NEED HELP ? ADVICE ? GRENADE ?

Guys I just recently rebuilt a 1987 mazda b2000 2.0 engine. I will give you a quick rundown on what I done : had crank polished and checked standard, rods checked and were also standard, replaced all bearings and plasti gauged, new head with new cam and rockers, new oil pump ,bore was standard with new rings installed,new water pump and new seals ,gaskets and new everything else that bolts on outside of engine. Today driving home oil pressure light came on and truck developed slight knock. So I just removed pressure switch and installed a gauge I had and readings are 4 psi at idle and 38 psi at 3000 rpm. Any input on what I need to look at would be helpful. Thanks Dustin

What weight oil are you using, and what was the temperature? Like was it real hot out, and what was your temperature gauge reading? City driving or highway? Is your oil level at the "fill" mark.

I do agree that your oil pressure at idle seems to be low. I'd like to see 10 psi at idle, and about 10 psi for each 1000 of rpm.

10w30 Valvoline, 85 degrees out ,temp gauge in middle running a new two row radiator, 35mph - 50mph country roads 6 mile trip. oil level perfect. oil was crystal clear last week and black now.

Is there a chance that you installed a bearing backward?
If you have a knock, I'd pull the pan and have a deep look inside.
Check the oil for a metallic sheen after draining it. It would be an indicator that something bad has happened.

All bearings installed correctly. I am meticulous on engines. all holes in bearings align . Forgot to mention engine has been driven 750 miles since overhaul.

Also I forgot to mention no knock at idle but start hearing it around 2500 rpm and engine has to be up to operating temp to hear it.

Ok just drained oil and cut filter open no chunks of metal but oil does have a very prominent metallic sheen to it. Possible spun main ?

I'm confident you did, but just to double....maybe triple-check, you got the thrust bearing in the proper location (3? I think)? ABE has found a couple them in the number #1 position making metal soup.

Is there any chance that the pick up screen to oil pump connection is loose?

That being said:

Your clearances for the mains are probably out of spec and what you are hearing now is rod knock. Maybe you didn't clean out the grit from the crank passages after it was polished and that sanded the mains and rod bearings right after you started up the vehicle. On an '87 you probably should have cut the crank down to take out the wear that would have been part of 27 years of operation.

You're probably in for a crank kit (reground crank and matched bearings) at this point. But make sure you wash the passages out of the journals before you install it, or you will be doing this over AGAIN.

I did clean all crank passages after polishing. Whole engine checked out standard size and all bearings were within spec using plasti guage . The engine I rebuilt only had 65000 original miles on it but did have a blown head gasket.