Hi. changed the mass air flow sensor. no improvement. new symptoms. added GM top engine cleaner through as suggested by local dealer mechanic. he thought maybe carbon was blocking intake valve. Now engine idles at 1500 rpm when first started, then comes back to 500 or so. also stays at high idle when letting off gas, have to fight brakes
to stop truck, idle settles back to normal after about 15 seconds after stopping. still misfires, code 303, but seems to smooth out when accelerating. Loss of power on acceleration seems more like lean condition, , code 171 and 174 come up, rather than cylinder missfire. All new ignition parts, good compression all around, new injectors, crank relearned, new ecu, o2 sensors, fuel filter, fuel pump, checked egr, pcv, brake booster. considering pulling intake manifold to change gaskets. Did a leak test with carb cleaner, but I know the bottom of gasket could be leaking. no antifreeze leak, and gaskets may have been change many years ago before I owned. any thoughts?