6.2 flexplate chatter

[EJohnson]

I recently installed a new Powerbuilt/Reviva 6.2L w/Banks turbo in my 1990 GMC Suburban. This has increased a problem which has occasionally occurred in the past which is a chatter/ping sound under acceleration. This started with my previous custom built engine which was installed in 2001. It now occurs regularly under even light acceleration. The chatter disappears immediately at any road speed when the throttle is lifted or the transmission is shifted into neutral.

It appeared that the starter Bendix drive was periodically contacting the flexplate ring gear. Reasoning that the drive was not fully retracting, a new starter was installed and the chatter became near constant under even the gentlest acceleration. Close investigation of the starter/flexplate area with my mechanic revealed that the flexplate deviates from true run-out by as much as 3/16" during powered running rotation. A second new flexplate was installed and it was close to 1/4" out of true. The first flexplate has the greatest deviation in the area of the balance weight, the second is 90 degrees off that! I am pretty sure that the first flexplate is the one the original 6.2 w/factory option Banks turbo was built with in late 1989

Despite a lot of web searching I have been unable to find much discussion of this type of problem, causes or solutions nor any source of extra stiff/trued flexplates. Lots of stiffened/thicker and balanced plates available for high powered gas engines but none for our 6.2/6.5.

Has anyone else run into this after boosting the HP & Torque on these engines? Any ideas for causes solutions? Thanks for your help.

[Robyn]

Has the tranny been rebuilt??
What I am suspect of is that you have a convertor that was rebuilt and the snout welded back on out of concentric with the front pilot. This is a condition that pops up ever so often. We used to see it every so often with a few companies that did questionable work.
If this is the issue the convertor will wobble like a button on an outhouse door. The stress on the tranny is horrific as well as the crank in the engine.
Get the rig on a rack and drop the cover and run the engine. If the convertor is wobbling too which it almost has to, herein lies the issue.
The front pump bushing and seal must be replaced in the tranny if you replace the convertor.
I will assume you have a TH 400
In rereading your post I see that once you replaced the flex plate the area of runout moved, this is due to the converter being indexed differently.
While you have the beast on the rack, Unbutton the convertor from the flex plate and rotate the convertor free of the flex by hand, I will be willing to bet that you will see that some of the bolt lugs are not contacting the flex plate and only do so when pulled up with the bolts. The lugs should all contact the flex when the converter is slid forward lightly by hand. When you rotate the convertor with the lugs touching you will most likely see some go by that are not touching the plate.
I am highly suspect of that converter.

Good luck and keep us posted
Robyn
Robyn

[EJohnson]

The TH400 tranny was rebuilt about 8-9 years ago and has been flawless since with the fluid (Valvoline Maxlife) and filter changed every year. The first 8 years of it's life it towed a 31ft Airstream in the winter and since Dec 1999 it has only towed a 4k lb BSA Scout troop equipment trailer about 6 times a year.

Bruce and I had the Sub on his rack under power with the cover off and did not notice converter wobble but did see it on the flexplate. The run-out measurement was done with the flexplate off the tranny and on a lathe. We are presently speculating that the FP may have gotten tweaked in shipping or engine bay insertion.

[Robyn]

The converter would straighten it out pretty much. There are 6 bolts holding the plate to the converter.
There is another possibility and that is the thrust main in the engine has gone away allowing the crank to move back and forth too much.
Check it with a Bar at the front behind the ballancer. If you can move it back and fort more than .010" there is an issue.
I have seen cranks go but usually there are big time issues.

[EJohnson]

Yeah, the fact that it's bolted at 6 points halfway out the plate has got us scrathing our heads as well. The engine is a reman with less than 1k miles on it. It is a complete drop in with everything from the injection pump to the oil pan supposed to be new except for the crank which has been fully speced. This is my 4th 6.2 between 2 Subs and is by far the most powerful and smoothest engine of all. It is so smooth most of my friends thought I put in a gasser! which is what makes this really frustrating.

[Robyn]

Run the engine with the converter unbolted and shoved back out of the way and see what you get.
This can only be a couple things.
Robyn

[gmctd]

Could be a warped starter ring gear, but you might also try a shim between the starter and the block - some require it, some don't, blocks and starters.

Available in several standard thicknesses.

[Robyn]

The ring gear is running out 3/16 to 1/4 the starter gear or shim is not an issue here. The converter has to be running out or something else is amiss.

[EJohnson]

We shimmed both starters early in this effort and all that did was make the teeth mesh real loose and noisy on engagement. The chatter/ping on throttle application still occurred. Remember this noise only occurs on throttle application no mater what road speed, even in drive on the rack. When the transmission is in neutral the engine can be revved and no chatter/ping occurs. I suppose the FP could exhibit one pattern of distortion when bolted to the converter and another when off and just center clamped to the lathe spindle. Can torque converters expand/distort under load but exhibit no other problems?

[Robyn]

yes.
Its been a long time but converters have been known to "balloon"
Check your line pressure via the pressure tap on the tranny with a gauge that will read to 300+ pounds. Do this under operating conditions and see if the pressure is responding to the throttle.
You should see the pressure drop off at idle and peak under acceleration.
I am still very suspect of the convertor.
If the "flex plate/ Converter" is running out of concentric it almost has to be the converter.
I have seen these many times though and never seen one with the gear hitting the starter teeth.
Uncouple the converter and slide it back and see if the plate still wobbles badly with the engine running. If it is fairly true then its the converter.
I hate to ask this??? but are you sure you have the flex plate bolted on to the crank properly, (Proper side to the engine) I have seen this once.
Then weight on the flex plate should be on the engine side.
I am not even sure its possible on a 6.2 to get it wrong.
Look these things over and see.

Check for converter runout with the bolts out and turn it gently by hand and note the proximity of the bolt lugs to the plate

Run with the converter unbolted and slid back to see if plate wobbles.

Check to see if plate is on correctly

Keep us posted

Robyn

[gmctd]

Have you checked the plate for cracks - where it bolts to the crank?

If not, I'd as soon not even hint at the other possiblility, just yet

And, btw - trying to start and run the engine with the tc unbolted would not be a good idea

[Robyn]

Running the engine with the converter unbolted wont hurt a thing for a short test.
Its done all the time. Keep fingers and hair and clothing away though.
I have done this many times to diagnose bad converters.
We are only talking a couple minutes.

[gmctd]

A flex plate does not have enough mass to compensate for the 400psi x 8 compression stroke in a Diesel engine - if it would even start.

Gassers - possibly excluding those with 12:1cr, but then again, they have large-overlap camshafts - cannot even begin to duplicate the rotational inertial forces encountered in a Diesel engine.

That's why the flywheels are so much heavier than gasser 'wheels, and the tc is the largest GM offers - the D4.

Even larger and heavier with the 4L80E for the 6.5

Just not a good idea to attempt that test - imo, of course

[Robyn]

The engine will start fine.
The flywheel does add mass but does not effect engine ballance.
The engines are all ballanced and the rotational mass needed is in the crank shaft counter weights.
We are not talking an extended run time here, maybe a minute if that.
I have been there done that. The engine would run fine even without the flex plate other than part of the ballance weight would not be there.
Only issue there would be a way to get it started.
This fellow needs to figure out what is causing the flex plate to wobble.
This can usually only be cause by two things. (Not counting a broken crank)

Converter poorly remanufactured with hub and pilot out of concentric

Flex plate on backward with weights to the back causing the converter to bolt up wrong bending the flex plate

[EJohnson]

Thanks all for the input. I have not been able to return to this project and problem as I have been busy with work and dealing with the AGP440.sys XP lock & hang problem (a nasty known to MS flaw that goes back to Win 2K) so I was without my computer for most of last week.

It continues to chatter and it will probably be early next week before I have anything further to report