Mercruiser Oil Pressure

[fireman15]

Just got my 89 sunrunner back from the shop and took it out on the water. The first thing I noticed was that the oil pressure was off the chart (past 80) (Mercruiser 3.0). I called the shop and his answer was "that is a high pressure/flow oil pump we put in because that is what they sent us for replacement. I can change the gauge if you wish". SERIOUSLY? If I normally run between 40 and 55 for pressure and this thing is now over 80 what is it going to do with my motor? I wouldn't think that kind of pressure would be acceptable for my seals. Any responses from you is appreciated.

 

[alldodge]

Re: Mercruiser Oil Pressure

I think the gauge is off, or somebody put the wrong oil in it; most likely the gauge/sender. Can you get a hold of a mechanical gauge to install and check?

 

[Bt Doctur]

Re: Mercruiser Oil Pressure

Better not be a hi-vol pump, you will destroy the engine from lack of oil.

 

[Bondo]

Re: Mercruiser Oil Pressure

Just got my 89 sunrunner back from the shop and took it out on the water. The first thing I noticed was that the oil pressure was off the chart (past 80) (Mercruiser 3.0). I called the shop and his answer was "that is a high pressure/flow oil pump we put in because that is what they sent us for replacement. I can change the gauge if you wish". SERIOUSLY? If I normally run between 40 and 55 for pressure and this thing is now over 80 what is it going to do with my motor? I wouldn't think that kind of pressure would be acceptable for my seals. Any responses from you is appreciated.

Ayuh,.... Welcome Aboard,... Who is They,..?? Who ordered the new oil pump,..??

Why is the oil pump bein' changed in the 1st place,..??

Plumb in a mechanical gauge to cross-check it,...

 

[flipbro]

Re: Mercruiser Oil Pressure

Bt doc .Not trying to hijack But could you please explain why a hi vol pump would destroy an engine!! Thanks phil

 

[Bt Doctur]

Re: Mercruiser Oil Pressure

most engines have a little less 4 qts in the pan. a standard pump can handel max rpm with no trouble because the oil has a chance to flow back into the pan and get re-pumped.
A Hi-Vol pump, unless your running 10-18 Qt oil pans or dry sump oil pans will pump the slightly less than 4 qts into the engine so fast it wont have a chance to return to the pan.So at 4000 rpm flying across the lake you run out of oil to pump into the enginebecause it never made it back into the oil pan.

 

[MarkSee]

Re: Mercruiser Oil Pressure

Darn good explanation BT, I was wondering the same thing.....having heard that term since the late 70's but never been into engines enough to know.
Thanks

Mark

 

[fireman15]

Re: Mercruiser Oil Pressure

Better not be a hi-vol pump, you will destroy the engine from lack of oil.

AS a matter of fact it is a high volume pump. I didn't think it was normal to run that high of a pressure, the gauge worked before I had engine work completed. New oil pump, camshaft, ect.

 

[fireman15]

Re: Mercruiser Oil Pressure

Ayuh,.... Welcome Aboard,... Who is They,..?? Who ordered the new oil pump,..??

Why is the oil pump bein' changed in the 1st place,..??

Plumb in a mechanical gauge to cross-check it,...

It had work done at Bob Feils boats and motors in Wenatchee Wa. Had the boat on the water 45 minutes (just bought it the day before) and the engine went. Needed new camshaft, oil pump, and $1800 more work completed.

 

[fishrdan]

Re: Mercruiser Oil Pressure

I called the shop and his answer was "that is a high pressure/flow oil pump we put in because that is what they sent us for replacement.

That sounds like a weak excuse... Yeah it was the wrong part, but we installed it anyway, because that's what they sent us. :facepalm:

Not only is that pump going to suck the pan dry at extended high RPM, but it's going to put undue stress and wear on the distrubutor and cam drive gears. It will also rob the engine of some power, but that's negligible.

High volume oil pumps usually come with a (extra) low pressure bypass spring, that can be installed to lower the pressure of the pump. If they would have installed the weaker bypass spring the oil pressure would be lower and excess oil would be dumped back into the pan, eliminating most of the issues you have now. Excess gear wear and (minimal) power loss would still be there though.

I hate to say it, but the shop needs to put a normal stock oil pump in the engine...

 

[alldodge]

Re: Mercruiser Oil Pressure

[I mean no disrespect and no offence] Been doing a bunch of research on the high/vol pump. Everything from emptying the pan to wearing distributor gear and everywhere I look it all comes back to a myths (no offence). The oil channels will only pass so much oil per revolution as we all know liquids cannot be compressed. Pressure is controlled by the relief spring in the pump. Pressure can be increased with stiffer springs I have found no location where a pan can be sucked dry by a high/vol pump. Only indication of a pan being sucked dry has been from clogged ports. Most racers using very deep pans do this because of oil moving from side to side where high g' loads are encountered, this is why they use windage trays to keep the slop down.

10 PSI per 1000 RPM's remains rule of thumb

Not saying my research is 100%, I just cannot substantiate the claim. My opinion only and not without flaws I'm sure

 

[81 Checkmate]

Re: Mercruiser Oil Pressure

Id still go with what Bondo said.....Check ( Or have them check) with a mechanical gauge to be sure your gauge is right.

I know when i change my oil it has a higher oil pressure reading than the old oil........After it thins some the reading comes down.

 

[fishrdan]

Re: Mercruiser Oil Pressure

Everything from emptying the pan to wearing distributor gear and everywhere I look it all comes back to a myths (no offence).

10 PSI per 1000 RPM's remains rule of thumb

I had one of those mythical worn out distributor gears,wipe out a cam gear, after installing a high volume oil pump in an old hot-rod... Problem went away after installing the low pressure bypass spring,,, but this was an automotive engine, not marine.

This is more of a "marine" issue as the engine is turning high RPM all the time, unlike an automotive engine that spins at relatively low RPM most of the time. Not many (if any) cars/trucks run the engine at 4000+ RPM constantly.

I agree with the 10PSI/1000 RPM, but his engine is over 80PSI. I'm not sure what his idle PSI is, but I'd suspect 40-50 with the high pressure spring installed, enough pressure for WOT at idle RPM.

 

[fireman15]

Re: Mercruiser Oil Pressure

Engine idle psi is 80, anything higher than that and it's off the charts a bit. No way of telling as there is a pin keeping it from going farther. I called the shop again yesterday and the mechanic said "so what's the problem, you wan't it high versus low don't you. Don't worry about it as this will not harm your engine in any way".

 

[Volphin]

Re: Mercruiser Oil Pressure

[I mean no disrespect and no offence] Been doing a bunch of research on the high/vol pump. Everything from emptying the pan to wearing distributor gear and everywhere I look it all comes back to a myths (no offence). The oil channels will only pass so much oil per revolution as we all know liquids cannot be compressed. Pressure is controlled by the relief spring in the pump. Pressure can be increased with stiffer springs I have found no location where a pan can be sucked dry by a high/vol pump. Only indication of a pan being sucked dry has been from clogged ports. Most racers using very deep pans do this because of oil moving from side to side where high g' loads are encountered, this is why they use windage trays to keep the slop down.

10 PSI per 1000 RPM's remains rule of thumb

Not saying my research is 100%, I just cannot substantiate the claim. My opinion only and not without flaws I'm sure

NASCAR oil pumps are stockers. Just sayin.

 

[Volphin]

Re: Mercruiser Oil Pressure

And please take Bondo's advice. Hook a real diagnostic oil pressure gauge to the engine and see what the reading really is.

 

[Bondo]

Re: Mercruiser Oil Pressure

Engine idle psi is 80, anything higher than that and it's off the charts a bit. No way of telling as there is a pin keeping it from going farther. I called the shop again yesterday and the mechanic said "so what's the problem, you wan't it high versus low don't you. Don't worry about it as this will not harm your engine in any way".

Ayuh,..... This is a Diy site, not a whine 'bout yer Mechanic site,....

Do ya wanta fix yer motor, or even see If there's a problem, or not,..??

We're tryin' to steer ya to the answer, but We need yer help too,....

All We can tell ya to do at this point is to drag it back to the marina, 'n have Them verify the oil pressure,...

 

[fishrdan]

Re: Mercruiser Oil Pressure

And please take Bondo's advice. Hook a real diagnostic oil pressure gauge to the engine and see what the reading really is.

Agree with Bondo and Volphin, though I would use a 0-200PSI gauge to see exactly how high the oil pressure is going.

I checked Mercruiser Manual #10 and the oil pressure should be 4psi minimum at idle, then 30-60PSI at 2000RPM. 80PSI at idle is way off the charts.

I wonder if the oil pump is defective and the bypass valve is stuck closed.

 

[Volphin]

Re: Mercruiser Oil Pressure

I think the gauge is off, or somebody put the wrong oil in it; most likely the gauge/sender. Can you get a hold of a mechanical gauge to install and check?

Missed this earlier, but AllDodge (great handle BTW) makes a valid point. Oil viscosity can effect your pressure readings, but IME only 7-10 psi. Any chance of an oil additive like STP or wrong oil in the crankcase?