2000 gmc sierra 2500 p0300 code

Question

Ok so I have changed the intake manifold gasket cause it had a small leak. I changed the spark plugs and wires, I have changed the mass airflow sensor and maps sensor. I have also changed the fuel filter. I have cleaned the throttle body as well. I still have a p0300 code that will set around 90-100 km/hour.
I had a K&N air filter that caused me some problems with fine oil getting into the intake and his is why I had to changed the mass airflow sensor and map sensor and clean the throttle body. I also replaced all 4 02 sensors as well. Truck seems to run well but still sets the code.
I am not sure what else could be the problem unless I have a catalytic issue causing an increase in back pressure or a fuel pump on its way out. I seems to be intermittent though so maybe an electrical issue..? Any idea of what else I should do to solve this issue...?
Any help would be appreciated.

Thanks.

Answer 1

Loosen the bolts to the catalytic and retest , on both banks and you can retest, , These also have a known problem with the fuel pressure regulator leaking fuel into the intake , take the vacuum hose of the fuel pressure regulator and start the engine if fuel comes out of the regulator vacuum connection replace the fuel regulator