Engine Accelerates On Its own

[Lmillsap]

I have a 2020 Silverado LTZ Z71 with the LM2 engine.

A couple of weeks ago I actually took it out and drove a forest service road that needed high ground clearance and 4-wheel low in order to negotiate some rocky areas. I was in 4-wheel low and geared down to L1. As I'm walking over the rocks at idle (300 RPM), everything is fine. As I'm tip-toeing through the goofy terrain, I would work the throttle up and down. If I touch the throttle and the RPM slowly increases to 500-600 RPM, the truck responds accordingly. However, when I creeped the RPM up into the 800-900 RPM range, all of a sudden, the RPM jumped up to 1,500 RPM on its own (not good when you're trying to negotiate dicey terrain)! I took my foot off the throttle and the engine continued at 1,500 RPM. It took some aggressive braking to slow the engine down. Once it got below about 1,000 RPM, it dropped back down to idle. This behavior was very consistent. It didn't matter if I was going up hill or down, forward or reverse. The same thing happened. I haven't been able to reproduce the problem in 4-wheel high or 2-wheel drive. Anyone have any ideas on what's going on?

I have a theory (no facts to back it up) that the turbo is doing nothing at idle and low RPM, but, for some reason it's kicking in once engine RPM hits about 1,000.

Thanks.

 

[gemarsh]

Do you have a throttle controller?

I'm thinking like you, something is kicking in, just off idle. I am wondering if throttle lag is causing some of your issues also?

Did you have Hill Decent engaged also? That will try to maintain your speed downhill only. I have experienced it more like a cruise control going down hill.

Let us know if you figure it out. That is a strange one.

 

[Lmillsap]

I bought it pre-owned from a Chevy dealer in January 2022 with 26,000 miles, still under factory warranty. As far as I can tell, the previous owner only added a 1”-2” lift so he could put slightly larger wheels on it, and retractable running boards. I don’t use hill descent since it only rides the brakes.

I understand throttle lag (on old carbureted gas engines, you punch it from idle for quick acceleration, but the sudden drop in vacuum pressure from the throttle plates opening up, momentarily starves the engine of fuel until the RPM increases enough to re-establish the venturi effect to pull more fuel into the intake manifold). This is just the opposite. If I’m crawling at 700 RPM and ease the throttle ever so slightly (aiming for 850-900 RPM), once it hits 900, it jumps to 1,500 RPM. After it’s at 1,500 RPM, if I take my foot off the throttle, it continues running at 1,500 RPM.

 

[BoostN]

Seems a bit dangerous.. The only relation I can say I have is when it's in regen mode it seems to be a bit toucher on the throttle and can seem like it's jumping. But I'm talking about a few hundred RPM's, not nothing massive like you're mentioning.

 

[Lmillsap]

Interesting theory. The regen sprays more diesel into the catalytic converter(?), not into the cylinders. That would lower the instant MPG. Not sure if it would affect performance, but my guess is that performance would be less if anything (typical with most SMOG controls).