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Knocking


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1990 MJ, 4.0, aw4, np231

 

truck recently started knocking, very loud and from underneath. Flushed the oil with fresh oil Incase it was an oil issue, no change. Engine running absolutely perfectly I should add. Checked flex plate bolts and one was loose, tightened up and still knocking but not as bad, still bad though. While underneath tightening the flexplate bolts and revolving flex plate, it sounds as if something is interacting with the thin aluminum spacer that is inbetween the trans and engine, sounds like a small pop at about 12-1oclock when rotated by hand. Any chance the flexplate would not be square, for lack of better term, or wobbled itself out of square with the loose bolt? Any benefit to loosening all flexplate bolts and retightening in a cross cross pattern? 

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When my flexplate cracked, it was in a jagged ring just along the edge of that "spacer". It's less a spacer and more a load distribution plate, basically a fancy washer that gets all the bolts in one piece. When I first looked at it there was just barely one shiny hairline that made me think there was a crack, and I was correct. I was also able to turn a couple of the torque converter bolts, but I wouldn't have called any of them particularly loose. Like maybe 1/4 turn or so with a 1/2" ratchet. 

What I'm pretty sure lead to the flexplate cracking was that my bellhousing to engine bolts had backed off, and variously wiggled out and/or sheared, or pulled out threads. It had been knocking a little at specific rpm and loads as the engine and trans banged against each other, then when the next-to-last bolt went, it was still driveable but made a hell of a racket. 

If you have a stethoscope or screwdriver, length of hose, etc, you might be able to better pinpoint the source of the knocking. 

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I’m really hoping it’s not the flexplate as I just changed that about a year ago now, haven’t pulled the trans to see for sure but no visible cracks That I can see from the inspection cover area. I have been battlting an issue with a top bell housing bolt backing out but I don’t think this is related as that bolt hasn’t caused an issue in a few months now, but I suspect that while loose it could have allowed the spacer to move/bend/tweak. Weird part of this knocking is when stone cold first thing in the morning the knocking is not there until warmed up. I’m fairly certain something is bent and interacting with the flexplate bolts, that’s why I’m thinking it’s the spacer between block and bellhousing, to be clear I’m not talking about the washer that bolts from the crank to flexplate.

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