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!

Larry

Sorry, I know I'm a little late with this reply. I hope you have figured out the problem and back on the road. If not here are a few of
the first things I check when dealing with a mystery driveability problem.

Fuel pressure: many early obd2 cars will not kick on the check engine light over fueling issues. A fuel pressure gauge attached at the rail is a great place to start. It won't tell you if it's a fuel pump, or injector. But you can start narrowing down the problem.

Engine grounds and voltage: problems with either one of these will make a car act up in the strangest way.

Hope this helps.

My buddy is looking at a cheap 2001 Cavalier Z24 as the next project. The current owner mentioned it was scanned twice and only the scanner from the local shop thrown a code. Guess we have to use a better scanner then. We'll just finish installing the new tires and aftermarket dirt bike exhausts on the current Honda CRF450 this week before we start working on the car.

Is it making any noise from the catalytic converter area? One of my 1996 2.4L Cavs had a cat failure. The honeycomb catalyst biscuit broke loose. At first it ran fine, but then sometimes I had a hard time getting it to go to high-revs. I guess the biscuit rattled around and changed locations, sometimes blowing back and clogging the exhaust as the higher-RPM (stronger-exhaust-flow) pushed it to clog the exhaust. It ran fine at low RPMs. I installed a new cat and downstream O2 sensor. All is well.