Tohatsu 18HP 4Stroke Fuel Starvation

[wlbacco]

I recently bought a boat that has a Tohatsu 18hp, 4 stroke motor on it. Model MFS18B2EPL. I am having a issue with it periodically running rough. I have traced this down to the fact that it appears to not be getting enough fuel.

Prior to starting the engine, I pump the ball and the fuel filter is 1/2 to 3/4 full. As the engine runs the fuel filter slowly empties down to almost nothing. Periodically a burst of gas comes into it. It runs fine when idling, since it is not using a lot of gas. However, if I increase the rpm, it will eventually be starved of fuel and start to sputter. Squeezing the ball at that point fills the filter and the engine runs fine again. I have examined and replaced the fuel line from the gas tank to the motor and all seems clear and sound. I have also replaced the fuel filter and verified that the tank is vented properly.

I am wondering if the fuel pump might be bad. If I manually pump it, it seems to be putting out sufficient gas, however it seems to be unable to draw enough gas from the tank. Another possibility is a check valve issue. This is not the original motor on this Bass Tracker. There is a check valve in the fitting just out side of the gas tank. When I inspected the line the check valve was free and seemed to be working properly. However I am wondering if this motor's fuel pump just doesn't have enough draw to open the check valve.

Any help you can provide is appreciated.

Thanks, Bill

 

[pvanv]

Re: Tohatsu 18HP 4Stroke Fuel Starvation

Those aluminum anti-return (anti-siphon) valves can cause a huge vacuum in the fuel line. I have even seen the pump suck the priming ball flat on more than one occasion. Since your priming bulb already has check valves in it, the aluminum anti-siphon is probably redundant, so I would delete it.

 

[wlbacco]

Re: Tohatsu 18HP 4Stroke Fuel Starvation

Paul,

Thanks for the quick response. I removed the check valve and replaced it with a regular hose fitting. I am still having the same problem. I suspect that the issue is the fuel pump. Is there any way to confirm that?

Thanks, Bill

 

[pvanv]

Re: Tohatsu 18HP 4Stroke Fuel Starvation

Bill,

Most common in a case like yours is a leaky o-ring at a hose connector, or a connector thread that is leaking at the tank, allowing air to enter the fuel line under suction. This is especially true if the tank is quite low (say 3 feet) below the motor. Does this happen if the tank is above the motor height? You can test the fuel pump vacuum with a gauge. We use the one that Tohatsu sells. Any dealer (including me) can get one to you:
FUEL PUMP VACUUM TESTER
Description:
Vacuum and Fuel Pump Tester
Check for leaky valve timing, fuel pump vacuum and gas line leakage
Large easy to read gauge with 3 1/2” diameter face
English and metric scales
Vacuum: 0-30 inch Hg. and 0-600 mm Hg.
Fuel pump pressure: 0-10 Psi and 0-7 Kg/Cm2
Heavy duty tubing and vacuum fitting included
Part # 99998V701M
List Price (MSRP): $29.58

 

[wlbacco]

Re: Tohatsu 18HP 4Stroke Fuel Starvation

After I found no issue with the the fuel lines and connections, I ended up replacing the fuel pump and that corrected the problem. Thanks Paul for your help on this.

Bill