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Old 09-06-2019, 10:28 AM   #1 (permalink)
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Mystery knock

Trying to figure out a knock that goes away. Truck is a 91 suburban, 350, 700r4, 4wd, 260k miles or so. Put a new starter in this summer, when I fired it up it started knocking, didn't knock last fall. Prior to me owning it the p.o. parked it after they had a massive oil leak they didn't want to fix. After I fixed the leak and changed the oil last fall no knocking. Its definitely louder from under the truck.

Last weekend my dad was here and we were looking at it. He noticed as soon as I put it in gear the knock totally stopped. Had him rev it while in gear and no noise, drove down the drive while I walked along and no noise.

Just pulled the inspection cover off the transmission, torque converter bolts are tight. Nothing was sitting in the cover. Started the truck, knock in park, looked underneath and couldn't find anything hitting anything, in gear no knock. Not fully committed to pulling the trans to see if it's the flex plate bolts yet.

Any thoughts on what would knock in drive but not in gear? Seems like if it was a rod or something in the engine it would be constant.
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Old 09-06-2019, 10:32 AM   #2 (permalink)
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Cracked flex plate, pretty common
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Old 09-06-2019, 11:12 AM   #3 (permalink)
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Quote:
Originally Posted by KWTMECH
Cracked flex plate, pretty common
I've had a flex plate go bad to where the starter couldn't reach to engage the teeth before. Got back under with a flashlight and started inspecting. The only thing I could find was a spot up by the crank, maybe 3/4" that could be a crack. Didnt see anything else anywhere.

[IMG][/IMG]

Best of the bad pictures I could get of it
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Old 09-06-2019, 05:47 PM   #4 (permalink)
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With the trans access cover off you should be able to get a pretty good look at the flex plate. If you don’t have a flywheel turning tool, you can use a screwdriver on the teeth to rotate the engine and look for cracks. While you are under there pry between the flexplate and engine block to see if there is any front/back movement on the crank.
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Old 09-06-2019, 07:45 PM   #5 (permalink)
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The spot in the picture is the only thing I could find resembling a crack. I did pry on the flex plate against the torque converter, not the engine, but didn't see any noticeable movement.

Any harm in driving it as is? I wont be able to get to it for at least a week but I'd like to use it for a few 5ish mile trips to hunt geese.
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Old 09-06-2019, 09:09 PM   #6 (permalink)
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If it does have a cracked flexplate, it will get worse. Check the bellhousing bolts also. I had a SBC 400 that started knocking. 3 bellhousing bolts were missing and the others loose. I had just bought it 2 weeks before. Flexplate was cracked also.
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Old 09-06-2019, 09:32 PM   #7 (permalink)
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Eventually the flex plate will crack all around, and be undriveable.
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Old 09-06-2019, 10:02 PM   #8 (permalink)
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Looking at that pic it looks like a circular crack near the end of the crank by the bolt holes, but yeah, you have to turn it and look

Last edited by KWTMECH; 09-06-2019 at 10:02 PM.
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