I have a 1999 502 MPI. Recently started having problems with oil pressure guage. At idle it's around 20-25 psi. Once I get into the throttle it goes to 0. I put on a new oil pressure sender and it was fixed for about a half a day. Then same thing happened. I hooked up a mechanical oil pressure guage and everything looks normal. 20-25 at idle and 50-55 psi at WOT. What's wrong? The guage is a Livorsi. The sender is stock Merc.

I've been experiencing a similar issue with mine as well. If you find a solution post it. Ill be checking back.

I Know This Is Going To Be The Typical Reply But A Ground Wire Can Cause Almost Anything Crazy To Happen........the Gauge Could Be Screwed Up Too, Especially With All This New Electrical Stuff Thats Out Know....check All Connnectiones To Gauge With A Test Light Good Luck

Ditto on connections! I haved worked on this crap for a long time, start with the engine harness plug. It is common to find the clamp has been way over tightened which can screw up a connection or two. Disconnect it and carefully look at the pins you wil see they have slits, if you hake a very thin blade you can carefully spread the tips this will give better contact within the plug. Make sure it looks good and clean, when you put it back together just snug up the securing clamp. Coming off the engine side of the harness is a heavy black #12 ground wire follow it to its end make sure the connection is clean and tight. After checking and cleaning the battery connections "but leaving the positive wire for now" clean tighten the grounds at the block, look over the engine for any other ground connections ditto on clean and tight. Done with that and the battery still "disconnected" get you butt under the dash and carefully fiddle with all connection to see if the are secure. Pay extra attention to slip on spades they tend to loosen over time, loose ones can be "gently" tightened up with a pair of pliers. This sounds like a lot of work, NOT you should be able to go through this whole dance in about an hour. It is far better to do the whole thing at least once a season. Now connect the positive battery lead turn the key and all should be fine!

I will start checking every ground I can find and also the harness plug and guage connections. How can I check the sending unit to see if it's any good? Since it worked for a little while and then quit, I'm wondering if it shorted out or something? Thanks for the info.

I had the same problem and it was driving me crazy thinking that I was melting down a motor due to an oil problem. Mine turned out to be the connection right at the sender. I tightened it up a bit and everything is perfect.

Rene

Oldandtired, you were right on. The wire on my sender is a push on type and I just crimped the connector a little tighter and that fixed the problem.Thanks to all who responded with some great advice. Finally a simple fix to a problem
Steve