D6 8U engine knock

Hello; hoping for some guidance before digging into an engine knock problem on my D6.

Tractor is D68U9088 which has the D318 engine. I was doing some backfill work with her the other day -- ran fine in the morning, left her to idle over lunch and when I throttled up after lunch to go back to work it had a knock (or rap, or hard tick depending on one's perspective.) Knock is in time with engine but, I think, slower than the crankshaft. Engine does not miss, no smoke out the stack, no blowback or noise from the air intake, no blow-by out the crankcase breather or oil fill. Knock occurs at all throttle positions except dead slow idle.

I pulled the oil filters, dissected them and found no foreign material. Drained the oil, sieved it and nothing there either. Next I let her idle at about 1/4 throttle and cracked the injector lines one by one to try and isolate the knock to a cylinder -- no luck. I got the expected RPM drop (and fuel shower) from opening each line but the knock persists unchanged.

I'm concerned it's a connecting rod bearing although I would have thought there would be junk in the oil pan. If I'm right and the knock is slower than crank speed then I would think it's a valve but if so it should be missing.

Before I start tearing things down does anyone have some ideas? I really don't want to have to pull the belly pan and oil pan if I don't have to.

Thanks in advance,

Pete P.
Harborcreek, Penna.

It may be a loose liner, they will make a ticking noise when loose. No air bubbles in the radiator tank? Other thing may be a stuck valve. It will have a lisp in the exhaust pipe tone.

One fairly easy thing you could check the valve clearances. Other than that all I can think of is a wrist pin which I assume would only let the piston slap on exhaust stroke TDC.

You can get in the crankcase through the side covers without too much trouble. It would be well worth the effort to avoid flying bits of shrapnel! I think you should be able to feel any slack in a rod either top or bottom end that was allowing the piston to hit. Depending on which cylinder was affected, you might even be able to pull the head and remove a piston working through the side covers.

I worked on an Allis Chalmers HD11 once that had a fairly loud clicking knock but ran pretty good. Took a while to find it but it turned out to be a broken valve spring.

Rods usually knock under heavy load, wrist pins knock when lightly loaded. If it's at less than crankshaft speed I'd be checking valve train.

What about injector nozzles? Can’t they mimic a knock?

He tried that, no diff.

Well, that settles it then. Off with the valve covers, check the springs and measure clearances then off with the side covers.

I'll post back with what I find.

Thanks all

Pete P.

Did you ever find the problem? My D69u has developed a knock as well. Sounds similar to your situation