Engine revs up and down

[BILLWH01]

I have a Yamaha 2010 250hp outboard (sits on the back of a 23ft Jones Brothers Cape Fisherman). Recently when offshore, the engine would not hold to a specific tach setting. The oil pressure gauge (digital Yamaha command center) registered one tick mark down, then would move one tick mark up, etc. When in neutral, the gauge simply moved between 1-down, the middle, 1-up, then start over again. The wind had come up, and we had a time con*****ing the engine to get home. Although, in spurts, it worked OK, we came home early. My mechanic is finding no codes with the engine. I have a fly by wire throttle, and no codes register for the throttle either. Any ideas? Thanks.

[boscoe99]

Do you have the optional water pressure and/or speed sensor installed?

Please provide the complete model by the way. A Yamaha 250 is a two stroke. Since you mention having a digital throttle I know that it is not a two stroke. It will be an F250 something or other. We need that something or other.

[BILLWH01]

Hi

The boat is at the beach with the mechanic, so I can't read the number from the engine. But, I believe it is the 6 cyl F250 offshore engine (4.4L, the heavy one) that was built for years, and replaced with the lighter version about 2010, I think. I once found the entire model number, where letters on the end reflected that fly by wire was available on the engine.

Yes. It has a Yamaha thru hull speed sensor (as well as a Garmin 7212 with thru hull transducer) on the boat. I can read the speed sensor while looking at the Yamaha digital gauge. I'm not aware of a water pressure sensor, but I don't doubt it. The boat was bought used about a year ago, and was used as the boat show boat by Jones Brothers. I can ask the mechanic if I have the water pressure sensor if you like.
thanks

[boscoe99]

If installed inspect the sensors for the presence of water or corrosion on the terminals. Early sensors were known to fail. Failure of the sensor with water getting onto the pins/sockets will cause all sorts of anomalies.

I am not talking about a thru hull sensor. The motor has provisions built in for a pitot tube derived speed sensor or a engine water pressure sensor. These are what I am referring to.

[rodbolt17]

pay attention to what Boscoe is trying to tell ya.

I have had a few of the later style speed/water pressure sensors fail.
they will typically load down the 5V ref that is used for speed,water pressure,oil pressure and the baro/intake air pressure.

this can lead the ECU to belive its operating at 15-20K ft above se*****l.

and yes for some odd reason the ECU is mapped to run at 25000 ft.

[BILLWH01]

Thanks much. I will have the mechanic inspect the speed/water pressure sensors that are found within the engine itself. I don't know if I have this, but would not be surprised. Thanks again.