Cylinder 6 misfire. code P0306 1999 gmc jimmy, still have 150 pounds of pressure coming out.

What is a "teach" That said you have an engine misfire, have the engine control system scanned, it will give a code showing what cylinder or cylinders are misfiring.

A P0300 diagnostic code indicates a random or multiple misfire. If the last digit is a number other than zero, it corresponds to the cylinder number that is misfiring. A P0302 code, for example, would tell you cylinder number two is misfiring. Unfortunately, a P0300 doesn't tell you specifically which cylinder(s) is/are mis-firing, nor why.

Multiple failures will contribute to this symptom. Both mechanical and electrical issues need to be looked at. Check for "Engine light" and any active or stored engine codes. Running a cylinder compression test, fuel line pressure test, exhaust pressure test, check the timing, check for misfire, check for clogged fuel and air filter, defective MAF or MAP sensor, check for vacuum leaks. The more details you can provide. I may be able to help you more.

Lets start at the beginning. Did anyone check fuel pressure before and after replacing the pump ?
Has anyone checked the fuel pressure regulator and the injector spider ? The 4.3w engine uses a spider with one injector that supplies all 6 cylinders. And it runs at 65psi or more.

replace fuel filter, air filter, spark plugs, wires, distributor cap /rotor, if you have one. new oil with your filter ? just thought I'd ask, have a auto parts store pull code to see why your service engine soon light is on. this will tell you what sensor needs replaced, I'll bet its the oxygen sensor !

Work on the cylinder misfires first

If the converter is 12 years old or more, then
it may be a problem

Assume you know how old the gas filter is

On the vortec engines they require 56 pounds of fuel pressure to start a few pounds shy of that you have a dead vortec you will need a fuel pressure guage to verify

Sounds like you have a couple of leads on the wrong plugs or in the wrong place on the cap.

Prob needs a o2 sensor or oxygen sensor they are called

P0303 is a misfire on cylinder 3.