I developed a very weird oil pressure problem a little bit ago and have not been able to figure it out.


Its a 502 with 465hp mercruiser and I get a strange problem where the oil pressure gauge moves erratically. It first started only over 3500 rpm, and now its doing it even at idle. At WOT i would be at around 40-50psi and it would tick back down and fluctuate to maybe 30psi erratically and randomly. Now at idle it usually idles around 40 psi and now its ticking 5-10 psi lower and goes right back to 40. There is no rhythm to it and its completely random. I swapped the sender, and I swapped the gauge thinking it was an electrical problem but the problem remains.

I notice the oil pressure seems to be more consistent after a fresh oil change

The boat runs beautifully and the motors only have maybe 300-400 hours on them.

Any thoughts on what the problem may be?

I just noticed the same thing on my friends boat yesterday. I asked him about it and he just shrugged his shoulders.

I've seen that happen before. Replace the sender and see if that helps. Sometimes the sender goes bad or a small piece of debris get stuck in it. Also make sure your oil level isn't too high, airated oil will do the same thing.

i swapped senders on the engines and the problem stayed with the engine. I also swapped gauges from motor to motor and the problem stayed with the engine.

as far as the oil level, its very strange. When the boat is cold its about 1 quart high. After it warms up to operating temp its at the full mark. One time I made the mistake of filling the oil to the full mark when it was cold, and when I checked them warm they were about 2 quarts too low. When the motor with the oil pressure issue was 2 quarts low, the oil pressure gauge was ticking more erratically and severe.

Have the motors been redone? It's just one motor right? Did they weld the pick up on? Check all the wires to gauge to see if one is shorting.
Check distributor gear but doubt that.

the motors were purchased brand new from mercruiser they are the 502/465hp

...and I will try to take it one step further. (Check all the wires to gauge to see if one is shorting)

Using either test gauge or take one of the gauges you have and wire it direct to the sender with jumpers. If you get the same results, you have, at least, taken that part out of the equation.

Not that many years ago we had a sport fisher with very similar symptoms. After many different efforts to resolve the decision was to do a major on the engine.

Luckily, one of the first parts removed was the oil pan. However it happened (second owner) the screen on the oil pick-up was caved and had a "sludge" on it that created the erroneous readings. rather than risk a partial fix, changed the entire pump assembly.

The mechanic was "old school". He used gun cleaning equipment and hit all the passages he could get to with the wads. The engines were same year, within 3-4% the same time, service had always been the same, but the sludge in one was much more significant than the other... go figure!

Have you verified a good ground at the gauge? Also, sometimes the harness plug at the engine the connection is loose. I've had to separate the pins on older boats to get a good connection. If you do that you must be very careful not to break a pin.

Maybe replace the oil filter on that engine?

I'd try eliminating all the simple stuff first before looking at the engine.

Which sending unit did you change?

Where was it located?

You could also run a mechanical guage temporarily to pinpoint it is indeed an issue with the engine itself.