Yamaha Outboard VST Problems & How To Fix Them!

If you have a Yamaha, or any outboard for that matter. You might be wondering what kind of VST problems you could come across.

Yamaha Outboard VST Problems? The most common VST problems include an empty VST, a failed High Pressure Pump, a dirty VST Filter, dirty High Pressure Pump filter, a faulty float and needle, or a bad pressure regulator.

Each one of these components inside the VST are super important, here is everything that you need to know about each one of them!

What Is The VST On A Yamaha Outboard?

On all fuel injected and direct injected Yamaha Outboards. There is a component called a Vapor Separator Tank or VST for short.

And really on pretty much any brand outboard that has fuel injection. Though some will call it by other names, like a Mercury’s Fuel Supply Module, or FSM for short.

(But you can learn all about the inner workings and components of these items in this article here!)

Some of them will have a variance of the parts found in that article. As we are about to discuss now the items that are found in Yamaha’s VST.

But they will hold pretty true to any manufacturer. The purpose of the VST is to do a couple of things. First, it separates vapors from the fuel, hints VST.

Then it produces and maintains the fuel pressure that is necessary for the engine to run. A fuel injected engine requires specific fuel pressure at the fuel injectors for it to operate properly.

And this is the job of the VST. Where the high pressure fuel pump, the pressure regulator, a float and needle valve assembly, and some filters are found.

The worst enemy of the VST, is going to be dirty fuel! As well as unproperly used ethanol fuel. (You can learn all about how to properly run ethanol fuel in your boat in this article here.)

Being that dirt is number one for a VST problem, before going into each one of these components problems. Here is a video of cleaning a VST from our Youtube channel!

As well as a video on ethanol fuel.

Yamaha Outboard VST Fuel Pump Problems

Now knowing and having a better understanding about the necessity of having clean fuel going to your engine. Lets talk about your Yamaha outboard VST fuel pump problems.

Being most commonly one of two things. The first one, is where the filter that is on the bottom of the fuel pump is just simply clogged with a bunch of junk.

And isn’t allowing the fuel to get into the pump, for it to pressurize it and send it to the fuel rail.

Then the second reason is quite simply. That enough dirt and junk has ran through the pump, that it is simply failed. Burned up. Or just given up and is worn out.

These two reasons leave you with usually two options. One is to clean off the filter and put everything back together. Which just requires the time to clean it.

Leaving the other option to just replace the fuel pump. Which can range from $100-$750!

Depending on the engine and model. So the cost of the fuel pump problem can get pretty expensive just for the pump itself.

There is also one other issue that is a little less common, but it does happen. Whenever we have a problem with the wires going to the pump.

This can be caused by a blown fuse, but that is usually whenever the pump locks up that it will blow the fuse. Unless you have another problem going on in the wiring.

As well as a broken wire, being either the power or the ground wire going to the pump. And this does happen over time, so before getting to wild in searching for VST problems.

Make sure you have proper power going to the pump. (Here is another article about the signs and how to tell if you have a bad fuel pump.)

Yamaha Outboard VST Pressure Regulator Problems

A common VST problem that can be over looked is an issue with the pressure regulator.

The pressure regulator is a super important component in the fuel system. It is the component that makes sure that the fuel pressure stays at a specific PSI.

And when we are talking about our number one problem being dirty fuel.

Getting some dirt or debris in the pressure regulator will shut you down really quickly.

Pressure regulators are pretty durable though. Unless they get super dirty and damage the internals of them. You can usually just clean them out and put them back into the system.

Other than cleaning them, the other option is to just simply replace the regulator.

VST Float Problems

Whenever we are talking about a Yamaha Outboard VST problem. We can’t overlook issues that can be found with the float and needle valve assemblies.

Inside the VST there is float, just like in a carb. This float moves a needle valve up into an assembly, which stops fuel from flowing into the VST whenever it is full.

Dirt and debris can get clogged in here where the float won’t stop the fuel flow. From coming into the VST and overfilling it.

Most VST’s have a vent system on them. So if there is an problem with the float or needle. The VST will fill up and then overflow out of the vent system.

Depending on the model of engine you have, will determine where this fuel is routed. Some models send it out the bottom of the belly pan.

Others will send the fuel out from underneath the cowling latch in the back of the engine cowling.

So if you see a solid stream of fluid coming out of your engine. Then your problem is most likely with this float and needle valve assembly.

In order to fix this, you need to disassemble the VST. Inspect the float and needle assembly. If it is just dirty, clean it and put it all back together.

Or you can simply remove and replace the needle valve assembly. It’s usually not an issue with the float.

Most commonly its that the needle has disformed and is not making a good seal. But when talking about fuel and VST problems, anything is possible.

Now you know it’s something to look for though!

Yamaha Outboard VST Filter Cleaning & Replacing

The last problem that we can discuss is most likely the most simple one. Being an issue with the fuel filter.

A VST Filter is a general term that people like to throw around. So it’s important to understand what exactly we are talking about.

On a Yamaha Outboard VST, there is a “U” shaped filter on the top of some VST’s. This is a simple part, you really can’t clean it, it’s more of a remove and replace.

Really, you don’t want to necessarily “clean” any fuel filter. Unless you are in a pinch, these are items that you want to remove and relace.

Then on other models, like an HPDI. That have a “canister” fuel filter on the high pressure side of the VST that goes to the mechanical high pressure pump on the top of the engine.

These again are a remove and replace type of item, but both of these filters can commonly be called a “VST filter.”

There are also some models that have a “strainer” that is inside of the VST. These again can be cleaned, but are commonly called a “VST filter.”

Spraying these off with contact cleaner, they can be reused. But they can also be simply replaced as well.

Then there is also a strainer on the bottom of the high pressure fuel pump that’s inside the VST. These can be sprayed off with contact cleaner as well.

These are commonly thought to be a “VST filter.” But they are really part of the fuel pump and can’t be replaced. Outside of replacing the fuel pump.

All of these filters and strainers can be replaced. The Yamaha VST filter cost therefore can range from $20 to about $100!

Based on which one of these “VST filters” you are trying to replace.

Check Us Out!

We hope that this has been insightful and allowed you to understand a little bit more about Yamaha Outboard VST Problems!

And all the components that make up the VST that delivers the fuel to the engine.

If you have more questions or want to learn more about your boat and its different systems. You should consider joining our Boating Academy where we have created HUNDREDS of video courses teaching you basically everything about your boat!

There are also even more helpful videos on our Youtube channel where we create even more boating videos!

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Here are some other super helpful articles that you might find interesting!

Aaron Hilligoss

Aaron has been working in the Marine Industry for over a decade and holds certifications for Yamaha and Mercury Marine. It is not uncommon for him to own and be working on at least three different boats at any given point in time!

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