OM606 Running Rough – Injectors, Pre-Chambers and Why Diesel Noise is Normal

OM606 running rough is one of the most common complaints on built diesel engines and most of the time the cause is straightforward. Today we cover the real causes of rough running, what to check first and why most diesel noise concerns are completely unnecessary.

Diesel Noise is Normal – Stop Leaning Over the Engine

A 2008 Volvo D5 common rail diesel with 280 horsepower sounds like a 1950s tractor from inside the car despite being a modern vehicle. From outside it is nearly silent. This is normal diesel engine behaviour.

A built OM606 with an eight millimetre pump, upgraded injectors and no plastic engine cover sounds like a diesel engine. This is also normal. Standing over the open engine bay leaning in to listen to every knock and tick is not useful – it just makes normal diesel sounds seem alarming.

Put the plastic cover back over the injectors. Ensure the underbonnet soundproofing is intact. Take two steps back. You will barely hear anything except a dark rumble from the exhaust. Close the heavy Mercedes door – that sound alone tells you more about build quality than any forum post ever will – and drive the car.

A 25-30 year old diesel engine making 400 horsepower does not need to sound like a new car at idle. It needs to run correctly and make power. These are not the same thing.

Actual Causes of OM606 Running Rough

When the engine genuinely runs poorly rather than just sounding like a diesel there are two primary causes.

Faulty injectors

Injectors that are worn, incorrectly calibrated or delivering fuel at the wrong timing will cause rough running. The fix is straightforward – send the injectors to a specialist diesel shop for testing and calibration. There are shops that specialise specifically in these injectors. This is not expensive and it is the first thing to check on any rough running OM60x engine.

Worn injectors also cause combustion chamber soot build-up over time. This soot eventually sticks to the valves and causes slight valve sealing issues. Not catastrophic but it contributes to rough running and is another reason to keep injectors in good condition.

Leaking pre-chambers

Pre-chamber leaks allow combustion gases to escape past the pre-chamber seating in the head. This affects compression in that cylinder and causes rough running. A compression test will immediately identify if one or more cylinders are significantly below the others – low compression on a specific cylinder points to the pre-chamber or the valves in that cylinder.

Less Common Causes

Worn valve stem seals – these cause oil consumption and some smoke but do not typically cause rough running. Old engines with 500,000 kilometres still show original honing crosshatch marks in the bore – ring and bore wear causing rough running is extremely uncommon on these engines.

Worn camshaft – possible on high mileage engines but again uncommon given the durability of these components.

The Correct Diagnostic Process

Before building or modifying any engine verify the base condition:

1. Compression test – all cylinders should be within specification of each other. One low cylinder indicates pre-chamber, valve or ring issue in that cylinder.
2. Injector testing – send to a specialist. Have them flow tested and calibrated to specification. This alone fixes rough running in the majority of cases.
3. Visual inspection – check for obvious oil leaks, fuel leaks and pre-chamber seating condition when the head is accessible.

If compression is good and injectors are calibrated correctly these engines run exceptionally smoothly. Smoother than a 2008 Volvo D5 common rail diesel in fact.

Valve Lapping – Simple and Effective

If compression testing shows valve sealing issues valve lapping restores the seating surface. Thirteen minutes of work with a lapping stick and lapping compound and the head is essentially new. Not a complex job and the valve and seat quality on these engines is so good that lapping is all that is ever required in most cases.

NA engine valve seat sink – occasionally reported on naturally aspirated versions but not commonly seen in practice. If it occurs lapping and seat inspection resolve it.

Summary

• OM606 running rough is almost always injectors or pre-chamber leaks
• Diesel noise from a built engine is normal – stop leaning over the open engine bay
• Test injectors first – specialist diesel shops calibrate and flow test these injectors routinely
• Compression test identifies pre-chamber or valve issues by cylinder
• Valve lapping restores sealing quickly and the quality of these components makes this sufficient in most cases
• Ring and bore wear causing rough running is extremely uncommon on these engines
• Verify engine base condition before building – good compression and calibrated injectors mean smooth running

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