How to Detect A Faulty Yamaha VST Filter?

Your Yamaha outboard engine has a vapor separator tank next to the electric fuel pump.

The filter in VST treats the fuel before sending it to the next component in the system. Due to various reasons, it can get clogged and cause some issues.

In this article, I am going to talk about Yamaha VST filter symptoms and signs along with possible solutions.

What Signs Should You Look for To Catch Faulty Yamaha VST Filter?

Due to the complex mechanism of the fuel line, you can’t pinpoint a component for a specific problem.

However, let me tell you when it is time to suspect that the VST filter is no working properly.

If you are having trouble starting the engine or it starts bogging down, you will need to inspect the health of the filter.

Now, that is one obvious sign, and let me tell you some more.

What Are the Symptoms of a Bad Yamaha VST Filter?

Here I am about to tell you some real-life experiences. It will help you understand when you should check, clean, or replace the VST filter of your Yamaha outboard engine.

RPM Maxes Out at 4500 or Less

A clogged filter sends less fuel to the cylinders leading to low power output.

Lots of owners have experienced this problem of having 4500 or less RPM even though they run the engine at its full speed.

One or More Cylinder Not Working

From VST, gas goes into the fuel injector and to the cylinders after that. When the filter gets clogged and not performing to its best, the amount of gas released from VST gets decreased.

As a result, the fuel injector can’t provide enough fuel to all of the cylinders. And, one or more pistons don’t get the necessary gas to ignite and run the engine.

Not Starting at All

The above two situations happen when the VST filter is partially dirty. When it is completely blocked, the engine may not start at all.

How Should You React If You Detect A Bad Yamaha VST Filter?

You know the nature of a faulty VST filter of the Yamaha outboard engine. And it is time to take some actions to solve it.

Check the Filters

  • First of all, buy a fuel pressure gauge which should cost you no more than 50 bucks.
  • Connect the gauge to the Schrader valve (tire stem). You will find it on the fuel rail.
  • The next step is to take three different measurements; key on, idle, and 1500 RPM. Also, make sure to hook up the motor to water muffs before you begin.
  • If you see a reading of 40 PSI, you will know that neither fuel pumps nor VST filters are clogged. Otherwise, follow the below procedures one by one and troubleshoot.

Remove and Clean

It takes 10 Philips head screws, 3 mounting bolts, and 2 bolts on the bottom to separate the lower half of the vapor separator tank.

You will find the filter on the bottom of the pump inside the VST.

In case the component is almost clean, using only a carb cleaner will be enough.


Some boat owners may have their filters very dirty. In such situations, you will have to replace the whole thing.

Tip: Make sure that you change the 10-micron water separator annually. It would increase the longevity of the filter.

Treat the Fuel

As today’s gasoline can break down within weeks, you should treat every tank of fuel. Initially, it may seem like costly maintenance work. But you will save a lot of money in the long run.

  1. According to Yamaha, add high-quality marine-specific non-alcohol-based formula. It works effectively in moisture-rich environments that are common to boats.
  2. Furthermore, use cleansing additives Ring Free PLUS which can prevent copper, brass, and silver solder in your fuel system from corrosion.
  3. Also, make sure that the gas you are buying is fresh. So, avoid the shops where the sale is slow.

Tip: Both ethanol and non-ethanol fuels have merits and demerits. The best action against the gas problem is to keep the boat running. You should use the engine as many times as possible. Also, check and clean the filters and fuel injectors regularly.

Are Yamaha VST Filters Expensive to Replace?

Yes, the VST filter on your Yamaha outboard engine is very expensive to replace compared to other filters on the fuel line.

The filter itself can cost you around 50 bucks or more.

Furthermore, you have to add the servicing charge to the cost which varies from state to state.

However, being able to do the replacement by yourself can save a lot of money. There are lots of online videos showing how to do it like a professional.

I am saying this because hiring a certified Yamaha mechanic can cost you more than 500 hard-earned dollars.

How Often Should You Replace VST Filter On Yamaha Outboard?

According to the official user guideline, it is ideal to check or inspect the VST filter every 300 hours of operation.

Also, one should not wait in case the filter goes bad, and replace it immediately.

At the same time, service and maintain the 10-micron external and initial on-engine filters.

Final Thoughts

I have described the signs and symptoms of bad Yamaha outboard VST filters.

You should take action by examining the state of the filter to maintain the life and performance of the boat engine.

As it is expensive to replace, the manufacturer recommends its users continuously service and maintain the first two filters in the engine’s fuel line. Also, treat the fuel in the tank to increase the life of the VST filter.

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