Hi Folks, OK, I need some advice.
Back history, 2016 I rebuilt the motor with everything new. It's been flawless ever since. One day I get in for a short trip to the post office and it shifted to second, (automatic) at a much higher rpm than normal one time. Next day I went out to do the same drive and it now sounds a bit louder, low power, won't rev past 3K in neutral or when driving it. When driving and flooring it, when I let off the throttle I hear what sounds like a blow off valve air sound. BTW, I get no codes or check engine light. All the gauges work properly and there are no additional lights lit up on the gauge panel. And I have checked all 4 runners of the exhaust manifold with a temp gun. All are showing about the same temps. So all 4 cylinders are firing.
I've sonic cleaned the throttle body, cleaned the idle air motor, changed the TPS, but no change. Service manager at Chevy suggested taking the upstream O2 sensor out and see if it runs better with less back pressure in front of a possibly plugged Cat. That didn't change anything either. So I made up a fitting and put it in where the O2 was and hooked up a gauge to measure back pressure just in case. It's just under 1lb of pressure even revving it to the 3K limit. So it doesn't seem that the exhaust/Cat system is plugged up. I have run it with the air filter off and on, and even unplugged the air flow sensor. No change.
If you have experience with this kind of problem could you please advise on what may be the problem so I can get it running again? It's due for a smog check on the 28th of this month. If I don't have it figured out by then I'll have to register it with DMV as Non OP. I really don't want to do that!