My oil pressure saga continues

M

majoma

Member
Oct 10, 2008
284
San Francisco
Boat Info
2000 340, V drive
Engines
Twin 7.4
2000 Sea Ray 340 with twin 7.4. As I mentioned in my old post, I was hoping to have the oil pressure under control. When I left my slip the pressure was about 40 psi on both engines. After about 20 minutes of running at 2,800 RPM the pressure dropped to about 25 psi on both engines. I got back because I was getting concerned. I was under impression that the oil pressure should increase with higher RPM. Any idea why this would happen?
Thanks
 
In cases like this I would always want to confirm the readings by the use of a mechanical OP gauge. You could get more accurate readings by installing a TEE in the block where the sending unit is mounted and getting both electric and mechanical readings. Harbor Freight sells a decent mechanical gauge that is not expensive.
 
See you are in salt water. If the coolers for oil and transmission have not been changed in 5 years they should be. Mr cool sells them for about $100 each. I just did mine. Three were fine and one looked 50% plugged and had lots of green (Copper) oxide in the tubes.
http://www.mrcool.us/
 
I agree with Northern, I checked my trans coolers recently and were quite blocked. I was able to rod some of the tubes but some were too far blocked. I investigated after one of the trans would moan on startup and after rodding the noise stopped. Have new ones to instal at next opportunity. Suggest you check all your cooolers.

The other thing to consider is possibly your engine oil has sheared and losing viscosity and therefore pressure.
 
Both engines at the same time. Sounds electrical to me.
 
Agreed on the mechanical gauge for verification. Has it always been this way or did it just start? Oil pressure will drop as the oil heats up. And while 25 is a little low for my liking most engines only 'require' about 10 psi per 1000 RPM. If this is a new problem then I would investigate and get it figured out before you cause any damage. If it has been like this or the boat is new to you then I would have a good mechanic take a look to make sure everything is good.

I have also seen where different brands of oil filters can affect oil pressure due to passage sizes and filter material.
 
We use SAE 40. Oil pressure drops 10 PSI from cold to hot. At 3800 RPM for extended periods pressure remains the same as at 1600 RPM One engine reads 40 the other 50. Once oil pressure dropped to 10 when I had an oil cooler fail and I lost oil.
 
Is the oil cooler cooled by the fresh water or raw water? My water temperature is always at 170 deg.
 
I tend to agree that the low oil pressure is due to the oil being too hot and the coolers not working properly.
My last question is (I promised to be my last): after running for about 20 minutes, the pressure stays at about 22 psi at any RPM for the rest of the day. In your opinion, is that risky or can I go with that until the end of the season before changing the coolers. Thanks again for your suggestions.
 
I would fix the problem.
If the pressure does not come up at idle it is more than a cooling problem. When my oil cooler failed I put in a piece of pipe so no oil cooler for the engine at 1200 RPM pressure was 40 PSI at 1600 RPM It started to drop. At 1800 it was 20 or so. Ran the boat at 1200 for 50 knots and pressure stayed at 40 PSI. Replace pipe with repaired cooler and all was well.
 
Seeing low oil pressure would drive me nuts.

Lets hope it's an easy fix.

My 1998 5.0's never go below 40 psi.

If there's a way to confirm with a mechanical style oil pressure gauge that would be the thing to do.
 
I had the mechanic check with the mechanical gauge and it came out about the same.
Next I will change the oil and see if it gets any better.
 
Unless I'm missing something these oil pressure threads seem to always involve Mercruiser engines and almost never Crusaders. I realized Brunswick owns both Sea Ray and Mercruiser so you only see Crusaders in older boats but I almost never here about Crusaders have OP issues. I wonder if the problem is in the oil cooler design that has the coolers sitting low along side the oil pan allowing sediment to accumulate in the coolers. Crusaders have U coolers that sit vertical in front of the directly after the raw water pump which would improve cooling of both engine and transmission fluids.
 
hottoddie said:
Unless I'm missing something these oil pressure threads seem to always involve Mercruiser engines and almost never Crusaders. I realized Brunswick owns both Sea Ray and Mercruiser so you only see Crusaders in older boats but I almost never here about Crusaders have OP issues. I wonder if the problem is in the oil cooler design that has the coolers sitting low along side the oil pan allowing sediment to accumulate in the coolers. Crusaders have U coolers that sit vertical in front of the directly after the raw water pump which would improve cooling of both engine and transmission fluids.
Click to expand...

I agree it does seem to be a lot more of Merc's but there are a lot more Merc's than Crusaders in genereal so I'm not sure if that is why. However I'm a big fan of Crusaders having them (8.1's) on my last boat and would choose them every time if I could.
 
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