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2010 Tohatsu 8hp w/Low Oil Pressure Light

reddingnative

New member
I have a 2010 Tohatsu 8HP outboard. I have been getting an intermittent low oil pressure light followed by a slight drop on RPM. The light goes out after 5 seconds or so and the motor runs fine again. I followed the troubelshooting steps in the service manual and currently have a oil pressure test gauge attached to the motor. I went out on the river and oil pressure is about 30 in gear at 960 rpm. Pressure is above 50 at around 1800 rpm. I have the motor mounted on a drift boat, so I never max out the rpm's. In keeping an eye on the oil pressure it is constant at whatever rpm I am running at. That was two days ago. Went out again yesterday and monitored oil pressure, no change, but the low oil pressure light came on briefly and went off once. Went out again today and low oil pressure light came on and went off while motoring. Oil pressure remained constant.

There is no reference in the manual for next step to determine cause of low oil pressure light coming on when oil pressure is good. Any help or advice would be appreciated.
 
Mike,

A 2010 8 hp should be model MFS8A3. The precise model is a lot more important than year of manufacture.

The oil pressure sender switch should keep the LED OFF at all pressures above about 4 PSI. Below that threshold, the switch closes, grounding the sender wire, triggering the LED (and oil light in the tachometer if so equipped) to warn you of low oil pressure. RPM is then also limited, to reduce the chances of hurting bearings at low oil pressure. The sender is quite sensitive, and can react very rapidly to blips in oil pressure... possibly faster than a mechanical gauge on a flexible hose, which can have a "cushioned" reading due to the elasticity of the hose.

Maximum pressure is regulated by a relief valve (Tohatsu Japan calls it the Plunger), and typically will not exceed about 50 PSI warm, though it could go a little higher cold. 20-30 PSI at about 1000 RPM is fine; a max of 50 when warm is also good.

If the oil is over full, the motor can aerate the oil, due to splashing of the connecting rods, crankshaft, etc., putting bubbles into the oil; that air is compressible (whereas oil is not), so if that is the case, you can experience fluctuating oil pressures, especially at higher RPM's, which could trigger odd/intermittent LED flickering. If the over-filling is extreme, the oil foams up, and you can also get extremely high crankcase pressures, which can cause all sorts of difficult-to-diagnose problems. Good, NMMA-certified FC-W oils are resistant to foaming.

Assuming that you have a tee fitting that is letting you use the oil pressure sender switch as well as the mechanical gauge, and the readings are good (your numbers are), you likely have a problem with the sender switch. It's possible to get a bit of debris in the inlet to the switch, but that's rare in a well-maintained (regular oil changes) motor. The other possibility is a flaky CD module, but that's very rare.

If I had the motor in our shop, I would swap out the pressure sender (and the boot that goes with it -- the boots sometimes dry-rot), and re-test. If you are running the proper oil, and it is at the right level (about 1/2 to 3/4 on the dipstick), odds are you have a flaky sender. The parts I would use are:


3H6076110MSWITCH, OIL PRESSURE$16.17

3H6076120MGROMMET, OIL PRESSURE SWITCH$1.93
 
You are replying to an old post about a different motor.
You should start a new thread.

The plunger on an MFS15 is on the underside of the block.
 
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