Mercruiser 350 5.7 Low oil pressure after running

[Meisenhuth5]

Hoping to tap into the vast knowledge of folks here.

I picked up a Wellcraft Nova 23 XL over the winter as a project boat and have confirmed some of the issues from the previous owners statement. The previous owner stated that after cruising the oil pressure would drop and the idiot light would come on. With that statement I immediately plumed in a permanent mechanical gauge.

Now for the nitty gritty. When i took her out thursday, the cold idle pressure on the mechanical gauge was 35psi and maintained that while cruising through the no wake zones. After getting on plane and running for 10 minutes (pressure was up to about 65) i decides to slow it down and put it neutral, the pressure dropped to about 3 psi and the idiot light came on for a split second. the pressure will slowly build again if left alone, or will jump right up if i give it a little throttle. What are you thoughts on this, bad bearings? How long would somebody run with this issue? I was hoping to get a little enjoyment out of the boat before tearing her down this off season to repair some soft spots in the floor and possibly re-wiring the boat neatly and correctly.

If bad bearings would you recommend just pulling the motor and replacing bearings or get a reman'd motor / getting this one machined?

Thank you in advance for helping me out.

BTW i also just changed the oil with quicksilver 25w-40 with new filter. The pressure was the same on muffs before and after the oil change.

 

[alldodge]

Hoping to tap into the vast knowledge of folks here.

I picked up a Wellcraft Nova 23 XL over the winter as a project boat and have confirmed some of the issues from the previous owners statement. The previous owner stated that after cruising the oil pressure would drop and the idiot light would come on. With that statement I immediately plumed in a permanent mechanical gauge.

Now for the nitty gritty. When i took her out thursday, the cold idle pressure on the mechanical gauge was 35psi and maintained that while cruising through the no wake zones. After getting on plane and running for 10 minutes (pressure was up to about 65) i decides to slow it down and put it neutral, the pressure dropped to about 3 psi and the idiot light came on for a split second. the pressure will slowly build again if left alone, or will jump right up if i give it a little throttle. What are you thoughts on this, bad bearings? How long would somebody run with this issue? I was hoping to get a little enjoyment out of the boat before tearing her down this off season to repair some soft spots in the floor and possibly re-wiring the boat neatly and correctly.

If bad bearings would you recommend just pulling the motor and replacing bearings or get a reman'd motor / getting this one machined?

Thank you in advance for helping me out.

BTW i also just changed the oil with quicksilver 25w-40 with new filter. The pressure was the same on muffs before and after the oil change.

:welcome: to iboats

You were getting 65 on plane and the engine was I'm sure at full temp. The 65 pressure looks great but the 35 at idle and cold looks low. Is the mechanical gauge you installed at the helm or back close to the engine? Reason for the question is the length of tubing needed to run to the helm. If it's at the helm did you bleed out the air in the tubing, also what size tubing (1/16 ID)?

My initial thought is either need a new oil pump, oil sump is loose and may be sucking some air, or you need a rebuild. But still need to know if there is air in the line and size of line. While 1/16 line is good, the reaction to changes in pressure is not as responsive as 1/8 ID line.

 

[Meisenhuth5]

AllDodge, Thank you for the welcome and your response. The mechanical gauge is located just above the motor and has approximately 15 inches of 1/16ID tubing. With that length of tubing, should i still bleed the air out? I can visibly see where the oil/air line is in the clear tubing. Also, do you think it is wise to run this in short spurts?

Thanks

 

[Southtowns27]

Any idea when the last oil change was? What oil is in it now? Might be worth your while to change it. Try running some Rotella straight 30 weight with a new premium oil filter.

 

[z00m]

Keep in mind that most of the mercruiser small blocks have a specified minimum oil pressure of just 4psi.

 

[alldodge]

AllDodge, Thank you for the welcome and your response. The mechanical gauge is located just above the motor and has approximately 15 inches of 1/16ID tubing. With that length of tubing, should i still bleed the air out? I can visibly see where the oil/air line is in the clear tubing. Also, do you think it is wise to run this in short spurts?

Thanks

No need to bleed air with it being that close. While Merc does indicate a minimum oil pressure of around 4psi, this should only be seen after coming off plane with the engine hot. After a short time of idling, the pressure should start coming up a bit more. Something isn't right because the pressure goes from 65 to 3, that is a very large swing. Using the 20w40 should be plenty to provide sufficient oil pressure. Only other issue that comes to mind is the oil pumps internal relief valve may be hanging.

What year is the engine and how many hours are on the motor if known?

 

[Meisenhuth5]

What year is the engine and how many hours are on the motor if known?

AllDodge, That is a million dollar question. From what I can tell the motor has been remanufactured and there is not an hour meter onboard. The casting on the block state its a GM 350 manufactured from 87-95. From your statements above, I might try to get this season out of the boat and pull the motor this fall and have a good hard look at the pump and bearings. With is getting good pressure anything above idle, am I playing with fate still running it?

Thanks!

 

[alldodge]

AllDodge, That is a million dollar question. From what I can tell the motor has been remanufactured and there is not an hour meter onboard. The casting on the block state its a GM 350 manufactured from 87-95. From your statements above, I might try to get this season out of the boat and pull the motor this fall and have a good hard look at the pump and bearings. With is getting good pressure anything above idle, am I playing with fate still running it?

Thanks!

It's a roll of the dice, looks to be mostly OK but the 3 psi isn't good. I cannot say with confidence that something will or won't happen. What might be worth a try to get so more pressure up is change oil to 20w50. Don't think it will actually change anything at this point but increasing the viscosity may increase the idle pressure. As before the 65 looks great, but the 3 is not. My 502 will not even put out that mush pressure on initial startup, but I will say I don't have a mechanical gauge. Since your electric agrees with your mechanical gauge, then we should be able to rule out a faulty gauge. Wish I could add something that to further define the issue

 

[alldodge]

I need to add that your not hearing valve clatter from collapsing lifters at the 3 psi. This indicates there is sufficient pressure to keep them pumped up, a good thing

 

[Meisenhuth5]

I need to add that your not hearing valve clatter from collapsing lifters at the 3 psi. This indicates there is sufficient pressure to keep them pumped up, a good thing

That's my only saving grace from just pulling the motor now. There is no noise from the valve train. Thank you for the advice.