knock at low rpm

[AvlAndrew]

I just repaired a 90 B2 engine, that had a bent intake valve due to a bad fuel injector
so re-installed the engine, its running smoothly, but there is a knock at idle from what seems like the bottom end where the oil pump is located. this knock goes away at 2k rpm. I am inclined to replace the oil pump.. any suggestions appreciated.

 

[MaicoDoug]

Well you know there are other things that make knocking sounds down there for instance there is always the infamous "rod knock" connecting rod not happy with it's fit with the wrist pin and / or the crankshaft. The first thing I would do is install some heavy weight additive oil and listen again. The increased viscosity should change the sound if it's oil related. New oil filter installed I hope. Also looking for an extended "dwell" at TDC watching at the piston with the head removed with a dial indicator. I really hope these helps with the noise, something tells me that it will. The distrubritor also has stuff moving around down there like it's drive gear if so equipped. Excessive shaft end play is how that wears out.

 

[AvlAndrew]

i installed a new oil pump, will fire it up tomm and see if that was the issue.

 

[AvlAndrew]

still same knock, possible crank shaft bearing cylinder 3

 

[MaicoDoug]

Try increasing the oil viscosity with a bottom end treatment such as motor honey or the like. That will make the sound go away quite a bit. Doing this should help you trouble shoot the crankshaft and connecting rod excess clearance knock issues.

 

[AvlAndrew]

Update, it ended up being a cracked flex plate.
Knock at Idle gone!

 

[alwaysFlOoReD]

Thanks for the update.

 

[rusty ol ranger]

Update, it ended up being a cracked flex plate.
Knock at Idle gone!

I got nervous...that woulda been one of a very few 2.9 bottom end failures ive seen around here lol

 

[Blmpkn]

I got nervous...that woulda been one of a very few 2.9 bottom end failures ive seen around here lol

Yeah yeah yeah.. I never need to worry about a cracked flex plate with my lima hehe

 

[rusty ol ranger]

Yeah yeah yeah.. I never need to worry about a cracked flex plate with my lima hehe

You could replace the flexplate on a lima with a piece of cardboard and it wouldnt have the torque to break it