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Old 05-22-2022, 10:06 PM   #1
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Default Low Oil Pressure situation

Hello,

2 years ago after running my 1995 SR 2000 wide open for about 20 minutes, i slowed down to dead slow in gear and the oil pressure alarm started to go off. I checked the gauge at it was reading 20 psi. After 5 minutes the oil pressure came back up on about 32 psi and the alarm stopped.

I changed the oil, changed the plugs, and nothing changed. If I increase the rpms to 1100 the oil pressure alarm will stop.

Cold start PSI is around 75. While running it flat out I can watch it drop to about 45 psi after 5 minutes and then it will stay there until i bring it back down to idle where the alarm will go off again. The oil pressure always comes back after about five minutes.

The cooling system is functioning properly, the engine is not overheating. There are no weird noises and it pulls great.

Has anyone ever had this before? Should I just disconnect the alarm? Should I put in a thicker engine oil?

Thanks for any help,

Paul
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Old 05-22-2022, 11:02 PM   #2
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Welcome aboard.

First problem is these engines are not designed to operate at full throttle for more than a few minutes. Running longer will damage the engine as you found out. Remember these are basically car engines modified to be cooled by salt water not high performance.

Answer you should not operate over 4000 rpm.
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Old 07-07-2024, 08:10 PM   #3
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Briefly describe the situation.

On May 18th, I imprudently drove at high speed (34 miles per hour at 4200 RPM) for a prolonged period (10-15 minutes).
The oil pressure during this time was 55 PSI (the oil was recently changed).
Then, my chartplotter signaled low voltage, and I checked the instruments. Along with the low voltage, the oil pressure also dropped to 10 PSI (the alarm did not go off). Frightened by the situation, I added 2 more liters of oil and returned to port at low speed.

Today, I decided to run some tests.
I left the marina and started moving at 3000 RPM (speed according to the chartplotter was 18 miles per hour). After 38 minutes and 6 miles, the oil pressure sensor showed 10 PSI (the alarm did not go off). I turned off the engine and checked the oil level - it was above normal.
After 15-20 minutes, I started the engine, and the oil pressure was 36-38 PSI.
I set off back at 1900 RPM, at a speed of 7-8 miles per hour.

After 1 hour and 7.41 miles, I reached the beach where I anchored. The oil pressure did not drop, and the level remained the same.

Now, the questions are:

At what oil pressure should the alarm sound?
How can I check the oil pressure alarm?
How can I properly diagnose the engine myself? If at low RPM, the oil pressure is almost normal (36-40 PSI), but at high RPM, the oil pressure drops to 10 PSI.
Thank you
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Old 07-08-2024, 12:24 AM   #4
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Quote:
Originally Posted by Ivan Horvat View Post
Briefly describe the situation.

On May 18th, I imprudently drove at high speed (34 miles per hour at 4200 RPM) for a prolonged period (10-15 minutes).
The oil pressure during this time was 55 PSI (the oil was recently changed).
Then, my chartplotter signaled low voltage, and I checked the instruments. Along with the low voltage, the oil pressure also dropped to 10 PSI (the alarm did not go off). Frightened by the situation, I added 2 more liters of oil and returned to port at low speed.

Today, I decided to run some tests.
I left the marina and started moving at 3000 RPM (speed according to the chartplotter was 18 miles per hour). After 38 minutes and 6 miles, the oil pressure sensor showed 10 PSI (the alarm did not go off). I turned off the engine and checked the oil level - it was above normal.
After 15-20 minutes, I started the engine, and the oil pressure was 36-38 PSI.
I set off back at 1900 RPM, at a speed of 7-8 miles per hour.

After 1 hour and 7.41 miles, I reached the beach where I anchored. The oil pressure did not drop, and the level remained the same.

Now, the questions are:

At what oil pressure should the alarm sound?
How can I check the oil pressure alarm?
How can I properly diagnose the engine myself? If at low RPM, the oil pressure is almost normal (36-40 PSI), but at high RPM, the oil pressure drops to 10 PSI.
Thank you
The good news is your engine bearings are not the issue since you have good oil pressure at low rpm. I donít know at what oil pressure the alarm sounds but I do know that the alarm and gauge use separate sensors. The alarm sensor is just a switch that is open when the oil pressure is low or basically zero. The gauge sensor is a variable resistor so you can put an ohmmeter on it to ground and verify its resistance is correct per rpm, this procedure can be found in the Mercruiser manual for your engine. This manual is under Documents of this site. If the sensor is at the correct resistance at the higher rpm where you are seeing low rpm I would suspect the oil pump or its pressure relief valve is the issue. You may want to temporarily install a automotive manual oil pressure gauge to verify things.
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