Please check my thinking

[MichaelD]

BL 4588 with twin Hinos about 1800 hours. Well maintained.

Recently this thing happens on port engine only. At about 1400 rpm and above, oil press gauge suddenly either drops to zero or starts flopping around. It does this instantly rather than at pace a change in real oil pressure would occur. When it occurs, the tach also becomes a bit unstable, usually rising a couple hundred rpm, but the engine sound does not indicate a real change. Sometimes changing from 1400 to say 2200 will clear it for awhile. Then it may recur, sometimes not though. Otherwise, the engine runs perfectly.

So, I’m thinking this has all the earmarks of an electrical issue vs oil pressure issue. When I looked I found connection on oil sensor a little loose and tightened it. This seemed to help for a while, but the problem recurred. I’ve found no other obvious connection problems. Maybe the sensor is having an internal failure. But, am I right in thinking it’s electrical? I don’t see how pressure/flow issues could produce the instant on/off indications on the gauge.

I’d appreciate any insights.

 

[O C Diver]

It seems electrical to me if both the RPM and oil pressure are effected with the engine seeming to be running at the same RPM. Maybe a loose ground in the gauge panel?

Ted

 

[Comodave]

Agree with Ted. Check every electrical connection for corrosion and tightness. Don’t forget the grounds. They may look good at first glance but give them a tug and see if the wire comes out of the connector. If it is a good connection the wire won’t pull out. Look for corrosion on the connections.

 

[kchace]

It definitely does not sound like an actual oil pressure issue. If a wiring issue turns out not to be the cause, then I would change the oil pressure sending unit. They are notorious for failing. One of the ways they fail is to become very erratic.

Ken

 

[tiltrider1]

Most boats have a low oil pressure alarm. You don’t mention it going off so I assume it didn’t. This would leave me to assume the oil pressure is good. I am assuming you checked the oil level and found it to be in the correct range.

As others have said, it appears to be an electrical issue, especially if the tach is acting up at the same time. Some where there is most likely a bad or corroded connection causing your issue.

 

[Comodave]

Since it is two things acting up I would lean towards it being a ground issue.

 

[Max Simmons]

Same at both helms?

 

[MichaelD]

Response to 3 comments

Great inputs. Thanks.

Yes, same at both helms. So it seems like it’s at the sender end vs the guage end.

I agree it sounds like ground vs signal side since two sensors are affected. I don’t know but assume it’s likely that oil and tach senders would share a ground. No other gauges show any affect.

And, correct, the oil pressure alarm does not go off, but it does when I shut down, so it’s working.

So, tomorrow, it’s into the ER I go (not the unpleasant one I hope).

 

[O C Diver]

After substantial thought, and a glass of wine, you're likely looking for a common positive wire between the gauge console and the ignition, most likely on the gauge panel.

My logic is as follows:
The oil pressure gauge sender gets its negative from the engine block and adjusts the voltage through a potentiometer in the sender, so it's not likely a ground issue at the panel. Both the tachometer and oil pressure gauge require a positive at the panel.

The other remote possibility is the wiring harness multi pin connectors. On many manufacturers engines, there is both a multi pin connector on the engine wiring harness and one at the gauge panel. All the gauges and ignition circuit run through the harness. There's a possibility that there could be corrosion on the pins (most likely the one near the engine). It's unlikely this is the problem as both pins on the connector would have to fail at the same time.

A loose positive wire on the panel would be what I would look for first.

Ted

 

[JDCAVE]

I had this same problem and it was the sender. Gauges were erratic. Look for loose wires at sender or gauges. If that doesn’t fix it, it’s probably the sender. You could check impedance at the sender and see if it is also erratic. If it is, it is almost certainly the sender. You will need to replace with a “like” unit, and for two stations. You will need a sender with the same impedance rating as the current sender.

While you’re at it consider putting in a “T” and installing a mechanical gauge above the engine. That’s what I did and it provides confidence in measurements.

Edit: I read through this more carefully and agree it is likely a suspect wiring issue, if the tach is also erratic.

Jim

 

[SteveK]

Oil sender probably, positive wire at dash, but what makes the tach go from 1400 to 2200 rpm with loose + connection.

 

[Slowmo]

The fact two gauges are affected indicates that power supply or grounding could have a poor connnection. I don't see how the sender would affect the tachometer, but a power problem would.

 

[MichaelD]

Completing this thread in oil pressure fluctuations

This thread started a while ago, but I just finished my work on it. It took so long because tech I asked to work on it kept putting it off, so I finally found a new oil pressure sender, waited through supply delays, but received and installed it. Problem solved. I suspect the reason the tach got wonky when the oil pressure did was that it was pulling down voltage on a shared supply to each. Anyway, all readings are now stable.

Thanks for all the thoughtful inputs.