If your outboard is sputtering at high speeds or dying right when you give it some throttle, a yamaha 115 4 stroke fuel pump replacement is likely at the top of your weekend to-do list. It's one of those jobs that sounds a lot more intimidating than it actually is, but if you've spent any time on the water, you know that fuel issues are basically an initiation ritual for boat owners.
The Yamaha F115 is a workhorse—probably one of the most reliable engines ever built—but it's not invincible. Between the ethanol in modern pump gas and the way these engines sit during the off-season, the fuel system takes a beating. Usually, the first sign of trouble is the "bog." You go to get on plane, the engine roars for a second, and then it just sags. It feels like it's starving for a drink, and that's because it probably is.
Is it actually the fuel pump?
Before you go ripping things apart, you've got to be sure it's the pump. I've seen plenty of people do a full yamaha 115 4 stroke fuel pump replacement only to find out they had a cracked fuel line or a clogged $10 inline filter.
A quick way to check if the pump is the culprit is to have a buddy squeeze the primer bulb while you're running the engine and it starts to act up. If the engine smooths out when they manually pump the bulb, you've got a delivery problem. That bulb is doing the job the pump is failing to do. Also, keep an eye out for "making oil." If your oil level is rising and smells like gasoline, that's a classic sign that the diaphragm in the low-pressure pump has a hole in it, dumping raw gas directly into the crankcase. That's not just a performance issue; that's an "I'm going to ruin my engine" issue.
Identifying which pump you need
The Yamaha F115 actually has two different stages of fuel delivery. You've got the low-pressure pump, which pulls gas from the tank and sends it to the VST (Vapor Separator Tank), and then you've got the high-pressure pump sitting inside that tank that feeds the injectors.
Most of the time, when people talk about a yamaha 115 4 stroke fuel pump replacement, they're talking about the low-pressure one on the side of the engine block. It's cheaper, easier to get to, and more prone to mechanical failure. However, if your boat has been sitting for three years with old gas, that high-pressure pump inside the VST might be gummed up with green varnish. Today, we're mostly focusing on the low-pressure swap, but the logic is pretty similar for both.
Gathering your gear
You don't need a massive rolling tool chest for this. A basic socket set (mostly 10mm, because Yamaha loves 10mms), some needle-nose pliers for those annoying hose clamps, and a few rags are usually enough. You'll definitely want the rags because even with the engine off, there's going to be some residual pressure and a bit of "oops" gas that leaks out when you pull the lines.
The step-by-step swap
First things first, disconnect the battery. It's a boat, there's gas, and you're messing with metal tools—don't be the guy who starts a fire because of a stray spark.
Accessing the pump
Pop the cowling off and look at the port side (the left side if you're standing behind the boat looking forward). You'll see the fuel lines snaking around. The low-pressure pump is typically bolted right to the block. It's a small, square-ish component with two fuel lines attached to it.
Removing the old unit
Take your pliers and slide the clamps back on the "in" and "out" lines. Give the hoses a little twist to break the seal before pulling them off. If they're stuck, don't go crazy with a screwdriver or you'll gouge the plastic or rubber. Once the hoses are off, tuck them upward so gas doesn't keep siphoning out.
Now, just unbolt the two mounting bolts. The pump should come right off. Take a look at the back of it—there's a little plunger that's driven by the camshaft. Make sure the gasket or O-ring didn't stay stuck to the engine block. Clean that mounting surface up so the new one sits flush.
Installing the new pump
Slide the new gasket onto the new pump and line up that plunger. Sometimes you have to wiggle it a bit to get it to seat properly against the cam. Bolt it down, but don't go "gorilla tight" on it. It's an aluminum block, and stripping those threads will turn a 20-minute job into a weekend nightmare.
Reattach your fuel lines. This is a great time to check if those lines are getting crunchy or soft. If they look bad, just replace them. A few bucks in hose is cheaper than a tow back to the ramp. Slide the clamps back into place and make sure they're snug.
Dealing with the VST (The "Other" Pump)
If you do the low-pressure swap and the engine still runs like garbage, you're looking at the VST. This is a bit more involved. You have to remove the intake manifold on many F115 models to get to the tank.
Inside that tank is a tiny filter (the VST screen) and the high-pressure electric pump. If that screen is clogged with debris, the pump can't do its job. If you're already doing a yamaha 115 4 stroke fuel pump replacement on the low side, it's not a bad idea to at least drain the VST using the drain screw at the bottom. If the gas that comes out looks like orange juice or has bits of black junk in it, you know where your problem lies.
Priming and testing
Once everything is buttoned up, don't just turn the key and crank it for a minute straight. You'll burn out your starter. Use the primer bulb! Squeeze it until it's rock hard. This fills the lines and the VST so the engine has something to eat the moment it fires.
Hook up the muffs and a garden hose, or back the trailer into the water. Fire it up and let it idle. Check for leaks around the new pump immediately. If everything looks dry, let it get up to temperature. The real test is under load, though. A fuel pump can often provide enough gas for a smooth idle but fail miserably when you're trying to push 4,000 pounds of boat against the tide.
Why do these pumps fail anyway?
It's almost always the fuel. Ethanol is the enemy of boat engines. It attracts water, and water leads to corrosion inside the pump. Over time, the internal diaphragms lose their flexibility and eventually tear.
To keep from having to do another yamaha 115 4 stroke fuel pump replacement next season, try to use ethanol-free gas if it's available in your area. If it isn't, use a high-quality fuel stabilizer every single time you fill up. It's a bit of an extra expense, but it's way cheaper than buying new pumps every two years.
Wrapping it up
Honestly, doing this yourself is one of the most satisfying "win" moments you can have as a boat owner. It saves you probably $200-$400 in shop labor, and it gives you the confidence to know how your engine actually works. The Yamaha 115 is a legendary motor for a reason—it's simple enough that a regular person with a few tools can keep it running for decades.
Just remember to take your time, keep things clean, and don't force anything. If you're organized, you'll be back on the water and hammering the throttle in no time, with that annoying bogging issue finally a thing of the past. Happy boating!