Red warning light

Motoring back to the marina, my Tohatsu 9.8 electric start red oil warning light came on. I reduced speed and made it back to the slip. I shut down the engine and checked the manual. It says I have an oil pressure switch problem. I put the engine into the fresh water rinse bucket, started it up, and the red light did not go on. I kept it running til the gas ran out, still no red light. Anyone know what's happening?

I assume you have an MFS9.8A3, right? How old is it, and how many hours are on it? What is your oil level? What oil are you using, and when was it last changed?

The oil pressure sender is a simple "ground out to turn on the light" arrangement, and comes on around 4 psi and below. It's located near the thermostat housing, on the starboard side, near the top of the block. Normal oil pressure when warmed up and running 10w-30/40 is about 14 PSI at 950 RPM, and about 48 PSI at 5000 RPM and above, so the LED typically is only on while the system is pressurizing (right after starting) for several seconds. The actual oil pressure can be tested by unscrewing the sender and attaching an oil pressure test gauge to the motor.

If the oil is overfilled, even a drop, it can cause aeration of the oil, which can result in low oil pressure (as well as excessive crankcase pressures). Likewise, if the oil level is way too low, you can be sucking air into the oil pump occasionally, resulting in low oil pressures. If the oil has a lot of blow-by dilution (especially prevalent on low-hour motors that idle a lot in cool water), the oil will be thinner than it should be, and the pressure will be lower. Typically that is accompanied by overfilled oil. Also, if there is sludge clogging the oil pickup strainer, that can result in low oil pressures.

It's not unheard of to have an oil pressure sender that fails when hot, or fails intermittently. Used to happen on US cars a lot. Likewise, if the wire to the sender has managed to ground out somehow (routed too close to something hot or sharp), it will trigger the light. An oil pressure test will confirm the actual pressure.

I would verify oil level. If the motor is not fully broken in yet, I would consider changing the oil, especially if it the level is on the high side. Likewise, if the oil has 100 hours on it, it's time to change it. Be sure to use (NMMA-certified) FC-W rated oil. 10w-30 is fine; 10w-40 is OK if you run in very warm weather all the time. Since some old oil will remain in the motor when draining, be sure to verify the level on the dipstick when filling, since you will not need the full 800 ml, and you NEVER want to overfill.

Thank you, Paul. The engine is five years old, has about 50 total hours, was last serviced a year and a half ago. I'm wondering why the red oil alert light didn't come back on when I flushed the engine.

I assume the light is working OK now... coming on for several seconds at startup? If your oil was over full, then running at speed may have aerated the oil, causing those symptoms, yet not show up at test tank speeds. Likewise, if there is an intermittently bad sender, that may (or may not) give you a false indicator, depending on the conditions at the time. Have you verified/changed oil?

My oil level was too high so I pumped some out and fired up the engine: no red light on. Apparently there's a pressure problem if the oil level is either too low or too high. Mine was too high.

If the oil was over full, either it was filled too much, or the motor is making oil. If the latter, I would change it. In either event, too much oil causes aeration (bubbles), which results in low pressure. It also causes radically high crankcase pressures and can blow a seal or gasket, in addition to sending the excess out of the breather.

Regardless of hours, the oil should be changed at least annually. Use FC-W rated 10w-30 oil.

pvanv said:

If the oil was over full, either it was filled too much, or the motor is making oil. If the latter, I would change it. In either event, too much oil causes aeration (bubbles), which results in low pressure. It also causes radically high crankcase pressures and can blow a seal or gasket, in addition to sending the excess out of the breather.

Regardless of hours, the oil should be changed at least annually. Use FC-W rated 10w-30 oil.

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Will the engine start if the red light is on?

Yes, it will.

Would seaweed cause oil light to flash….20hp Merv efi 2019?

Low oil pressure, yes. Poor cooling, no.