Mitsubishi ASX: 4N13 1.8D DPF Oil Dilution from Incomplete Regeneration

The 1.8-litre 4N13 diesel uses post-injection fuel pulses during active DPF regeneration; on vehicles used predominantly for short urban trips the regeneration cycle is repeatedly interrupted before completion, causing unburned diesel to wash down the cylinder bores and accumulate in the engine oil sump. The oil level rises above the maximum dipstick mark, triggering an oil level warning, and the diluted oil loses its lubricating properties. This mechanism is extensively documented on Mitsubishi owner forums and a Denso 4N13 DPF fault thread on MHH Auto, and in severe cases leads to premature bearing and bore wear.

• Oil level above maximum on dipstick with no external oil leak evident
• Oil smells of diesel fuel rather than normal lubricant
• Check engine oil level warning on dashboard
• DPF regeneration warning light activating very frequently — every 50–100 km on short-trip vehicles
• Rough idle or increased smoke at start-up on vehicles with severely diluted oil

1. 1

   Check the oil level on the dipstick — a reading above maximum with no obvious external leak is a strong indicator of fuel dilution.

2. 2

   Smell the dipstick; diesel-diluted oil has a distinctly fuel-like odour.

3. 3

   Ask about trip-length history.

4. 4

   An extended motorway drive prior to purchase can temporarily lower the oil level by burning off some dilution, so insist on checking the oil before any pre-sale servicing.

Parts involved: Diesel particulate filter.

$600 to $4,500 at New Zealand independent workshop rates. That is for this job on its own, not for the car.

An estimate, in NZD, for the repair as normally carried out. What a specific car costs depends on which part has failed and what it damaged on the way — treat this as the floor for a negotiation, not a quote.

• “Has this vehicle been used primarily for short urban trips, and is most driving highway or city?”
• “Has the oil level ever risen above maximum without an obvious explanation?”
• “Has the DPF ever been professionally cleaned, forced-regenerated, or replaced?”

The failure described above is documented in published sources — owner forums, technical service bulletins, recall notices and workshop write-ups — held in our fault database. The inspection instructions and the cost estimate are KickTyres’ own interpretation of them, and it is a starting point for an inspection rather than a manufacturer statement.