than a second turbo.