Hey guys, we've been chasing an issue with our 4.3 Vortec powered 09 Silverado. Essentially what's happening is that under acceleration (especially over 30 mph) the engine RPM will increase but the vehicle will barely accelerate. It will even enter passing gear and proceed to go nowhere. Initially our mechanic figured it was a fueling issue and changed the pump, nothing changed. Then we looked towards the torque converter or tranny itself, we ordered one and put it in after waiting two months for it to come off of backorder. The problem continued.

The vehicle has no codes currently outside of an intermittent EVAP issue and a knock code that appeared once when we floored it out of frustration. There was a lean code once that showed fuel trim in the 25% range which made me concerned about vacuum leaks, but none were detected. The live data that I've seen since then shows relatively normal fuel trims with long term being below 10%. We have checked fuel pressure with the new pump, we have checked exhaust back pressure, changed the O2 sensors, the MAP, MAF and TPS sensors, and we've checked for vacuum leaks. At this point, we're completely dumbfounded. It feels like a slipping converter that's not putting any power to the wheels. The motor is no spring chicken, but it starts right up, idles smooth, runs smooth and has good oil pressure. If there was something wrong mechanically I'd be shocked.

Thanks for reading any help would be greatly appreciated, thank you all.

 

I have to admit I am no expert with automatics, but thats where I'd be looking if I were you.

 

Yeah, the tranny was our second guess and we replaced it with a brand new 4L60 straight from Chevy. I know parts come in bad at times, but for the same issue to be present on both transmissions the odds would have to be infinitesimally small.

 

Yes I agree.