oil pump removal?

i have a 1990 chevy 2500 4x4 with a 350 in it. i had a problem with the oil pressure gauge readings so i replace factory unit with a good mechanical gauge. but to my dismay, the readings were the same on the aftermarket gauge. so i have assumed that i need to replace the oil pump. my question is, is it possible to replace the oil pump by droping the pan without removing the engine? or does anyone have any other explanations on why my oil pressure reads fine at start up, till engine warms up, then when i come to a stop it drops to zero. never heard any kind of noises indicating engine is starving for oil. PLEASE HELP!!

how many miles on that engine/pump? If its over 200K, it sounds like if it's not the pump then there is a lot of clearance. If the oil is cold and thick that will show a higher reading. Did this just start over night, or has it been gettng worse. If it just happened one day then the pump is a good place to start. On my 91 4.3L you do not need to pull the engine. The 4.3 is much like the small block that way, but you would need to find out.

the motor has about 50k on it (not original motor). the problem started about a year ago but truck wasn't driven much till recently. tied to put a thicker oil in it but did not seem to cure the situation. is ur 91 4.3 a 4x4? that is the question that i am running into about pulling the motor. mine is 4 wheel drive. thanks for the info

As to whether you need to change the oil pump, I would say yes if you're getting less than 10 psi at idle (in gear) while hot with a gauge and sending unit you know is reading correct. Be sure to check the oil pressure sensor, mine went bad and was reading exactly like you are describing. Typically these 350s will run 25 to 30 psi at idle hot and sometimes 50+ cold (my '88 350 still runs these numbers with 170K on it.) As to pulling the engine, no. I changed the oil pan gasket on my '88 K1500 a few months back and got the pan to clear the flywheel (barely) and front differential by pulling the motor mount bolts and lifting the engine until the transmission was touching the tunnel. Just be sure that all wires and hoses (including the heater hose) are clear and don't get pinched or pulled. The top of the fan shroud will have to go too.