Mitsubishi Pajero: 4M41 3.2D Engine Oil Cooler O-ring Failure and Coolant Contamination

The rubber O-rings sealing the oil cooler stack to the block on the 4M41 engine harden and crack with age, allowing engine oil to migrate into the coolant circuit (producing a milky, mayonnaise-like deposit in the overflow tank) or allowing coolant to dilute the engine oil. The NT and NW generations (2006 onwards) are most frequently affected due to the age of the vehicles and the original O-ring compound used. If not caught early, the contamination accelerates bearing wear and can lead to overheating as the coolant loses its heat-transfer capacity.

• Milky or brown emulsified residue inside the coolant reservoir cap
• Engine oil appearing frothy or overfull on the dipstick
• Slow unexplained coolant loss with no visible external leaks
• Oil staining or weeping around the oil cooler housing on the engine block
• Overheating in sustained towing or low-speed running

1. 1

   Pull the oil dipstick and look for a milky or frothy residue.

2. 2

   Inspect the oil cooler housing for weeping oil stains.

3. 3

   Check coolant level — a slow unexplained drop without visible external leak is a warning sign.

4. 4

   Remove the coolant reservoir cap and inspect for a brown or cream-coloured emulsion on the underside and in the coolant.

$900 to $2,800 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 the oil cooler or its O-rings ever been replaced, and if so when?”
• “Has the coolant ever appeared discoloured or has the oil ever looked milky?”
• “Is there a current overheating issue or has the temperature gauge ever spiked unexpectedly?”

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.