So I have been searching and reading this site for the past few days. I had a problem with my 03 z71 suburban after towing my boat. I got it home and the engine started missing at idle and setting a P0300. I changed the plugs and wires and that didn't help. I then checked the compression and the #7 cylinder only had 50 psi of compression. I got an 03 lq4 and put it in this weekend but had to use my intake and injectors over because of the flexfuel injectors and wiring harness. It still idles rough and throws a P0300. I checked the missfire counter for all injectors and 1, 3, 5, 7 all show misfires but 5 is way less. I checked the spark on all four cyl and they have spark. I also checked for battery voltage at the injectors.
Could my #5 injector be stuck open and causing misfires on the other three cylinders?

 

I also tried a different computer. Still runs like crap from idle all the way up to 3000. Also did crank relearn.

 

I just did the injector balance test and all injectors are the same. Could the o2 sensors be causing a problem?

 

Not the front o2 sensors either but at least I got two new ones now!
Anybody have an idea?

 

you check or change the coil packs??

 

I did check for spark and was ok

I finally figured out where the problem started, I Think. I checked backpressure at b1s1 and found I had 6 psi at idle and above 10 at 2000. Mitchell said the backpressure should be no more than 1.25 at 2000. I removed the down pipes and cut the cats open and found the front part of the cat to be turned 90 degrees to the flow. It was rounded off on the edges so it had been doing this for a while. I think the decreased flow was causing the o2 sensor to read rich and therfore leaning out bank1. I drove it home tonight and it is running great. Tomorrow I will get a tune and be ready to go!

 

Got my tune and this thing is way better now!