Mercruiser 5.0 oil pressure question

TheBlizzard · Jul 8, 2011

Today while I was out on the water I noticed that when I was up on plane (3000 rpms - 4600 rpms) my oil pressure would be pegged at almost 80psi. At idle it would be around 40 psi. As soon as I would give it some throttle it would jump way up. Also I noticed while checking the oil that there was a small amount of oil in some water in the bilge area.

Is this normal? Is the oil level too high? Is the sending unit going out? I checked it and it seems normal with the boat in the water.

The boat in question is a 1989 Celebrity 20ft Cuddy with a Merc 5.0.

I searched the forums and found that some people said from 50-60 psi it normal under load.

Thanks in advance.

dirtdog38 · Jul 8, 2011

Re: Mercruiser 5.0 oil pressure question

yes its higher than i like .check the gage might be bad to much will steal hp. its only norm to see oil in ther if u got a leak or u spilled it

Sharp Shooter · Jul 8, 2011

Re: Mercruiser 5.0 oil pressure question

Did it stay at 80ibs after it reached normal operating temps?

TheBlizzard · Jul 9, 2011

Re: Mercruiser 5.0 oil pressure question

Sharp Shooter said:
Did it stay at 80ibs after it reached normal operating temps?

Yeah it was totally warmed up after an hour or so of using it. Went from 40 to 80 under load.

TheBlizzard · Jul 9, 2011

Re: Mercruiser 5.0 oil pressure question

dirtdog38 said:
yes its higher than i like .check the gage might be bad to much will steal hp. its only norm to see oil in ther if u got a leak or u spilled it

If it was not a spill where are some spots it could be leaking? COuld the high oil pressure be causing some leakage? As far as robbing horsepower; it turns a 19 pitch 14 1/2 3 blade to 4600 rpm and goes 51 mph (GPS).

The gauge seems like it is working though, is there a sending unit that could be acting up?

Saqbcq · Jul 9, 2011

Re: Mercruiser 5.0 oil pressure question

What weight oil?

Sharp Shooter · Jul 9, 2011

Re: Mercruiser 5.0 oil pressure question

Is your guage electric? If yes, try a mechanical guage. Saqbcq is asking a good question. You aren't running straight 50 wt oil are you?

TheBlizzard · Jul 10, 2011

Re: Mercruiser 5.0 oil pressure question

Saqbcq said:
What weight oil?

5-30 is what is in there.

TheBlizzard · Jul 10, 2011

Re: Mercruiser 5.0 oil pressure question

Sharp Shooter said:
Is your guage electric? If yes, try a mechanical guage. Saqbcq is asking a good question. You aren't running straight 50 wt oil are you?

The gauge is electric. I am going to go and mess with it today and see what is up.

dirtdog38 · Jul 10, 2011

Re: Mercruiser 5.0 oil pressure question

at 4600 u need less than 50 anything more is taking hp but in what u have its no big deal. i was just saying
.but id say u have a junk gauge like i said the other day

dirtdog38 · Jul 10, 2011

Re: Mercruiser 5.0 oil pressure question

or yes a sending unit

TheBlizzard · Jul 10, 2011

Re: Mercruiser 5.0 oil pressure question

Determined the oil was actually leaking from the front main seal, pulled everything apart and put a replacement seal kit in it just a little bit ago. Took about an hour to do total, I ran it on the muffs for about 5 minutes and it didn't leak a drop that I could tell.

Hopefully that fixed it.

The oil pressure issue still remains though, it reads 60 psi at idle and goes to 80 psi at 1500 rpm. I am sure it is just misreading somewhere so I will have to put a mechanical gauge on it to see if its the sending unit or the electronic gauge on the dash itself.

TheBlizzard · Jul 14, 2011

Re: Mercruiser 5.0 oil pressure question

Edited above post with recent info for reference.

Small oil leak from the front case = main seal (in this case)

Saqbcq · Jul 15, 2011

Re: Mercruiser 5.0 oil pressure question

5W-30 is pretty thin, might allow oil to move easier creating higher pressure.They recommend 25W-40, 20W-40, SAE 30, and 20W-50 from best to worst. High oil pressure could cause oil to get past seals main bearings are right on the other side. If you have a manual there is an easy way to test the guage. NAPA sells Sierra Marine parts oil sending unit is probably pretty cheap Sierra part# 18-5899 except Dual application. Good luck

TheBlizzard · Jul 15, 2011

Re: Mercruiser 5.0 oil pressure question

Saqbcq said:
5W-30 is pretty thin, might allow oil to move easier creating higher pressure.They recommend 25W-40, 20W-40, SAE 30, and 20W-50 from best to worst. High oil pressure could cause oil to get past seals main bearings are right on the other side. If you have a manual there is an easy way to test the guage. NAPA sells Sierra Marine parts oil sending unit is probably pretty cheap Sierra part# 18-5899 except Dual application. Good luck

Thanks for the info. I just re-checked and 20-50 was put in it last time around so I think it has the right oil in it.

When I replace the main seal the old one pulled out very easily, I could have pulled it out with my finger most likely so I am thinking it was the problem.

TheBlizzard · Jul 16, 2011

Re: Mercruiser 5.0 oil pressure question

Ran it on the water today for 45 mins, didn't leak a drop, so it was the main front seal that was bad. Oil pressure still reads from 40-80psi but I attribute that to a fault gauge/sender, im not too worried about it.

cr2k · Jul 16, 2011

Re: Mercruiser 5.0 oil pressure question

I would check the oil press for sure. If you run that high and go to WOT or fast cruise you could float some bearings and spin them. Major crank and block damage.

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