Knock in bottom end at idle only, help with diagnosis

[pdq67]

If it has stock cast Pistons it is a broken piston skirt.

Usually a cracked or broken piston skirt doesn't stop above an idle. And as always, jmho..

pdq67

[fdicrasto]

Sounds like you have located the problem. Hopefully it is not a crank snout issue and replacing the balancer will get you back on track.

[Circlotron]

You don't mention if the engine is a fresh build and noisy from the start, or has been running okay for some time and the noises developed. In the early '90s I had a junk 179 in an even more junky HQ (weird combo, LOL) and when the fibre timing gear stripped I pulled the cam and gear out and replaced it with a cam that I had lying around that had an alloy gear. For the first few hundred miles it whined quite a bit, and when that finally settled down you could hear a knocking sound at about 1000rpm between the fuel pump hole and the back of the timing cover. After several thousand miles it finally calmed down and eventually was as quiet as the OEM timing gear. Must have had a high spot on one of the gear teeth.

[Belgian1979]

FWIW, if you have a knock sensor on the engine, connect it to a pair of headphones and listen. You wouldn't believe the noises of individual parts you are able to distinguish this way.

[68maliblue]

I had a harmonic balancer with a crack in the key way that did the exact same thing as you describe. Replaced it with a new balancer and problem solved. If your balancer is moving at all that would be the first thing I would check. Pull it off and check the key way. The crack will be obvious if it's making as much noise as you describe.

[xanadu]

Hope you secure the problem. Just on topic of noises with a stethoscope, had a Windsor in the shop with a loud clunking noise at idle, not quite like your noise, but close. Turned out that the aftermarket oil pan stated 8L of oil fill. But should have been 6.5L of oil. Turned out that the extra 1.5L of oil in the pan (I can only guess) was enough to cause massive windage / counterweights slapping oil around to cause a dull knocking noise at idle.

[TEROL]

FWIW. Back when I knew little but thought otherwise I assembled an AMC 360/390 with PAW (long gone Performance Automotive Warehouse) parts. It had a nearly inaudible tick at idle. After several attempts to locate the issue my upset friend took it to a professional for diagnosis. The results were poor quality parts not dimensionally proper. The piston skirts were touching the crank counterweights. Lesson learned.

[PackardV8]

FWIW. Back when I knew little but thought otherwise I assembled an AMC 360/390 with PAW (long gone Performance Automotive Warehouse) parts. It had a nearly inaudible tick at idle. After several attempts to locate the issue my upset friend took it to a professional for diagnosis. The results were poor quality parts not dimensionally proper. The piston skirts were touching the crank counterweights. Lesson learned.

For true; I had the same issue with a Packard V8. Egge Machine was the only supplier of pistons and one skirt was just ticking a counterweight. It got louder as more heat got into the piston. The interference was so slight it was almost impossible to see. Finally, my partner noticed a silver streak on a counterweight.

[benno318]

FWIW. Back when I knew little but thought otherwise I assembled an AMC 360/390 with PAW (long gone Performance Automotive Warehouse) parts. It had a nearly inaudible tick at idle. After several attempts to locate the issue my upset friend took it to a professional for diagnosis. The results were poor quality parts not dimensionally proper. The piston skirts were touching the crank counterweights. Lesson learned.

For true; I had the same issue with a Packard V8. Egge Machine was the only supplier of pistons and one skirt was just ticking a counterweight. It got louder as more heat got into the piston. The interference was so slight it was almost impossible to see. Finally, my partner noticed a silver streak on a counterweight.

i reco'd a head for a mitsubishi diesel inline 6, 6d16, about 7.5 litre capacity - customer and his mechanic mate were doing an inframe rebuild of it -anyway the customers mechanic calls me all nasty telling me i have done the head wrong and valves must be hitting pistons etc. as when it warms up it gets noisy...

generally i cut all my seats to the minimum depths on diesels to allow longest service life, maximum compression etc. and i had done just that on this head, i wondered if the specs i had been given were wrong etc.
anyway he brings the head to me and i see piston alloy on valves etc, but i also saw the part numbers of the pistons along with the shape of the crowns, embedded into the face of the head!

asked if he even measured piston protrusion and the reply in a condescending tone "i have built hundreds of diesel engines and i have never had to do that before"
i lent him a dial gauge and off he goes to measure, came back with the pistons, man were there some self machined valve notches in them!! and a spec he wanted me to machine off the crowns, and then off he goes all embarassed, so much so that he never paid the bill for that work i did!!
from memory it was something like 10 thou removed off most pistons, and the noise wasnt there until warmed and loaded up, so goes to show the growth of alloy...