Mercedes-Benz C-Class: OM651 Oil Cooler Housing Seal and Injector Leak-Off Pipe Oil Leaks

The OM651 has two pervasive external oil leak points: the oil cooler housing O-ring seals, which harden and shrink with heat cycling allowing oil to weep from the block face; and the plastic high-pressure injector return (leak-off) pipes, which crack at their banjo fittings or along the pipe body after repeated thermal stress. Both faults allow oil or diesel to deposit onto hot exhaust manifold and turbo components, creating a fire risk in addition to progressive fluid loss.

• Blue-grey smoke from the engine bay caused by oil burning on the exhaust or turbo
• Oil puddle forming under the engine after parking
• Burning oil smell noticeable during or shortly after driving
• Oil level falling noticeably between service intervals
• Visible oily seepage around the oil filter housing or near fuel injectors at the top of the head

1. 1

   With the engine warm, look under the car for fresh oil on the underside of the block.

2. 2

   Trace the oil cooler housing (mounted on the right side of the engine near the oil filter) for signs of seepage or residue.

3. 3

   Inspect the injector return pipes at the top of the cylinder head for cracked plastic or oily residue around banjo fittings.

4. 4

   Check oil level after the test drive and note any burning oil smell from the engine bay.

Parts involved: Fuel injector.

$400 to $2,000 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.

• “Have the oil cooler housing seals been replaced? Do you have receipts?”
• “Have the injector return leak-off pipes been replaced? They are a known failure point on this engine.”
• “Is there any oil residue on the underside of the engine or near the exhaust manifold?”

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.