can some explain the nailing process?
 

I understand the knocking of a gasser.
I don't understand what/where/causes the nailing sound.
My 3.5 dsl nails for about 2 minutes at idle when temps are below 40F.

Along the same subject, what the hell is nailing?

My impression is it's caused by leaking injectors putting fuel in the chamber before the compression stroke. The fuel detonates under compression, similar to a gasser, but worse because diesel compression is higher and diesel fuel has lower octane.

sometimes it's caused by injecting too much fuel on one cyl as compared to the rest. We do this on purpose with locomotive engines to determine if that cyl is firing. If you cracked an injector line to do a balance test you would wind up bathed in diesel fuel.

Why only when cold?
Then if I retard the injection timing, will it be off when the motor heats up?...which can't be right since the timing is fixed.

Nailing is a much louder than usual "pinging" you usually hear. The springs inside the injectors may weaken over time allowing fuel to be sprayed earlier than required. The new CDI diesels are able to pulse electrical injectors very quickly and for multiple times over the combustion event to almost eliminate the diesel ping.

I drove a new CDI a couple of weeks ago and it was really quiet. Although you you could still tell is was a diesel.

Nailing is caused by too much fuel igniting all at once. This can be caused by many things, such as poor injector spray pattern, low compression, carbon in the prechambers, or low-cetane fuel. Cold temperatures will exacerbate the problem because a cold diesel has more delayed combustion compared to a warm diesel. I think only in rare cases is it the injector springs causing the problem. I've tested injectors that I believed were over 20 years old with all original parts, but the pop pressure was only slightly lower than where it should be and it was still within spec. Hope this helps.