6.5t Engine knock

[PBerger]

I have a 1994 Chevy 3500HD 6.5T engine.automatic trans. The orginal problem was compression leak into coolant. Replace heads with aftermarket new heads. Ended up pulling the motor and boring the block .30 and overhaul the motor.Three lifter replaced,bearings and injector pump overhauled,nozzles repaired. Lift pump on truck replaced,turbo check out ok. Truck ran 3000 miles developed a noise.Torn down and found that the bolt hole for the valve cover broke pieces out into the intake and scored right side pistons a little and stuck rings.Reringed and replaced all pistons,bearings,had heads repair all bolt hose by the intake side.Reassembled and I still have this extra noise of pinging or slightling louder than it should,played with timing does nothing,sound like in the center of motor higher up. The only wierd thing that nobody can explained is why I have a slight mark of intake valve on some of the pistons. Head ckeck out ok.

[john8662]

Was the block decked when you had it bored?

If the block was decked, you have to compensate with the .010 thicker head gasket or use .010 reduced height pistons. Otherwise you could have pistons touching valves.

If you're seing marks on the valves, that's most likely due to interference with the piston.

Other possible causes can include timing marks off on the timing set (chain or gears as it may be).

I hate to hear of the bolt holes in the heads not being deep enough to accomodate the bolt for the valve cover leading to FOD entering the intake. You mentioned aftermarket heads? I'm guessing these are the ACH heads that are (you may not know this yet) cast in China.

[PBerger]

The Block was not decked. The marks on piston are intake and not all off them had the marks,and the machinest said the valve are deep enough that the I should not need the .10 head gaskets

[john8662]

The next step to determine where the problem is will be to measure how much the piston protrudes from the block when the piston is at Top Dead Center. I don't have the spec on hand, but I'll look it up.

It would make sense to measure only the cylinders that have the piston with the intake mark on them to give you an idea.

The above metioned will require a dial gauge and a magnetic block to affix the gauge to the deck surface. Your machinist will have the ability to check this if you take the block to him.

[PBerger]

I just want to add more information on the problem. The engine came back to me after 3000 mile with this exact noise.A upper excessive knock or autoable noise that is hard to figure out where it is coming from you can cut out the noise a little by cracking fuel line. I tore into the engine and found the pieces of metal from the intake bolt had scored right side of engine. Not damaging the block but the piston. I replaced all the pistons,bearings,I thought that was the prolbem.But I was wrong, I have a hard time believing that the piston height is the problem were on the second set of pistons and the orginnal time it did not have this noise. Could somthing like hydralic cam roller be sticking or something else. The only thing that has not been replaced is the rest of the lifters. I guess when I get back on this engine were going to try to loosen up rocker shaft to see if that will change noise difference any other ideals would be helpful I really do not want to pull the engine again.

[john8662]

A collapsed lifter could sure make a knocking sound in the upper end. You'll be able to visually find the collapsed lifter, because you will be able to see the plunger stuck downwards after removing the pushrod. Yes, I realize you can't see much with the heads installed, but you can see if you really really try.

A weak lifter (bad spring, etc internally) can cause a knock sound when starting the engine, I had one that sounded like a rod knock when you first started the engine, but it went away after running for a minute. So, if your sound doesn't go away, then it's either a completly collapsed lifter or something else internal in the engine.

One method I've been told to use to diagnose the lifter situation is to remove the valve covers and bar the engine over cylinder by cylinder and watch the rockers go up and down. When the rocker comes back up, take and press down on it manually and see how much play it has. If it's springy, then the lifter is not pumped up (could be bad, might just be from sitting). Basically you'll be going cylinder to cylinder looking for one that leaves slack between the pushrod and the rocker. So if you press down on the pushrod and it doesn't press back up, then you have a bad lifter below. You can replace the lifters with the engine installed, with some patience, a couple telescoping magnets, and some good light.