Rattle/Knock was flexplate, not timing chain

[drdavis_ks]

My 99, 4.0 SOHC, Auto, 145,000mi, Sport had a rattle/knock. I read plenty on timing chain death and was preparing for a massive overhaul. Upon total failure, found it was the flexplate. I removed the starter and found I could turn the flywheel (flexplate) with my finger.
The symptoms were a metallic rattle (like a heat shield or something) but only under 1000rpm. Over 1000 rpm it was smooth and quiet...normal.
It was very subtle, more annoying than loud. I'm pretty good but I could not isolate the rattle beyond engine-passenger side. Even changed spark plug wires figuring knock...they needed changing anyway.
Other than that, it ran and drove normal, no CEL or codes thrown. Never overheated or delivered unusual gas mileage. Failure was without warning. Tried to start up from a stop light...motor rev'd up but old green wouldn't move, kind of buzzing and sawing sound, expensive sounding. A daily driver, it went about 3 weeks from 'first noticed' to 'won't move'.
Flex plate is $100-ish, book says 5.5 hours to R&R transmission.
SO...rattle/knock does not always indicate a timing chain/tensioner issue.

 

[IZwack]

So why were you able to turn the flexplate - did the flex plate to engine bolts shear off or something?

 

[Diff Whack Daddy]

Been there done that. That's exactly what happens IZ. The flex plate basically just breaks apart in to two pieces. Imagine a donut, the flexplate being the donut lose in the bellhousing, and the donut hole still bolted to the crank.

 

[IZwack]

AH i gotcha, so the flexplate broke an outer ring and a center.

 

[Diff Whack Daddy]

When mine broke, it left about a 5 inch diameter circle atached to the crank. It looked like it started as a stress crack and slowly spead all the way arround. I didn't have the rattle, but when going into gear or from drive to reverse and visa versa, I would get a ping. The ping lasted for about 2 weeks then all of a sudden, there was a hard downshift out of overdrive to drive and the pitiful sound of clanking and banging.

 

[drdavis_ks]

Update afer repairs complete

Here is the final report. The bolts that hold the flex-plate to the crankshaft had backed out allowing the flex-plate to wiggle and rattle. As the bolts were loose, over time they wore about a 2 inch hole in the middle of the flex plate as they destroyed the 'hub'. When the last of the flex plate hub wore off, the crankshaft could spin freely and the torque converter could stop. The bolts to the torque converter were found to be secure. Since there isn't a lot of room in there, the crankshaft could kind of bang and scrape on the remains of the flywheel so it made the awful sound, but did not produce forward motion.
On reassembly, a generous coating of locktite was applied to the crank bolts.
A large pain was acquiring the replacement flexplate (flywheel). All the parts stores kept superceding to the 5.0 plate, and that would not bolt up.
Did it all in the driveway with jack stands...about 7 hours on a Sunday.
The hardest part of all was removing the rusty exhaust bolts without breaking them. Whilst scooting under and out many times, I noticed left motor mount had 3 bolts missing and one bolt remaining was broken. Engine was sagging about 2 inches! Jacked up engine and replaced those as well.

 

[derek98explorer]

I was wondering that rattle is it mostly heard coming from pass side underneath the truck because I have a 98 ohv engine and I have done everything plugs,wires,coil,seafoam,fuel filter,drive belt,and intake gaskets. Nothing helped,but I noticed when I put it into drive thats when the noise almost goes away,but while in neutral or park it rattles alot. So it goes away under load. And as you said reving the engine past 1000rpm and no noise is heard. I stumbled on this post and it sure hits home what you guys are talking about.

 

[derek98explorer]

I should mention that even before I did all those maintenece items the truck ran great and now runs just as great if not a bit more responsive. Does not miss a beat,it is very powerfull up hills tranny shifts smooth so Im stumped and frustrated that it kind of sounds like a diesel while idling.