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Old 07-07-2019, 18:19   #1
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Yanmar 3JH3E - Oil pressure drops as RPM's increase

Had some work done at Indiantown before starting the trek up towards St. Augustine last week, it's been brutally hot with no wind, so ICW all the way. Day 1 gets us to Stuart, don't notice any problems, but could have been preoccupied by finally being back in the water and moving again (and trying to make the last lock through at St. Lucie by 4:30 PM).
* Day 2, noticed the oil pressure gauge leaning precariously over towards 20, this while chugging along at 2500 RPM (rated max is 3200, so about 75%). Engine hours are low, 525 at start of the trip, with fresh oil and filter change (and fuel filters, and antifreeze flush and fill). Backed off on the RPM's to 2000 and watched the gauge climb to 30ish - just in the green. OK, so now we're doing 4.5 kts and I can live with that.
* Day 3, we have a little current against us and I'm finding @ 2000 RPM the gauge is ticking just over the 20 mark, I drop to 1900, climbs back up around 30ish, we make it a short day. I change the oil and filter that night to be on the safe side.
* Day 4, no better. Did I mention brutally hot? The coolant temp is steady, straight up in the green. Took the engine cover off and directed a fan into the compartment, no appreciable difference, except the cockpit is now a really miserable place to be.
* Day 5, lots of current against us and I'm down to 1750 RPM to just keep the needle at 20. It's a short day, the oil level is a tidge lower than it was.


All along the engine sounds the same, a bit loud, but it always has been, so no change there. Exhaust is clear, oil color is golden, temp is steady. Bad gauge? Oil cooler? Oil pump? Thoughts?
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Old 07-07-2019, 18:25   #2
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Re: Yanmar 3JH3E - Oil pressure drops as RPM's increase

Just one thought - engine oil takes a loooong time to reach maximum temperature, easily 40 minutes or more under near full power so the decrease in oil pressure might be due to the high ambient temperature.

An oil change won't hurt!
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Old 07-07-2019, 18:31   #3
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Re: Yanmar 3JH3E - Oil pressure drops as RPM's increase

If you have a mechanical gauge, my first step would be to verify the oil pressure.

Sounds like the oil is really hot. Is the coolant hot, too? Could be a plugged oil cooler, but usually the oil cooler is on the same plumbing as the coolant exchanger. Maybe you have a restriction in the raw water circuit and have a low flow of raw water, collapsed hose, dirty strainer, plugged exhaust loop, bad impeller, thru hull restricted or valve partially closed.
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Old 07-07-2019, 18:39   #4
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Re: Yanmar 3JH3E - Oil pressure drops as RPM's increase

You can tell oil temp with an IR thermometer and shoot the oil filter.

However I’d guess you have a stuck open oil pressure relief valve.

Even stupid hot oil as in low 200’s shouldn’t drop pressure that much.
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Old 07-07-2019, 18:53   #5
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Re: Yanmar 3JH3E - Oil pressure drops as RPM's increase

I would check the electrical wiring if you have a electric oil pressure sender. it would be really strange for the oil pressure to drop with increased RPM. But a bad sender or at loose wire somewhere might cause a electric oil pressure gauge to read low.

If you had a oil cooler I can see possibly a leak in that to seawater, causing low oil pressure as well.

I had a old 3qm Yanmar that exhibited low oil pressure. That was caused by a cracked valve rocker Tower. I will note that you can run the engine all day at 20 psi, with no damage to the engine.
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Old 07-07-2019, 20:25   #6
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Re: Yanmar 3JH3E - Oil pressure drops as RPM's increase

If you are running low on oil or oil return galleries are clogged or if there is a crack in your pickup tube you may get this.
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Old 07-07-2019, 21:21   #7
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Re: Yanmar 3JH3E - Oil pressure drops as RPM's increase

My guess is oil pressure is dropping because of increased oil temp. You say it's stupidly hot. I'd go with A64 pilot suggestion as to check oil temp with IR thermometer.
Oil temp will climb if you are fighting current, it's analogous to climbing a hill in your car. I would be a little concerned with that oil pressure given the low hours but do you know what the specification is? You dont say what model engine it is.
Agree with Sailorchick that engine can run all day at 20psi, many ancient tractors do.
What was the nature of the work done before you departed?
Has the problem occurred suddenly or has the oil pressure been slowly dropping over mths/years?
You could try a heavier viscosity oil if you cannot find a reason.
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Old 07-07-2019, 21:22   #8
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Re: Yanmar 3JH3E - Oil pressure drops as RPM's increase

Sorry engine type in title. Duh!
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Old 08-07-2019, 04:34   #9
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Re: Yanmar 3JH3E - Oil pressure drops as RPM's increase

“Upgrading Yanmar oil pressure gauge and sender” ~ by Harry Hungate
Upgrading Yanmar oil pressure gauge and sender - Ocean Navigator - October 2011

SKU #RM218026 - Oil Pressure Sender for Yanmar Marine Diesel, Replaces: 124060-39452
https://repowermarine.com/index.php/...060-39452.html
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Old 08-07-2019, 05:42   #10
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Re: Yanmar 3JH3E - Oil pressure drops as RPM's increase

Thanks all - I will start by replacing the sender unit, inexpensive and within the range of something I can do (thanks Gord for those links, straight to the part number and an Amazon click away).

The work I had done at Indiantown involved pressure testing the cooling system. The pesky little leak was coming from a drain plug at the oil cooler of all places, on this engine it is just a tubular ring that runs coolant around the base of the oil filter. I wonder if that plug is socked in too tight and threads now obstruct coolant flow. It's a PIA to get to, need to remove the oil filter to get to the plug, so will do that as the next step. I'm also going to get an IR thermometer and start collecting data. Any suggestions on make and model or other fun diagnostic stuff would be great.
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Old 08-07-2019, 05:54   #11
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Re: Yanmar 3JH3E - Oil pressure drops as RPM's increase

Quote:
Originally Posted by 1boat2many View Post
........ I wonder if that plug is socked in too tight and threads now obstruct coolant flow. It's a PIA to get to, need to remove the oil filter to get to the plug, so will do that as the next step. I'm also going to get an IR thermometer and start collecting data. Any suggestions on make and model or other fun diagnostic stuff would be great.
Or perhaps too much sealant was used on the threads and has caused some blockage???
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Old 08-07-2019, 08:16   #12
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Re: Yanmar 3JH3E - Oil pressure drops as RPM's increase

What weight oil is in it now? Shell Telus T6 in 15W40 might help. It's a diesel rated oil, in synthetic or dino, I'd just use the dino stuff.
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Old 08-07-2019, 10:01   #13
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Re: Yanmar 3JH3E - Oil pressure drops as RPM's increase

My thoughts exactly....I'd double check the oil quantity.
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Old 08-07-2019, 10:10   #14
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Re: Yanmar 3JH3E - Oil pressure drops as RPM's increase

doesn't make sense to me that the oil pressure goes UP with reducing rpms

I've gone through 2 oil pressure and temp gages the last 8 years
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Old 08-07-2019, 10:14   #15
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Re: Yanmar 3JH3E - Oil pressure drops as RPM's increase

Pump cavitation!
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