I've been troubleshooting this issue for 3 months now (but our outboard is only lightly used for pushing a dinghy - so the issue started at hour 10 on the motor and it has 20 hours on it now). It's a Tohatsu MFS6 which is mostly new but it was a 2016 floor model that we bought from a going-out-of-business sale (yeah, dumb move, but what's done is done).
Symptom is the oil pressure light comes on at idle throttle (only), after running the engine at load (>=1/2 throttle) for 5-15 minutes (light doesn't come on at idle on a cold engine, after starting). Everything else about the engine looks and sounds great, except that 5% of the time (1 in 20 runs) we get a weird power loss issue at 1/2 throttle - the engine starts jerking and revving like we're losing power or hitting an rpm limiter. That's happened 2 or 3 times and throttling down to < 1/4 throttle makes it go away and gets us home.
First thing I did was check the oil - it was slightly overfilled from when I changed the oil after break-in (I thought oil should be measured with the cap resting on the threads - Tohatsu clarified cap should be screwed in). Reduced oil to sit right at upper mark. That didn't resolve anything.
An outboard mechanic told me it sounds like an overheating issue (would cause the oil to thin out, triggering the pressure sensor and maybe the power loss issue). So I pulled out the thermostat like he suggested and ran outboard at load without thermostat in - oil pressure light never came on and no power loss. Tested thermostat in a hot water bath and it opened at about the right temps, going by the Tohatsu service manual I have on hand. The mechanic said the next thing he would check would be the impeller. So I pulled the lower unit, and the impeller looked perfect. We always get good cooling water telltale flow. I could run it without the thermostat in, but I understand that runs the engine too cool which isn't good in the long-run.
I don't want to continue replacing expensive parts arbitrarily. Already replaced the cheap ones (spark plug, fuel filter) and cleaned the carb, and all the parts have looked pristine and new. I could order a new thermostat, but since it tested fine in the hot water bath I'm not sure that's it? Still hoping to fix this myself - outboard mechanics here are under high demand and don't seem eager to take a small, frustrating job like this.
Does this sound like an overheat issue, fuel issue, or something else? Any clues on how I could narrow it down?
Symptom is the oil pressure light comes on at idle throttle (only), after running the engine at load (>=1/2 throttle) for 5-15 minutes (light doesn't come on at idle on a cold engine, after starting). Everything else about the engine looks and sounds great, except that 5% of the time (1 in 20 runs) we get a weird power loss issue at 1/2 throttle - the engine starts jerking and revving like we're losing power or hitting an rpm limiter. That's happened 2 or 3 times and throttling down to < 1/4 throttle makes it go away and gets us home.
First thing I did was check the oil - it was slightly overfilled from when I changed the oil after break-in (I thought oil should be measured with the cap resting on the threads - Tohatsu clarified cap should be screwed in). Reduced oil to sit right at upper mark. That didn't resolve anything.
An outboard mechanic told me it sounds like an overheating issue (would cause the oil to thin out, triggering the pressure sensor and maybe the power loss issue). So I pulled out the thermostat like he suggested and ran outboard at load without thermostat in - oil pressure light never came on and no power loss. Tested thermostat in a hot water bath and it opened at about the right temps, going by the Tohatsu service manual I have on hand. The mechanic said the next thing he would check would be the impeller. So I pulled the lower unit, and the impeller looked perfect. We always get good cooling water telltale flow. I could run it without the thermostat in, but I understand that runs the engine too cool which isn't good in the long-run.
I don't want to continue replacing expensive parts arbitrarily. Already replaced the cheap ones (spark plug, fuel filter) and cleaned the carb, and all the parts have looked pristine and new. I could order a new thermostat, but since it tested fine in the hot water bath I'm not sure that's it? Still hoping to fix this myself - outboard mechanics here are under high demand and don't seem eager to take a small, frustrating job like this.
Does this sound like an overheat issue, fuel issue, or something else? Any clues on how I could narrow it down?
