Knocking sound from flex plate??

[Brewtal66]

Backstory: my buddies truck needed the engine rebuilt. We pulled it, stripped it to a long block and the engine shop did their thing. The engine runs great, but has developed a knock. We took the truck back to the machine shop and they looked it over and said that it's either the flex plate(bent) or the torque converter.

Does that seem right? Would a torque converter or flex plate knock like a rod bearing? I'm racking my brain trying to figure out how a possibly bent flex plate or bad torque converter could cause a knocking sound. Mind you, the transmission works flawlessly too, so I'm thinking if it is was he's suggesting, it would have to be a bent flex plate.

Thoughts/ideas?

[MillerBuilt]

I wouldn't think a bent flex-plate would make that noise. You could setup a dial indicator down there on/against the flexplate to check.

IF it was the flexplate I would think it would be either cracked or the ring gear is separating at a point or two?

All I could think of...

[camcojb]

Originally Posted by Brewtal66

Backstory: my buddies truck needed the engine rebuilt. We pulled it, stripped it to a long block and the engine shop did their thing. The engine runs great, but has developed a knock. We took the truck back to the machine shop and they looked it over and said that it's either the flex plate(bent) or the torque converter.

Does that seem right? Would a torque converter or flex plate knock like a rod bearing? I'm racking my brain trying to figure out how a possibly bent flex plate or bad torque converter could cause a knocking sound. Mind you, the transmission works flawlessly too, so I'm thinking if it is was he's suggesting, it would have to be a bent flex plate.

Thoughts/ideas?

Does it knock all the time? In gear/out? Worse at certain times or rpms? Yes a cracked or loose flexplate can sound like a rod knock.

Does he have a mechanical oil pressure gauge? One quick test that points to a rod bearing issue is to snap the throttle open quickly and watch the gauge; if the oil pressure dips for a split second and then goes up it leans towards a bad rod bearing(s).

[David Pozzi]

You can pull the 3 converter bolts, push the converter rearward & run the engine to test it. Are you sure the bolts got tightened?

I had an engine with a shot thrust bearing. With the truck facing downhill it knocked. Facing uphill, it didn't. The front counterweight was smacking the block. You might pull & push the crank & check for end play. A rough finished thrust face on the crank could tear up the bearing, but I would suspect the flex plate more.

[Che70velle]

This has happened to me twice, and it was simply the dust shield hitting the flywheel both times. Don't know how it moves rearward, but they did for me. Two different vehicles, by the way. Unbolt the dust shield and start it...

[Brewtal66]

Originally Posted by JSM

I wouldn't think a bent flex-plate would make that noise. You could setup a dial indicator down there on/against the flexplate to check.

IF it was the flexplate I would think it would be either cracked or the ring gear is separating at a point or two?

All I could think of...

From what I researched last night, I think you're correct. There's other people that have had a "rod knock" type sound, only to find a cracked flex plate. Nobody mentioned anything regarding it being bent.

Originally Posted by camcojb

Does it knock all the time? In gear/out? Worse at certain times or rpms? Yes a cracked or loose flexplate can sound like a rod knock.

Does he have a mechanical oil pressure gauge? One quick test that points to a rod bearing issue is to snap the throttle open quickly and watch the gauge; if the oil pressure dips for a split second and then goes up it leans towards a bad rod bearing(s).

Since it's my buddies, I haven't been able to see it an diagnose it. It's actually a 2007 GMC 1500, and the engine had some weird issue, and got rebuilt. The engine rebuilder is the one that told us it was the flex plate or torque converter.

Originally Posted by David Pozzi

You can pull the 3 converter bolts, push the converter rearward & run the engine to test it. Are you sure the bolts got tightened?

I had an engine with a shot thrust bearing. With the truck facing downhill it knocked. Facing uphill, it didn't. The front counterweight was smacking the block. You might pull & push the crank & check for end play. A rough finished thrust face on the crank could tear up the bearing, but I would suspect the flex plate more.

That's a great idea. I'll try that first. Worse case I have to remove those bolts anyways. That would also tell me if its the flex plate or torque converter.

Engine has been rebuilt with less than 10,000 miles on it, so I'm going to guess no on the thrust bearings.

Originally Posted by Che70velle

This has happened to me twice, and it was simply the dust shield hitting the flywheel both times. Don't know how it moves rearward, but they did for me. Two different vehicles, by the way. Unbolt the dust shield and start it...

I'll have to check for that. In fact, I'll take it off, run the truck and see before I disconnect the torque converter and run it.


Thanks for all the replies. I appreciate it! I've just never heard of such a problem so it kind of seemed fishy to me, but it does seem like it's happened to other folks.

[Vince@Meanstreets]

I've had cracked flex plates do that. Sounds just like a rod knock. Loose flex plate to crank bolts too.

Do they still have dust shields on these? 07

[Brewtal66]

Originally Posted by Vince@MSperfab

I've had cracked flex plates do that. Sounds just like a rod knock. Loose flex plate to crank bolts too.

Do they still have dust shields on these? 07

I honestly can't remember. I don't think they do. I believe the bell housing wraps all the way around if I remember right.

I'm getting under the truck on Friday, so I'll be sure to update this post with my findings.

[Brewtal66]

Just to update and close out this thread...

It was indeed a cracked flex plate. Check out the picture...it was cracked completely on two sides, and the third side was almost all the way through.

Installing a new flex plate next week! Thanks again for all the help guys.

[waynieZ]

Great job on the diagnosis guys !