1993 gmc 2500 has good oil pressure when started but when at operating temp the pressure drops to zero when at idle in drive when in neutral it goes back to around 20 any idea why?

Answer 1

== == i have a 93 gmc k1500. i thought the same thing but it turned out to be the rear main seal. 15w40 only delays the engine blowing trust me i know. I would have the rear main seal replaced and then you wont have to worry

it could be low oil level.a bad oil pressure sensor switch. if there is no engine noise knocking or ticking i would doubt the oil pump is the problem and i would look at the oil pressure switch. I can almost guarantee that it's worn bearings and an oil viscosity that's too "light" for the age of the engine. You can probably raise the oil pressure by changin the oil to something with higher numbers. If you're using 5W15, change to 10W30 or even 15W40. After the engine bearings wear, the oil pressure ALWAYS drops. If you can get the oil pressure back up using the above described method, you should have plenty of life in the engine. The simple fact is, oil pumps SELDOM go bad before the engine needs to be rebuilt, but it's a common scam that's used by many mechanics to make a few extra dollars.