completely stumped on 502 oil pressure problem
 

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

GTOFFSHORE's idea is good...
 

...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.

piece of sh** oil filters can do that even brand new ones. Try a racing wix. you'll see a diff. on the gauge compared to a reg purolator/fram/etc

I do run the top of the line wix filters on the boat.

I tried swapping senders and gauges with the motor with the questionable oil pressure, and the problem continually stayed with the motor....so i'm thinking its the motor. I might do the mechanical gauge to just be sure, but I'm doubtful its gauge related.

+1 on the filter, a jumper lead to the other motors temp sender wire will eliminate gauge and harness questions. double check oil lines for sharp bends, but sounds like you'll be doing a motor soon.

Darko. Stop messing with senders and gage problems. Install a mechanical oil pressure gage(40 bucks from the zone) and you will know if it has a mechanical issue or electrical issue.

As far as sludge..... There is not any from the original owner. that i can say 100000%

Do you know that there is two sending units ? Did you change the right one ?

+1
That can take the guesswork out of it being electrical or mechanical.

+1
You could be swapping the alarm sender and not the gauge sender. When mine was a factory 502 the alarm was in the bellhousing area and the gauge sender was in the oil filter pad area.

2???? where are they at? I saw 2 bell senders with 2 wires going to them right next to the oil filter housing at the back of the motor


I also had someone tell me to replace the breathers on the valve covers. He said once they get saturated with water they can wash out the oil and cause the issue i'm having. Maybe one of the reasons a fresh oil change gets rid of the problem.

As others stated there are two sending units, one for the alarm system and one for the gauge. Make sure you are swapping the correct one. USUALLY the bell is larger on the gauge unit.

What you are describing sure sounds like a sending unit. Mine did that earlier this year and a new sending unit fixed it.

If it doesn't follow the sender then get a mechanical gauge on there.

As Fixxer22 said... The one on the back top of the motor is the alarm. The pressure sender is on the side of the motor down by the oil filter pad.

The sender changes its resistance to ground with changes in pressure a volt ohm meter can rule out any bad connections... or take the wires of both motors and swap them with a jumper and see if it changes