2014 GMC Sierra Downshift Engine Noise

[SteveCZ]

OK, so I've tried searching here and the interwebs for this, but only get stuff related to the transmission clunks.  A little back story....

I live on top of a hill, so every morning, to go ANYWHERE, I have to go down said hill.  Rather than use up my brakes, I let the engine "help" me slow down by manually down shifting.  What I'm noticing lately is, if I down shift to 3rd gear, and the RPMs jump up to say, 3K, I hear a quick "thrash" from the engine, almost like its trying to adjust the variable valve timing or something..... Then, POOF, its gone, and all is well.  It will repeat, however.

 

My question is, is this a sign of a bad lifter, or something else?  It's not just ONE lifter tick, and I would think it would do it all the time if it was a bad lifter.....

I plan on bringing it into the dealer shortly, but I'm figuring they will say what they normally say, "Normal operating noises".....

 

Truck has 84K on it, and has had Mobil 1 oil changes every time the oil life interval said to change it, short of the very first one I did early.... And the oil life has no bearing on the noise....

[m3n00b]

Do you have a video or sound clip?  Do you still have the exhaust flapper?

[SteveCZ]

No video or audio yet.... yes, have the flapper, but it doesn't sound like its coming from the rear, I'm 99% sure its from the motor..... It almost sounds the same as what some have found is the starter staying engaged after a cold start on a cold day.  Which to me, sounded like the valves weren't getting oil for a split second on start up.....

[goeb34]

Grade active braking?

[Tahoe02]

Grade active braking? Thats exactly what I was thinking.

[SteveCZ]

Negative.  I’m shifting manually.  Sounds like maybe a loose heat shield.  I’m hoping that’s all it is 

[SteveCZ]

Bump 

[Supreme Pizza]

Timing chain smack? 

 

Why not select the desired gear before it is needed?  That way you can still use compression braking, but without the sudden surge on the drivetrain? 

[SteveCZ]

On 7/16/2018 at 7:00 PM, starman8tdc said:

Timing chain smack? 

 

Why not select the desired gear before it is needed?  That way you can still use compression braking, but without the sudden surge on the drivetrain? 

Hmmm, possibly.... however, I'm noticing it is doing it on down shift, and also under hard acceleration (two gear kick down to pass someone).  Only happens in the range of 2500-3000 rpm, and for about a second or so, then goes away....

[Supreme Pizza]

I believe that your vehicle has "load limiting" software. When the transmission shifts under hard acceleration, the computer "lets off the gas" just for a split second as the transmission shifts, in order to prolong the life of the transmission. So when you stomp on the throttle, your engine quickly goes from power to deceleration then back to power. That may be changing the load on the timing chain from one direction to the other, then back again. It would be very brief though. That can cause wear on the timing chain gears over time, and can also contribute to timing chain stretch.

 

I do not believe that the Chevy OLM is the best option for your motor, unless you only want to get 100K miles out of it. I feel that it will definitely shorten the life of your engine as opposed to changing it every 3K miles with full synthetic oil (about $17 total). But that's another topic that has been discussed many times on this site. 

 

  This is just a theory about the timing chain, I could be way off. Its hard to diagnose and engine based on: "I hear a quick "thrash" from the engine,"

 

  A sound like that could easily be the valve train instead, which is directly controlled by the timing chain via the cams. 

 

 

Examples of some different timing chains and valves trains: 

 

(You can see why people feel that good clean oil will be beneficial)

 

 

 

 

[SteveCZ]

Well. Changed the oil and still have the same noise.... any other thoughts?