502 MAG mpi w Oil Pressure Question
 

I up jus t had my top end refreshed with new heads (stock 088) and new manifolds and risers. Today when I was running the boat I noticed the oil pressure drop when I got above 4000 RPM. My oil pressure is usually always at 40, but above 4000 rpm it drops to around 15-20 after a few minutes of running. When I pull back the throttle it comes back up. The engine is an early Gen VI 502 Mag mpi. Any ideas on where to start? After I shut the engine down and restart it goes right back to 40 and stays between 30 and 40 up until around 4000rpm and will then the gauge starts to drop.

I would verify what is going on with a manual gauge if ok replace the sending unit. Do you have Livorsi gauges? If so you might have to get a sender from them to read accurately. Randy

Make sure the oil level isn't over full. Nothing was touched on the bottom end of the motor, correct? Just intake, heads, and manifolds pulled?

Correct on just the top end. Gauges are stock. I have been running that engine for 3 seasons with zero issues, however right before I took the boat in for the top end, I did notice my oil pressure gauge drop to 15 when running above 4K. I immediately shut the engine off, checked everything out and restarted. Oil Pressure went back to normal and everything seemed fine. Yesterday the boat was running perfect, but I was watching that gauge very carefully and as soon as I got the RPM's up above 4K it would drop off. When I pulled back the stick it would go right back up? Obviously the bearings are somewhat of a concern, however if it was a bad bearing(s) I would think the oil pressure would stay low? Also, this problem seemed to happen suddenly after 3 years of it being good. Oil temp doesn't seem to matter. I can run at 3800RPM all day and the oil pressure stays right where it should be.
Oil is fresh and right at the full mark on the dip stick.

I agree verify the gauge is accurate. I have seen more bad sending units the last 3-4 years than the previous 15 years. It appears more and more are chinese junk!

I had my gauge drop to 0 psi after a certain rpm, I think it was 3200. Did it every time and dropped instantly. I was 99.9999% sure it was the sending unit but wasn't going to chance it so I went home. I added a mechanical gauge and put the old sending unit back on(which read a bit high) and never had another issue. I think I tried 2 or 3 different sending units in two weeks and they all had issues.

Thanks for the responses guys. I just dropped $6K on the top end refresh, so I am really not trying to pull the engine out right now. I am somewhat mechanically inclined, however I start to loose my confidence with oil pressure stuff. This is a rookie question, but is adding a mechanical gauge very difficult, and where is the sending unit on an early GenVI 502 Mag? The last thing I want to do is to have to dump more money into this engine right now, or I might also be needing to pay for a divorce attorney.lol

Because this OP issue happened so suddenly after being fine since the engine was completely rebuilt in 2008, is that any indication that this is not a catastrophic issue with the bottom end and might just be a sending unit issue? Obviously I need to get a mechanical gauge on the engine ASAP. But if anyone has some thoughts that would bring me from jumping off the nearest cliff I would appreciate it!

Very easy. The sending unit is on the rear port side of the engine.

You can pull it off(I'd replace it anyway) and take it with you to a hardware store to get necessary hardware to make a t-fitting so you can run 2 gauges. Or you may be able to pull a plug on your remote filter pad and install one there. Just keep in mind that you will get a slightly different reading there.

I permanently mounted mine in the cockpit. But, if you don't want to do that you could run enough tubing so a passenger could hold the gauge and then mount it in the engine compartment after.

Or, you could just try a new sending unit for now.