my 525sc has 60 lbs. pressure cold,idles at 45lbs. warm,
gage drops to 20lbs. Sometimes at full throttle.other
times it is fine.occassionally it varies from 5-60 lbs.
at idle.checked the sender with a meter,within spec,
what could cause oil pressure to vary so much,
also, i have run the boat on somedays and the pressure
is fine,gage steady

If it is dropping at WOT then you are probably getting foaming or windage. Try removing 1 quart of oil from the crankcase and see it that doesn't fix the problem. Of course if you do this you need to pay close attention to oil temp.

BT

I know of two cases, 1 above 3700 rpm the pressure would drop. turned out to be the pan was being pumped dry. Wrong pan from a rebuilder.Truck pan not a marine pan.The oil wasn't running back quick enough, Actually the pan wasn't deep enough. Same guy a few years earlier had a loose pick up on the oil pump. At an idle the pressure was fine. bring the rpms up a little and the level would drop enough to let the pump suck air where the pick up entered the pump. Both easy fixes,just labor intensive
Good luck

I second the mechanical gauge ck 1st. Also any excessive silicone floating around will be sucked into pump screen. Have any cover gasket"s been replaced ?

I had a similar problem. Both Gauge and sender tested out perfect. Put on a mech. gauge and pressure was actually fine. Turns out my Merc. sender and Gaffrig gauge didn't like each other. once I installed Gaffrig sender the problem was corrected. Merc. Will tell you their senders work fine with Gaffrig but I have talked to numerous people with the same problem I had. For some reason sometimes they work together and sometimes they don't, mine did for one season before suddenly went haywire.

I had the same problem. I hooked up a mechanical guage and all was ok

o-kay guys, took it out this evening, cold start oil pressure
65-70, idling through river 50 -60,nice gradual plane,
oil pressure dropped to 20,as soon as i back off to off plane
pressure went back to 50.put the motor in neutral idling at 50,
revved it to 3500+ oil pressure was dropping to 20,water
temp was 145,oil pan has 8 quarts 20w -50 valvoline vr-1,
fram hp-4 filter,motor has 76 hrs. on it,i hope the oil pump
is not bad? i have never seen a motor respond like this
even with bad mains or rod bearings,i need to know the
best way to check it with a mechanical gage,this web site
is a wealth of info-i am sure you guys can help me or steer me in the right direction,could i have a problem with a valve train,
motor revs and clearances increase,motor runs as strong
as it was new,if it indeed it ran good when it was new?
no body has a 525sc where i boat----------thank you-----joe
,

Duster - that is exactly what my oil pressure did with the sender problem. Here is what you need to do. Go to an auto parts store and buy a cheap mechanical oil pressure gauge. Unscrew the sender from the block (right side, back of motor about halfway up the block with one wire coming off the top), get a T fitting and install sender to one side and tube from the mech. gauge to the other. You can get the T and what ever other fittings you need from Home Depot. Run enough tubing from the T to the mech. gauge so you or a buddy can hold the gauge and compare.

Duster have you had the oil cooler lines off for any reason? If you have, make sure your return line back to the block is not hooked up wrong, which would be returning thru the outside fitting. It returns thru the center hole, just to clarify. This will cause the anti reversion valve in a Fram to push against the little holes in the top of the filter and close off most of the oil pressure. Also a pickup falling off or silicone is were good suggestions. If it was a rod you would hear the clanking. Longshot would be a bad cam bearing, have you changed the cam?

motor is as shipped from mercury racing,has never been opened
up,i have not had time to run it but i found the wire to the sender
was very loose where it connects to the sender,crimped it with pliers,pushed it on firmly,it might be a long shot but i will know
monday,family obligations mean i am on the trailer this weekend!!
thank you for all the help------joe