1990 B2200 Oil Light coming on at low rpm's

About a month ago my oil light started coming on after driving for a while when the engine returned to idle speed. Oil level is good and there is no oil leaking around oil pressure switch. Also at certain lower rpm's while going it makes a sound that sounds like metal clinking around. It loses power often but the light doesn't come on when it loses power. Mechanic is suspecting bad rod bearings and says I probably need a new engine. It's my daily driver with roughly 180k miles. Any thoughts?

I'd (1) either try a new oil sender switch or (2) thread in temporarily (and gently) a mechanical pressure gauge into the spot where the oil pressure switch was, see what that reads. I'll report that when I rebuilt my own B2200 engine at 183K miles (smoking, bad rings), that the crankshaft journals and rod bearings looked like brand new.

Yea my mechanic didn't hear any bearing noises at all. So I got a new oil sender switch for 8 bucks. When I take the old one out will I lose a lot of oil like an oil change? just curious if i need to just do a whole oil change.

No. Only a dribble.

For the real story, hook up a gage, then you have the whole ugly story. If the pressure on the gage is fair at cold start up (if not mains and or rod). Secure the gage out of harms way and either let it idle and warm up or drive as usual until it warms up. At this time if the idle oil pressure plummets, that is the story, bad main bearings Sharp banging in the engine, rods, dull, mains, tapping, oh that is just the lifters ;).

@ Cusser

The rebuild you did, was the pressure escaping by the rings? This is what I think is going on with my 87 B2000. I'm getting a mostly flickering but sometimes solid oil light only after the engine is warmed up. It does smoke a little but not that much, mostly at hugh RPMs. I'm also getting a little oil blown back in to my air breather as well . . . after 50 - 100 miles, the valve cover breather filter (the small filter in the air breather) is pretty well soaked.

Another question, do you think that an average Joe with a good collection of tools and a bit of knowledge could do a rebuild on this engine?

Did you see my write up at http://www.mazdabscene.com/wiki/65 ?

I was not aware on mine of having low oil pressure. Mine was smoking, even though compression readings were good, I didn't want to waste my time doing the head only. Yes, I think an average home mechanic can do what I did without removing the engine. Will help with the blow-by for sure, and smoking.

However, if the crankshaft journals look bad (mine were pristine), then the engine and crankshaft would need to be removed for machining. I borrowed ring compressor and cylinder hone from Autozone.

"I'm getting a mostly flickering but sometimes solid oil light only after the engine is warmed up." The oil sending unit grounds out at less than 3 psi and that occurs at idle when your main bearings surrounding the crankshaft heat up and expand away from the crank when the engine warms up. The solution is to replace the crank or remanufacture yours with underground bearings. You can replace the crank without pulling the engine, but if your rings are worn out too, it might be time for a short block.

Kinda misleading/over simplifying it.
The transmission, and all that that entails, will have to be pulled to removed crank from the engine. Might be able to replace crank bearings without pulling trans or engine, but that rear crankshaft seal is going to be as old as it is.
Could possibly be warn cam bearings, but less likely than main bearings.
How is the oil pressure on the engine cold start? Going to have to borrow or invest in an oil pressure kit. Can simply connect to the engine and lay on the fender well, do not have to do the install to check the pressure. Simply screw adapter into oil pressure switch port, connect line, connect line to the gauge, start engine, read pressure on the gauge. After engine has warmed to normal on the temperature gauge, compare pressure to cold engine pressure. Of course if the pressure stays good, sending unit is bad or wire to light is erratically grounding.

"I'm getting a mostly flickering but sometimes solid oil light only after the engine is warmed up."

You might have an oil pump that is worn out.
And it has a seal that is as old as the rear main unless a clutch R&R included the seal.

At 180k miles, you are going to need a clutch soon.
The transmission gets unbolted, and then slid slid up onto the crossmember, where it will sit while you unbolt the clutch and related hardware.
To remove the crankshaft the oil pan has to be removed, the transmission slid back, the timing belt and gear removed, the windage tray and oil pump and pickup screen removed. There is some other stuff I am overlooking, but no one said replacing a crankshaft was easy, just that is can be done with the engine in the frame. If you don't have an engine hoist this may be the only way to salvage your motor. The easier route is a rebuilt shortblock, IMHO and my .02.