GM 3.6 VVT

[Husker6.5]

Gee, you're saying that all those years that I worked repairing and designing chain drives on industrial and manufacturing equipment I didn't have a clue what I was doing? The whole point of using a chain drive is that there is no slippage like you would get in a V-belt drive. The crank rotates 2:1 to the camshaft in a 4-stroke engine. Therefore, the cam sprocket(s) have twice the number of teeth of the crank sprocket. That is a fixed ratio relationship that does not change, for every X amount the crank rotates there is always a Y amount the camshaft rotates in response. As a chain wears, there will be a slight delay between the crank sprocket turning and the cam sprocket responding as that slack is taken up - and that's compensated for by the tensioner sprocket on the leading (drive) side of the chain. Again, idler sprockets, fixed or tensioner, do NOTHING to affect the timing relationship between the drive (crank) and driven (camshaft(s)) sprockets.

Now, what WILL affect the timing, or cause mechanical interference between valves and rotating assembly, could be the timing chain slipping - but IIRC, you ruled that out early on as there were no signs of broken/excessively worn sprocket teeth or super sloppy chain.

The other possibilities could be:

1) The initial "dead timing" relationship between the crank sprocket and camshaft sprockets was off and that could be caused by keyways/flats off in relationship to the tooth position of the sprocket or to the shaft the sprocket goes onto, or the "timing marks" were off so that a sprocket in proper relationship to its shaft is rotated to a wrong position to align the timing mark.

2) All keys, flats, timing marks ARE correct upon assembly, but something causes/somehow it happens that the camshaft sprocket(s) or crank sprocket rotates on the shaft after installation during operation. And THAT is what VVT is! It changes the rotational position of the camshaft sprocket in relationship to its "dead timing" position on the camshaft in order to advance or retard the shaft's timing in relationship to the crankshaft.

From that one photo you posted earlier, it looks like one sprocket, although the chain mark looked right in relationship to it, the camshaft flat was off compared the other cam sprocket. Could it be possible that one of them was "stuck" from manufacturing and righted itself after oil pressure was built up and the ECM told it to actuate?


As for the initial bent valves you found on first disassembly, well everybody on here has gone through all the usual suspects, over rpm valve float w/piston strike, timing issue (either due to VVT malfunction/chain&sprocket) w/piston strike, failure due to material defect and of course Alien Death Ray - with no resolution as to why.

 

[ak diesel driver]

If it had only a single gear on it I would agree , add on the second gear to the idler and it's a different ball game.

 

[Twisted Steel Performance]

I wonder if a mark was off on 1 chain or gear if the more it turned the more off it would get, a progressive kind of thing?

 

[Husker6.5]

Oh yes I do. And Gunsmirk too.

And of course Rowdy Yates and Rawhide!

 

[Rockabillyrat]

So it ended up being the idler gear that was off? Im glad you figured it out! That was one hell of a head scratcher.

This is one reason I avoid aftermarket internal engine parts. But oddly enough Cloyes makes the factory guides for the GM 3.6. That last chain job I did i noticed their name on all them. Im not sure if they make the factory gears though.

 

[Husker6.5]

If it had only a single gear on it I would agree , add on the second gear to the idler and it's a different ball game.

Nope. Doesn't change a thing. You could put 20 driven sprockets with 19 idlers, one between each, and it wouldn't make any difference as to how one is "timed" to another - chain and sprocket tooth wear being ignored - as it is a fixed ratio. Like on the flaker oven at the organic ceral plant I did maintenance at. #100 double roller chain drive for the sprockets on the rollers at each end of the 40' long stainless steel mesh conveyor belt that carried the wet wheat flakes through the drier oven. That was over 84' of chain, with idler sprockets every 5' under both the top and bottom chains to carry the weight (about 400 lbs per chain!) of the moving chains. Everything stayed perfectly timed.

Or like the box assembly and stuffing machine. Multiple chains to drive sprockets that controlled the arms that grabbed the bottom flattened cereal box from the stack, unfolded the box and set them on the table where pegs attatched to the parallel chains in the table would come up behind the unfolded box and push it down the table where fingers would fold over the small bottom end flaps, fold up the lower bottom flap, hot melt bead then the upper bottom flap folded down to close off the bottom of the box as it moved down the table past the merge point where the conveyor carrying the sealed bags of cereal from the bagger machine came by and at the right moment the chain-driven cam would cause the pusher to shove the bag through the open box top into the box that moved along to the station that sealed the box top and then to the arms at the end of the table that put the box up on end and and queued them up for the packer that would stack 24 boxes into a case. At appoximately one finished box of cereal a second!

All of this driven off of one long center drive shaft with various sprockets on it that drove the chains that either directly connected to a station or to a 90°gear box to change the plane of rotation and often times had several idler sprockets and intermediate shafts with sprockets on each end to carry the power to an outboard station like the box stuffer. The sprocket tooth count controlled the speed of that individual station function, like if it had to perform a function twice during a cycle the station drive sprocket on the main shaft would have double the tooth count. Using a split hub, you could rotate the sprocket on the main drive shaft to adjust the "timing" of that station to allow for different sized boxes, like the leading small tab closing finger actuating earlier to account for the wider 16oz box whose leading edge is further away from the pegs pushing the box forward than on a 9oz box.

Once everything was dialed in during us mechanics setting it up for the next run and everything locked down, that machine would run pretty much non-stop until that order of 500,000, 840,000 or whatever boxes was ran and nothing would lose its timing the whole run.

So an engine crankshaft, two camshafts, and a tensioner and idler sprocket with one chain inherently going "out of time" without a mechanical failure of of some sort just doesn't fly with me. Especially since chain-driven (and toothed belt) DOHC configurations have been used for decades and probably hundreds of millions of miles of driving.

Now, a mechanical or manufacturing issue with the VVT phasers in the camshaft sprocket(s), that's entirely different

 

[WarWagon]

Nope. Doesn't change a thing. You could put 20 driven sprockets with 19 idlers, one between each, and it wouldn't make any difference as to how one is "timed" to another - chain and sprocket tooth wear being ignored - as it is a fixed ratio.

Looks like the idlers are not to take the slack up, but, control the relationship of the chain going to the cam from the chain going off the crankshaft. If a link is marked wrong or the teeth are machined out of time on the idler gear itself it would mess up the static timing.

They are not there to support the weight of the chain but drive a second chain.

The slack appears to be taken up by rubbing on wear blocks.

 

[ak diesel driver]

Talked to cloyes and they said that they did have some issues with the idlers a couple of years ago. They were interested in this thread, especially the pics so they could hopefully resolve the issue with the manufacturer. The did tell me that they would give me a new timing set to compensate me for my trouble. I guess I'll see what projects come down the pike

 

[WarWagon]

So they said the timing was now "ok" by the changes you made ... or are you are risking bending the valves again?

 

[ak diesel driver]

There never was a smoking gun as to why the valves got bent. Even though I've been told the phasers can't advance the timing far enough to bend valves I don't know what else it could be. The timing appeared to be right

 

[dbrannon79]

I thought I had read an article somewhere online about this where a faulty tensioner / chain guide combined with the phasers maxing out one way or the other could theoretically cause the valve contact. something about that GM was aware of the issue and knowing it was a common one too but didn't want to issue a recall or TSB on it.

I can't remember where I read that though.