Hello, I have a GMC 1999 Denali with 170K. Unsure of the previous maintenance although was told its was done pretty religiously. The car has been running extremely rough at idle and has experienced a loss of oil pressure when hot at idle. The SES light has been on and blinking at times with noted codes of P0300, P0307, P1131, P1137. I have cleaned the TB, Mass Air Unit, replaced O2 sensors, changed oil and replaced with thicker viscosity (20W50). Also have tried to test for a intake manifold leak by spraying cleaner near gasket but noted no change. I also notice a "knocking" in the motor that seems to go away at RPM's above 2000 or so. This knocking is there even when the engine is colder before the oil pressure begins to fall. Any ideas would be appreciated