1984 SN 2001 ... low oil presure

[n5891e]

I just changed the oil in 1984 SN 2001 351 w. Oil presure was fine until i ran it at about 2500 rpm around the lake for about 20 minutes. The oil pressure gradually dropped to just below 20. Any ideas?

[Mamigacz]

I had the same problem on my boat. Below are five possible reasons.

-Bad sender
-bad gauge
-worn crank, rod or cam bearings
-worn oil pump
-oil galley plugs that are loose or have completely came out (not likely)

Start with the sender and see if you problem changes. Then move to the gauge. You can buy a cheap oil pressure guage at a parts store (this would only be a test to see if your stock gauge is bad).

Unfortunately, on my boat it wad worn bearings. After my rebuild the oil pressure read 40 psi at idle after the engine warmed up. One thing to note, I did run the boat for 3 years with the same oil pressure readings as you had. The only reason I rebuilt it was to be proactive. Who knows it might of gone another five years without issues if I would of left it alone.

[TRBenj]

Ive read that 10psi per 1000rpm is acceptable pressure... so as long as that 20psi you were seeing was down at idle and not at speed, then youre probably ok.

That being said, none of my Fords drop oil pressure like that, even at idle. They run north of 40psi at all times. The Chevies on the other hand, drop down a bit at idle after a hard run.

What type and grade of oil did you use?

[n5891e]

I just bought the boat so I am not sure what type and grade of oil was in the boat before i changed the oil. The seller said that the lower was recently rebuilt and had only around 4 hours on the engine. I bought it from a local dealer with good rep. The oil presure never dropped below 35 at idle and running temp while putting an additional 2 hours on the engine. I changed the oil using 10w - 40 and ran the boat for about 30 minutes after the change and at idle the oil pressure was around 20. I am going to replace the oil with 20w - 50. Do you think that might bring it back up?

[Mamigacz]

As TRbenj said, 10 psi per 1000 rpm is acceptable. It don't think there is any concern about spinning a bearing with 20 psi at idle (thats assuming you have higher oil pressure at higher RPM's). Mine was actually around 8 psi at idle before the rebuild.

The only thing that seems weird is 20 psi does seem low for a fresh rebuild. 20w-50 weight oil might bring it up, but there is nothing wrong with what you currently have.

[Zach@n3]

My fresh 351 was set up clearence wise for 30w oil. After a long WOT run on a 80+ day it settles at about 25 psi during idle. Normal warmed up operating oil pressure runs about 55psi.

Like what Tim said I did notice out in my dad's 378 hour GT-40 the oil pressure seemed to stay up around the 40psi mark.

[n5891e]

Just another thought. Could over filling the oil cause a false oil pressure reading after running the engine for about thirty minutes? Not hard running but a consistent 2500 rpm for 20-30 minutes.

[Mamigacz]

No, overfilling does not have any effect on oil pressure. As long as:

-your oil pump is strong
-bearings are within specified clearances
-the pump pick up is submerged in oil
-pressure relief valve works
-oil galley plugs are in tact
-no cracks in the engine block

There should be no issues with oil pressure.

The only negatives of over filling, is its hard on seals. Oil level should not be above the pan gasket, front seal or rear seal.

[AirTool]

The only negatives of over filling, is its hard on seals. ...l.

And that it can foam in the pan and splash up onto the cylinder walls fouling the lower parts of the cylinders and pistons with coke.