Strange oil pressure fluctuations

[geoffschultz]

I have a 1997 Yanmar 3JH2-T(B)E turbo motor in my Freedom 40/40 and when I start it up, the oil pressure is low, but when I start motoring, it climbs to normal levels. Then after about 20-30 minutes, the pressure starts dropping to levels lower than I would like to see it. Quite often I can change the engine RPMs and the pressure will climb back to normal levels, only to drop again after some random amount of time. The pressure never drops to levels where the alarm sounds, but this is clearly concerning.

The motor has about 5K hours on it and is currently in Guatemala. I'll be headed down to the boat on 2/1, so I need to figure out what parts I may need to bring down with me. Any ideas?

-- Geoff

 

[kloudie1]

Oil level/condition correct (diesel in crankcase oil?).. then
first thing I would suspect would be bad/loose electrical connections in the gauge circuit.. Next likely suspect would be the pressure sender on the engine ..check and clean up the connections in that circuit all the way back to the 12V power wire.. if that doesn't fix it, I would get a cheap mechanical gauge and temporarily install that to see if it acts the same way.. if it does not fluctuate then your oil pressure sender is bad. .if the mechanical gauge still fluctuates, then the next thing to check would be the oil pressure bypass valve in the base of the mounting flange for the oil filter.. sometimes debris in the oil can jam the little ball valve causing strange pressure changes..

 

[geoffschultz]

I've replaced the sender, with no change. I would agree with you on the electrical circuit, but in my opinion that doesn't explain why making small changes to the throttle would alter the voltage/resistance that the gauge is seeing.
I will take a mechanical pressure gauge down with me to observe the behavior. The real issue is that once I'm in Guatemala, getting parts down there is next to impossible/very slow. Hence I was trying to figure out what parts, if any, I need to bring with me.

 

[BrianRobin]

I agree a mechanical gauge will be your best bet. If it still looks wanky, another possibility could be a plugged oil pump pickup or oil pump pressure relief valve.

 

[LeslieTroyer]

Low oil pressure at idle is a sign of large bearing gaps. On race and high performance engines this is normal. Oil pressure decreasing as it comes up to temp is a sign of worn bearings. Taken together you may have a lower end job in the future. That’s all assuming the gauge is working.

Les

 

[Rich Stidger]

In my experience, a cold idling engine will have a moderate oil pressure. Increasing rpm on a cold engine will increase the pressure. After the engine warms up to normal hot the oil pressure will drop back to a moderate level because the hot oil has thinned out a bit. Taking a hot engine back to idle will result in the oil pressure dropping to below the point when it was cold- again due to the thinner oil. In no case should the oil drop below the minimum specification or cause an alarm.
If you are seeing pressure fluctuations other than what I have just described, then maybe there is an issue, otherwise I think what you are seeing is normal.
The exception to this would be if the oil pressure fluctuations have recently changed.

 

[Gunni]

My Yanmar has two senders, one for the alarm and one for the gauge. Old oil pressure gauges are a pain in the neck. You don’t have an alarm, you have a funky gauge. Bottom line, I think you can chill and observe. Take a mechanical pressure gauge to get a real pressure reading. Make sure your turbo hasn’t blown an oil seal.

 

[DayDreamer41]

@Gunni if the turbo has blown an oil seal would there be oil in the exhaust, like a slick in the water?

 

[Gunni]

A blown turbo oil seal will send oil into the exhaust.

 

[geoffschultz]

Re the turbo: The exhaust is clean.

 

[geoffschultz]

By any chance does anyone know what are the thread dimensions for an oil pressure sender for any of the following engines as they all use the same sender:

YSM8, YSM12, 2QM15, 1GM(10), 2GM(20), 3GM(30), 3HM(35)(F), 3JH2-E, TE, 3JH3-E, 4JH(2)-E, TE, HTE, DTE, UTE, 4LH-TE, HTE, DTE,D27,AX, D36,AX,AY

I want to make sure that I've got the right fitting for the oil pressure gauge.

 

[kloudie1]

To your post #3, Geoff, a change in RPM can change the vibration in the wire harness and connectors and "wiggle" a bad connection into thinking it is good for a while..

 

[Gunni]

Call Mack Boring, ask for the parts department:
http://mackboring.com/contact/

 

[geoffschultz]

To answer my own question, the thread on the sender unit is 1/8" BSPT and not the US NPT.

 

[Slipin Away]

Jeff Steve from Betterdays here. On my return from the Rio to
Florida last month, my oil pressure gauge exhibited the same fluxuations.
I am in the process of diagnosing the problem only the boat is out of the water now
and will be until late Feb. Please let me know what you find out and I will keep you informed as to my progress

 

[MitchM]

my experience on calling mack boring was that mack boring quit dealing with human beings , now they'll just refer you to a yanmar dealer. so look up the yanmar GOLD dealer near you and ask their parts department. there's a yanmar GOLD dealer in coastal SW Massachusetts whose parts department guy is excellent. ( so excellent that i forgot their name. )

 

[MitchM]

try niemec marine in new bedford MA, ask for bob.

 

[Whatfiero1]

That electric oil pressure is just a electric meter. If you have a clamp type on board prefer a analog type with a needle. You can check volts as motor runs to see if it matches gauge