The truck used to run great prior to me, replacing the valve cover and intake manifold gaskets. It had visual and oil burning coming from valve covers leaking.


So, I've replaced the ignition coil pack, all 6 spark plugs and wires, mass airflow sensor, Idle air temperature sensor, all 6 injectors, bank one sensor one oxygen sensor, bank two sensor one oxygen sensor, all 6 cylinders have compression of 135-150 psi and performed a noid test on cylinder 4 injector signal wire which can is good and cylinder 4 injector resistance test is good (15mv). I still get a P0304 but only when speed is above 35-40mph (truck runs really rough and shaky) and won’t increase above 65mph or when the RPM is above 2000 parked. I have sprayed carburetor cleaner all over the intake to test for a vacuum leak but idle doesn’t change. It sounds decent when it is idling. Fuel pressure is 45 PSI. And sometimes I get a P0171. I had a mechanic shop test for vacuum leaks and he said there are no leaks.

I’m thinking about replacing the fuel pump. I have heard that a fuel pump that is lacking pressure at higher RPMs could throw the fuel trims off and cause misfires but I would assume misfires would happen across multiple cylinders, not just one.