6.2 heavy knocking when accelerating

[korjaamo]

Loud knocking and grey smoke when pedal is pushed. Idles fine and when accelerating slowly, no noise. No power on motor. Done so far:
Injectors checked at pro, injection pump reconditioned, new camshaft sprockets and chain, new glow plugs, main bearing clearance and big end bearing clearance checked, was at factory spesification maximum, valve train checked, combustion pressure checked, ignition advance tried 3-8 degrees. Works best 3 degree. Damper is fine, turbine is fine.
Problem started when owner driver car with broken valve spring about 50 miles. Valve springs renewed, then noise started. Owner takes cylinder heads off for inspection two times. Cannot be sure if done ok. One glow plug was with broken tip. Next job is to take heads off again?

[john8662]

Welcome!

Sounds like this engine has been apart a few times.

I'm guesing from the last statement that it ate a glow plug tip and that tip may or may not have made it out of the cylinder. The tip could be in the cylinder still!

My experience tells me that once stuff gets in the cylinder, that it doesn't simply escape easily, if at all besides removing the head and inspecting the cylidner carefully. I'd go as far as to remove the piston to make sure that it wasn't damaged along the wrist pin side.

Other sources for knocks can also be external to the engine, such as a broken flexplate/ring gear for the transmission, loose bolts. Injector lines that are touching other components will also make a knock every time that cylinder fires, so make sure that the rubber isolators are present and that the lines are clear from contact.

J

[korjaamo]

More info now. Heads removed , both head had 1 prechamber loose. They are now fixed. But before installing I measured piston to deck height. All but one piston are too low . 7 pistons stay under deck about 0.05mm and only one was over but only 0.1mm. Our book says that piston must come over deck 1.25mm, is this right?

[john8662]

By pre-chamber being loose, you mean that it practically fell out of the head when you removed the head? I've seen this quite often, it's really about normal, some fit better than others, I prefer them tight too.

As far as compression height.

The piston should protrude from the deck .125mm not 1.25mm, or .005"

If the pistons are below the deck, it will have lower compresion, due to reduced compression height.

You're checking all these pistons at TDC to get the height.

Are you checking this height with a dial gauge and a bridge?

Any sign of piston to valve interference? Any exhaust valve contact circles on any of the pistons?

Probably a little more investigation before installing the heads again, considering what this engine and been though.

[korjaamo]

I noticed my error, all pistons are close enough. All odd pistons are worn, #7 is very worn and maybe skirt is turned. next engine off and chek all pistons and tolerances.

[korjaamo]

Now disassembled the engine. One crankpin has too much clearance, crankshaft bearings are a mess, crankshaft is very scored. Bores are taper and out of round, and clearance as big as 0.20 mm

[john8662]

Sounds like it's rebuild time with larger pistons.

Crank needs to be turned and or find another standard diameter crankshaft.

If you're replacing pins, do them all.

The bore clearance is large, I'll bet the engine had quite a bit of blowby as a result.

But, how are you taking your measurements?

What's your next move?

[korjaamo]

Engine has now oversize pistons, crankshaft undersize. New pistons, pins, rings, bearings, camshaft bearings, crankpin bearings. Some assembling now.

[korjaamo]

Problem solved, engine runs fine and sounds nice. Also nice power now.