HELP! Knocking sound from 390

[FishCakes428]

Just shooting from the hip here and maybe barking up the wrong tree....................but if all parameters with the engine check out i.e., oil pressure, power, no misses or hiccups, compression, still pulls out hard etc., I would ask manual or automatic trans? The guys here have a whole lot more experience than me. Maybe if auto, a converter bolt, flex plate bolt, converter after trans fluid heats up? Manual trans, well maybe flywheel bolt, pressure plate bolt or such. Little too much crank thrust clearance??

Does the knocking follow the engine RPM's in park (auto) or neutral (manual)?? A broom stick would reach the trans or general area of the bellhousing from underneath the car. I have used a stethoscope myself and it's an invaluable tool to at least get you in the right neighborhood.

Good luck and stick to it. Not my intention to send you on a wild goose chase.

[Lenz]

You re-torqued the manifolds and changed the donuts and it went away.  I would highly recommend looking at your manifold the head gasket situation.  If when you re-torqued it went away a compromised gasket may have been temporarily silenced.  A heat and cool cycle or two would open up a bad gasket fairly quickly, and other than the donuts that is the only variable you messed with to get the "knock" to go away. 

[NWwildman]

Just shooting from the hip here and maybe barking up the wrong tree....................but if all parameters with the engine check out i.e., oil pressure, power, no misses or hiccups, compression, still pulls out hard etc., I would ask manual or automatic trans? The guys here have a whole lot more experience than me. Maybe if auto, a converter bolt, flex plate bolt, converter after trans fluid heats up? Manual trans, well maybe flywheel bolt, pressure plate bolt or such. Little too much crank thrust clearance??

Does the knocking follow the engine RPM's in park (auto) or neutral (manual)?? A broom stick would reach the trans or general area of the bellhousing from underneath the car. I have used a stethoscope myself and it's an invaluable tool to at least get you in the right neighborhood.

Good luck and stick to it. Not my intention to send you on a wild goose chase.

It is an automatic, I will give it a look over in the bolt department and triple check that everything is tight.  And yes the knock follows engine RPM directly in all gears and park and it really sounds gutteral like a rod knock, especially when I am WOT driving (only makes the noise above 3000 rpm(ish) when under load vs 2000 when in park revving.  again, quiet as a mouse at idle hot, cold or sideways.

[NWwildman]

You re-torqued the manifolds and changed the donuts and it went away.  I would highly recommend looking at your manifold the head gasket situation.  If when you re-torqued it went away a compromised gasket may have been temporarily silenced.  A heat and cool cycle or two would open up a bad gasket fairly quickly, and other than the donuts that is the only variable you messed with to get the "knock" to go away.

I have thoroughly investigated the exhaust and can't seem to find any leaks, I am not ruling it out but this knock is pretty intense when it does come on... Though JayB and some others have mentioned exhaust leaks being convincing as knocks.  Man this is frustrating.  It runs so darn good, I can't picture it being a wrist pin or rod bearing but on the other hand I just can't seem to narrow it down.  Starting to think I'm gonna have to get my wife to drive it while I ride on the fender like Stroker Ace with the stethoscope to find this damn knock.

[57 lima bean]

Broken flex plate.

[NWwildman]

Broken flex plate.

Yeah, it's a deeper tone than I've heard from a flexplate before but I probably should get out the mag light and have a real good look at it.  I've heard a bunch of chevrolet's with broken flexplates but never broke a Ford... nonetheless I'll have a look.

[cjshaker]

The car will make a slight knock very quickly when it first fires after sitting for a while

This is the key statement. A bad main bearing will typically make a noise when first started, then quickly go away as oil pressure reaches it. It could still be a rod though. You're not doing it any favors by running it at WOT, trying to figure out the problem. If you can't find any external issues, then it's time to start tearing into it and find the problem...before something worse happens.

[NWwildman]

The car will make a slight knock very quickly when it first fires after sitting for a while

This is the key statement. A bad main bearing will typically make a noise when first started, then quickly go away as oil pressure reaches it. It could still be a rod though. You're not doing it any favors by running it at WOT, trying to figure out the problem. If you can't find any external issues, then it's time to start tearing into it and find the problem...before something worse happens.

I realize the potential danger of WOT runs if there is a real issue at hand.  I have only done a couple in an effort to narrow down the rpm range/cause of the issue.  It does have a tiny knock on startup when warm that lasts just long enough to be audible then clears up.  I guess my quick fix used 390 will be anything but.. looking more and more like I need to halt my other projects and finish the engine I was already building for the car.  That said, I'm going to investigate the flexplate today or tomorrow.

[Tboggus]

Also, check harmonic balancer. I've seen one that the rubber was separating on and it would make a weird knock. Also, saw a brand new brand x pickup engine replaced under warranty at the dealership........ Only to find out driver's door mirror loose......