NAILING
 

I have seen posts that mention NAILING. What exactly is nailing and how does it sound. My 240D engine sounds like a diesel. How do I know if it is NAILING.
Anthony

Nailing is caused by worn injectors. When injectors get worn out they can leak fuel even when not at pop pressure. This causes excess fuel in the cylinder which creates hard combustion which is the noise you hear. It's an extra loud sharp clacking noise.

It sounds like someone driving a spike into a piece of oak, hence nailing.

I would say it sounds more metal-on-metal hammering but I guess thats splitting hairs

When I had the problem on one of my diesel MB cars of past -- It was almost like a "spark knock" you would get in a gasser...Replaced the injectors and noise went away...

Not to split hairs, but the hammer would be striking a metal spike.:D

nailing is a detonation at any undesirable time during the stroke.

like an explosion that is not absorbed by the movement of the piston ()think of catching an egg from an overhand pass()

If that's the case, the injectors may not be the only possible culprit, right? Incorrect IP timing could cause undesirable results. Would this sound like nailing?

there are certainly more culprits.

IP timing could do it as well.

Fue Injection Pump Timing can case Nailing, worn Injectors as well as air in your fuel system.

If you get your Engine started on a cold day without using the Glow Plugs you will hear the Nailing sound until the combustion chambers warm up enough for the Fuel to burn properly.

My Mitsubishi Fuso glowplugs only activate below about 35 degrees. When I start it at 40 degrees it nails like crazy until it warms up.

Is Nailing to be expected in a cold engine? Mine nails a little during warm up and is gone when the engine reaches 80C.

I wouldn't necessarily say expected, but certainly a common experience among higher-mile cars with questionable injectors so don't be too freaked out.

The Genesis of 'Nailing'
 

All of the following are from my personal experiences so please challenge me as you see fit. Some of this has not been written about or at least I have not come across it.

'Nailing' has many causes. The sound is produced from a secondary combustion event which is temporally and/or spatially separated (usually retarded in time) from the main explosion. Generally it can be caused by an Injector with an uneven spray pattern or with an imperfectly atomized trickle flow.

In a normal situation a coherent explosion with a smooth well defined flame profile occurs at the expected time. Nailing represents a perturbation to this process caused by one or more of the following:

• Assymetrical injector spray pattern
• Carbon build up in the pre-chamber and other explosion space
• Mechanical distortion or damage to the explosion space
• Damaged or distorted atomizer
• Air ingress into the hi pressure fuel lines (Many causes give rise to this including air ingress into the lo pressure side being referred into the hi pressure circuit sometimes but not always by the IP)
• Incorrect IP timing relative to crankshaft/cam timing
• Mal-adjusted valves
• Very low temperature in the explosion space caused by GP issues, low ambient conditions, etc.
• Low compression

I did this from my head so I may have missed some but you get the point.

As was pointed out it does acoustically sound somewhat like what is referred to as 'pinking' or 'pinging' or 'pre-ignition' where low octane gasoline is used in a hi-compression gasoline engine. It is a different phenomenon however in a Diesel engine since it is usually a retarded timing event that gives rise to 'Nailing'.

Is it bad for an engine?? Let us agree that it is not good but since these engines are extraordinarily robust it does no harm as long as it does not persist all through the operation. After warm up most nailing will disappear. 'Pre-ignition' in a gasoline engine however is very destructive. I have seen a piston with what looked like a bullet hole through its face from the effect.

Does it negatively affect MPG?? Of course the answer is yes. Energy is being used in an undesired manner that is not contributing to the desired power flow.

I would add mal-adjusted injectors to Dion's very complete list as well. Differing pop pressure between cylinders will affect timing and/or combustion. The spec on my engine is < 3 bar between cylinders.