Oil Light Comes at Idle When Engine is Warm

I looked in the thread history and didn't see anything specifically relating to this so I was hoping someone had some insight here. I have a 1974 with 81K on a 250 i6. I went through everything after my wife inherited it from her Grandma. It runs and drives great and I was looking to start taking it to work occasionally to stretch its legs more. The first time I took it to work, I noticed when it is at idle after being driven for 15 minutes, the oil light came on. I replaced the sending unit to eliminate that as the problem and I added gauges so I could better see what was happening. On a cold start, the oil pressure registers about 25psi but it will drop to the single digits eventually when it warms up. When the oil light is on and I give it gas, I am at 25-30psi and the temperature stays right at 185 degrees.

I dropped the oil pan and found the pickup was clogged with old gasket material. I replaced the pickup and there was absolutely zero metal on the magnet in the pan. While I could see the bottom of the engine, I looked for scoring, charring and anything else problematic but it all looked very good and clean. I put it back together and I am still having the same issue. I would think if no oil was being distributed, I would see at least a fluctuation in engine temp if there was any excess friction in the cylinders. I was wondering if any of you had experienced this and see if you did something to take care of it like use a lighter weight oil or if it was just time to bite the bullet and rebuild the engine.

Thanks in advance for your help.

 

Sorry, yes, I did replace the oil pump as well. I am running 10W-30 in there right now and there is no knocking or engine noise from the motor at idle or otherwise.

 

That is a good thought to take the valve cover off to make sure oil is flowing equally through the oil passages. It will certainly make me feel more comfortable knowing that is the case.

 

This & switch to 10W-40 or 15W-40 oil...

 

What is the idle speed? If it's low you could bump it a little to pick up more flow/pressure.

 

Your main/rod bearings are history. They probably look like these. If you don't change them run 20-50wt oil.

 

If you don't run it hard a lot you can probably get by for some time, but you'll want to start saving up for a rebuild. Engines which start with adequate oil pressure that drops once warmed up usually are losing it due to excessive internal clearances. It's not something that gets better, but for a street driven car it doesn't necessarily mean the oil pan is about to fail to keep the bottom end inside.

You've done your due diligence for things like spun bearings, you just have a tired motor.

 

Could be cam bearing wear. Had a '69 Mach I with this problem

 

Thirty plus years ago my daughter had a half motor (2.3) '86 Mustang that rattled at light throttle, oil pressure was low. Arranged for some lift time at a buddy's car lot to yank pan. Found something I'd never seen, the center bearing surface on each upper was chipped away for approx three quarter inch. Mains(at least two I checked) were fine. Headed up street to local auto parts store, bought four rod bearing sets, never forget $2.40 each. Installed bearings and reused orig rubber oil pan gasket. Total job less than $10, rattle gone, pressure much improved. Didn't leak. She drove it another couple years and sold it to her mom, who drove it another three or four years. Long enough that some time later I had to help my son in law rebuild the carb for his mother in law(my ex).

 

I pulled the valve cover and there is oil being distributed….less at idle and more with a little gas. My idle was at 650rpm. I increased it to 750rpm and it wasn’t enough to make the light go off. I think the engine is just tired. We are just going to drive it short distances until I have time to pull the motor. I will put some heavier weight oil in there in the meantime. Thanks everyone for your help.