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Old 26-05-2015, 04:40   #1
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Oil pressure gauge increasing with revs

Hi we seem to have in increase in oil pressure when the revs are increased it normally it sits at around 40psi and have run her longer than this in the past with out issue, we are currently cruising in the Med and this happened after being under engine for nearly 15 hours at no more than 2000rpm, I use 15/40 oil which I'am thinking of changing to a thicker grade due to the water temp here. we have a Perkins 4108 and help would be very much appreciated
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Old 26-05-2015, 04:55   #2
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Re: Oil pressure gauge increasing with revs

What you describe is normal, I wouldn't change to a thicker oil.
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Old 26-05-2015, 05:50   #3
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Re: Oil pressure gauge increasing with revs

Thanks, so Needn't worry about the gauge, I have a spare so I will try it. Just felt strange that it's never happened before. The situation was made worse we were 3 hours from our destination having been sailing for over 40 hours with no wind and a gale forecast . Thanks for your reply..
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Old 26-05-2015, 06:41   #4
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Re: Oil pressure gauge increasing with revs

I would actually be concerned about any change in behavior.

Of course oil pressure is supposed to go up with revs, and down with temperature. But why does it behave differently than before?
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Old 26-05-2015, 06:54   #5
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Re: Oil pressure gauge increasing with revs

The oil pump on the 4-108 is a direct drive unit that's rpm is matched to the engine rpm. With that, the engine's oil pressure will increase as the engine's rpm's increase which helps "float" the engine's bearings in a sheen of oil in the journals. As for oil, it was customary to use a "heavier" viscosity oil in hot weather/waters as viscosity is effected by temperature. These days, modern multi-grade oil, e.g. Shell Rotella 15-40, eliminates the need to switch to heavier oil during hot weather. On our boat (also a 4-108) our oil pressure sits at about 35-40 psi at low speed/rpms but increases to about 60 psi at "cruising speed" which, for us, is around 2500-2700 rpm.

FWIW...
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Old 26-05-2015, 06:57   #6
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Re: Oil pressure gauge increasing with revs

I too have the Perkins 4-108 and from day 1(1985) the oil pressure always went higher with higher rev. Seems like 40 on the gage is normal. Speaking of oil pressure, last month when going to the boat(a 6 hour drive) in my Jeep wrangler, the pressure gage pegged out high and stayed there. I was concerned and had an oil change and found out that the sensor itself was shot. The Jeep was only 15 years old whereas that Perkins is 30, so failure of sensors might be expected.
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Old 26-05-2015, 07:38   #7
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Re: Oil pressure gauge increasing with revs

Can you describe what is different? Oil pressure normally fluctuates with RPM, but if it used to run 60 PSI and now runs 40 PSI, then maybe you do need to look at that.
I'll get in trouble with saying this as engines are different, but as a general rule I don't get concerned if I get 20 PSI on a hot engine at idle, with 10 PSI per thousand or so RPM increase up to the relief valve set point of course. Cold oil can exceed the relief valve opening pressure on some engines.
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Old 26-05-2015, 08:03   #8
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Re: Oil pressure gauge increasing with revs

th eonly change i would make is to remove the air bubble from your oil pressure tubing.
funny how that happens. should be full to overflowing when reattached ... that way , no air bubble. bleed it out.
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Old 26-05-2015, 09:15   #9
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Re: Oil pressure gauge increasing with revs

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Originally Posted by zeehag View Post
th eonly change i would make is to remove the air bubble from your oil pressure tubing.
funny how that happens. should be full to overflowing when reattached ... that way , no air bubble. bleed it out.
I hope not many boats have a mechanical gauge with an oil "tube" going to it. The guy who lost oil pressure and had his engine seize should agree. Oil pressure gauges should probably be electrical with a sending unit. If you want a mechanical gauge it should have a proper flexible hose going to it, not a little 1/8" plastic or copper tube like a car might have. In addition to that, if you want a mechanical gauge the fitting on the engine should have a small orifice to limit oil loss if it fails.
In either case, the last bit of hose or the sending unit itself should be filled with auto trans fluid or some sort of light oil that does not coagulate.
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Old 26-05-2015, 09:24   #10
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Re: Oil pressure gauge increasing with revs

Oil pumps are gear pumps, or flow based, not like a centrifugal pump which is effort based. The fast it goes the higher the pressure until the relief valve lifts, at about 60 psi at the pump discharge..on big engines, the pressure is detected at the most remote bearing,which makes sense if you think about it...if it went low all of a sudden, i would be concerned about the relief.
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Old 26-05-2015, 09:58   #11
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Re: Oil pressure gauge increasing with revs

Quote:
Originally Posted by Guy View Post
I hope not many boats have a mechanical gauge with an oil "tube" going to it. The guy who lost oil pressure and had his engine seize should agree. Oil pressure gauges should probably be electrical with a sending unit. If you want a mechanical gauge it should have a proper flexible hose going to it, not a little 1/8" plastic or copper tube like a car might have. In addition to that, if you want a mechanical gauge the fitting on the engine should have a small orifice to limit oil loss if it fails.
In either case, the last bit of hose or the sending unit itself should be filled with auto trans fluid or some sort of light oil that does not coagulate.
Personally,I trust a mechanical oil & temp gauge way more than electric ones,but I am old & had bad experiences drag racing in 60s,when electric gauges were fairly new on market.
You are correct about the cheap plastic tubing supplied with today's mech. oil ga. Do not use it-car or boat-I know from experience.
If you can find 1/8" copper tubing & compression sleeves,it is OK.Just make a coil,like a spring,at each end to absorb vibration. Good practice with any copper tubing.
Better still,use 1/4" hydraulic hose. just my opinion. / Len

To OP Check that your engine oil pressure relief valve is OK
Change oil filter-some collapse,some have built in oil press. relief,etc
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Old 26-05-2015, 10:34   #12
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Re: Oil pressure gauge increasing with revs

Quote:
Originally Posted by JulianGullsway View Post
Hi we seem to have in increase in oil pressure when the revs are increased it normally it sits at around 40psi and have run her longer than this in the past with out issue, we are currently cruising in the Med and this happened after being under engine for nearly 15 hours at no more than 2000rpm, I use 15/40 oil which I'am thinking of changing to a thicker grade due to the water temp here. we have a Perkins 4108 and help would be very much appreciated
What are the pressures at idle and at cruising RPM, hot and cold?

IE, 4 numbers :

Idle cold
Idle hot
Cruising cold
Cruising hot

How have the pressures changed?

Without this information, no-one can help you.
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Old 26-05-2015, 10:39   #13
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Re: Oil pressure gauge increasing with revs

15w40 oil is fine, just make sure you change oil regularly.
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Old 26-05-2015, 11:30   #14
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Re: Oil pressure gauge increasing with revs

As far as I know, the numbers on a grade of oil, e.g. 10/W30, 15/W40 etc., refer to the range of temperatures for which it is suited i.e. 10/W 30 for use between -10deg Centigrade and +30deg Centigrade.
These are Multigrade oils, which are designed for use all year round. They were introduced in the 1960s to replace the oils they had then, which were either Winter Grade, or Summer Grade and had to be changed seasonally.
15/W40 should be fine for use in the Med but I personally would not deviate from using the oil specified in the engine's user manual.
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Old 26-05-2015, 11:36   #15
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Re: Oil pressure gauge increasing with revs

If your oil pressure is starting to go down (at the same RPM) from what it used to be, it means either the oil pump is wearing out -if you're lucky- , the bearings are wearing out, or the crank is getting worn. If the bottom end is wearing out (really, really low oil pressure) and you want to save the cruising season, I'd just go to 20/50 and maybe some STP. I've done that with my flathead powered T-Bucket on a 5000 mile road trip, and did the re-build over the winter.
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