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 (which made it run worse). No change when I plugged it back in.

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

 

Have you checked for leaks in the air intake?

 

No, but I'll do that right now with some starting fluid. I hadn't thought about a possible cracked plastic intake runner.

 

OK, no change in rpm when I sprayed around the intake runners. One thing I forgot to mention is a rattling noise in the intake area after I let off the throttle.
I took a short video of it, so I hope I can load it here, or at least the link to it. I have used a stethoscope and can only hear it through the intake manifold. Checking the head, block and all surrounding components, I don't hear it.

 

Have you looked at the fuel pressure regulator? If you pull the vaccum line is there gas in it? The rattle may be a loose exhaust clemp ex.

 

Fuel pressure regulator holds when I put the vacuum pump tester on it. No fuel in it. Nothing is loose anywhere. The noise is Only heard in the intake manifold. I can even feel it with my hand. Maybe it's time to pull the manifold off, but I don't see anything down inside the plenum that could be loose or rattling. I might pull the throttle body again and use my scope camera and see if that shows anything.

 

Well, I didn't pull the Throttle body to do the camera. There's no way to get it down and back up in the runners. I jacked up the car to look under at the exhaust. No cracks or leaks.
So I pulled the rear O2 sensor out and ran it unhooked. No change. BUT, the rear O2 is really black compared to the front one. To me, it seams to indicate that the CAT is not getting hot enough for some reason. Could this really happen and not show a code?
I'll let the motor cool down and start soaking the collector bolts at the manifold so I can pull the exhaust pipe down and run it that way to see if it's truly a plugged CAT. I don't want to cut the welds on it if I don't have to. It might be a couple days before I can do that though.
BTW, this cat was changed out about 10+ years ago with universal unit.

 

I don' know other than check the valve lash

 

What do the plugs look like?
Burned valve?
Do a compression check.
Is the throttle valve opening all the way?
If the valves are adjustable, check if any have too little clearance.