Taxi with rising oil level and repeated DPF regenerations
A 2021 BS-VI 1.5-litre common-rail diesel hatchback working as a city taxi has covered 96,400 km. The driver reports the DPF warning lamp appearing every 300–400 km, sometimes flashing together with reduced power. At the workshop the engine oil level is 12 mm above the MAX mark on the dipstick and the oil smells strongly of diesel; it was last changed 4,000 km ago with the specified low-SAPS 5W-30. The taxi does 6–9 km stop-start trips, nine hours a day, six days a week. Scan data show no active fault codes, DPF differential pressure 4 kPa at idle (spec below 8 kPa), soot mass 24 g, ash mass 0.9 g, and 41 completed regenerations in the last 8,000 km. Coolant reaches 82 °C on the road test. The owner wants the DPF replaced and the oil simply topped up so the car can earn tomorrow.
1. Explain what is happening to the oil and why replacing the DPF would not fix this vehicle.
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The extra 12 mm on the dipstick is diesel, not a overfill. During an active regeneration the ECU injects fuel late on the exhaust stroke; because the trips are only 6–9 km, exhaust temperature never sustains the 250 °C-plus needed for passive NO2-based oxidation, so the ECU keeps commanding active regenerations - 41 in 8,000 km is one roughly every 195 km. Each event puts fuel on the cylinder walls, some of which passes the rings into the sump. The consequences are a fall in viscosity and HTHS, accelerated bearing, cam and turbo wear, more oil vapour through the crankcase breather back into the intake, and in the worst case an uncontrollable run-on. The DPF itself is not the failed part: 4 kPa differential pressure at idle is inside the 8 kPa limit and 0.9 g ash is low, so the filter is not physically choked. The problem is duty cycle and regeneration frequency, possibly with a contributing fault such as a leaking injector, stuck EGR or low coolant temperature. Replacing the DPF leaves all of that untouched.
2. Set out the immediate workshop actions and the advice you would give the owner, with the checks you would use to confirm the repair.
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Do not top up and release the car. Drain the diluted oil, fit a new filter, refill with the specified low-SAPS oil and mark the new level. Before any regeneration, check the contributors: coolant temperature sensor and thermostat, EGR operation, intake and charge-air leaks, air filter, injector leak-off rates and rail pressure at cranking, because a dribbling injector adds fuel independently of regeneration. Then run the manufacturer's service regeneration with the car in the bay, watching soot mass fall and exhaust temperatures, and confirm differential pressure afterwards. Road test 30 minutes at 60–80 km/h. Tell the owner plainly: this engine needs one continuous 25–30 minute drive at 60 km/h or more every 300 km or weekly, no long idling, only the specified oil, and a weekly dipstick check. Ask them to return after 2,000 km; if the level has risen again, send an oil sample for fuel dilution and revisit the injectors before any DPF is condemned.
3. The owner insists the DPF be replaced under warranty. What evidence would you record, and what would justify replacement?
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Record the facts that separate a duty-cycle problem from a failed filter: oil level and fuel-dilution result, soot and ash mass before and after the service regeneration, the number of completed regenerations and distance travelled, differential pressure at idle and at high flow, coolant temperature behaviour, injector leak-off values and any stored or pending codes. Save the scan reports and a photograph of the dipstick. Replacement is justified only with physical evidence - a cracked or melted substrate seen on borescope, a filter that cannot be brought back below the soot threshold after a correct regeneration, or ash loading beyond the service limit with documented pressure rise. None of that is present here. A DPF fitted to this car would load and regenerate in exactly the same pattern within a few thousand kilometres, and the diluted oil would still be there, so the comeback would be yours.