Mercedes OM651: injectors, timing chain and the delete myth

The OM651 appeared in 2008 and went into the C-Class, E-Class, Sprinter, Vito and much else. It is a well-understood engine with a documented fault list, which makes buying and diagnosing one relatively predictable.

Shared reference: Mercedes-Benz common DTC codes and Mercedes-Benz engine diagnostics.

Injectors

The most documented item on this engine, particularly on earlier production.

Injectors wear, their spray pattern degrades, and their return quantity rises. The ECU compensates until it cannot.

The valuable part: this is measurable long before it becomes a symptom.

Injector correction values show how much the ECU is adjusting each cylinder. Values drifting steadily on one injector over months is that injector approaching the end of its life — visible while it is still a plan rather than a breakdown.

Injector correction values on EDC17, common rail injector diagnostics and P0263 to P0266 cylinder contribution.

Symptoms when they progress: rough idle, a knock that sounds mechanical, hard starting, and smoke.

A note on removal: injectors in these engines seize in their bores, and extraction can be difficult. That is worth knowing when comparing quotes — the labour is not trivial.

The timing chain

The OM651 uses a chain driven from the rear of the engine, and earlier production has a documented concern with chain and sprocket wear.

The warning is a rattle on cold start, and it is worth listening for on any used example.

Access is at the rear of the engine, which makes the repair substantially more expensive than on an engine with a front-mounted chain. That cost is the reason to check carefully before buying.

P0016 crankshaft-camshaft correlation and timing chain vs belt DTC codes.

Turbocharger

Some variants use a two-stage turbocharger arrangement, which is more complex and has more actuators to fail.

Underboost is usually a split pipe or a loose clamp rather than a failed turbocharger — check the cheap causes first: turbocharger boost leak diagnostics and P0299 and P0234.

Actuator faults on the variable geometry mechanism are a genuine item, often from carbon rather than from the actuator itself.

EGR and the intake

Carbon in the intake tract and the EGR system, as on every modern diesel. Short journeys accelerate it substantially.

P0400 and P0403 EGR flow, P0405 and P0406 EGR sensor and short trips and engine damage.

After-treatment

• DPF — track soot load and distance between regenerations: DPF light on and P2459 regeneration frequency.
• AdBlue and SCR on later models — the countdown is legislated: AdBlue warning countdown and P204F reductant system performance.
• Oil dilution from interrupted regenerations. Above maximum on the dipstick means change it.

The delete question, answered plainly

You will find offers to remove the DPF, the EGR or the SCR system from these engines.

It is illegal in every market that mandates them, fails inspection, and is detectable through the ECU calibration: ECU remap and chip tuning detection and exhaust modification and emissions.

It also affects resale, insurance and, in many jurisdictions, the vehicle’s legal roadworthiness.

A car offered for sale with a deleted system is a car with a documented legal problem, not a bargain.

Buying used

1. Read injector correction values if you can — the single most informative reading on this engine.
2. Cold-start it yourself and listen for chain rattle.
3. Check soot load and distance since regeneration.
4. Check the oil level for dilution.
5. Check for evidence of emissions deletion — calibration verification, and the physical presence of components.
6. Check readiness and permanent codes for a recent clear: Mode 0A permanent DTCs.

Used car pre-purchase OBD check.

This check needs no workshop — an ELM327 adapter and the free tier of the app cover it. Check soot load and the oil level together on any OM651. Those two numbers, tracked monthly, prevent most of what goes wrong.