EFI Outboard Vapor Separator Tank (VST) and Injectors - Symptoms, Filters and When It's a Dealer Job

Write down when it happens (how many minutes after starting, what RPM, how much load), whether the primer bulb (if you have one) goes flat, and any warning messages or beeps. A dealer can find a VST or pump fault much faster with this information, and it helps you separate a fuel problem from a propeller or ignition problem.

Take a sample from the water-separating filter or the tank into a clean glass jar. Water at the bottom, cloudiness, debris, a dark color or a sour varnish smell mean the fuel is the problem. Mercury's FAQ explains that no additive can remix phase-separated fuel: the water and bad fuel must be removed and disposed of properly, and the tank refilled with fresh fuel. If you find water or phase separation, assume the VST may have taken some in too, and tell your dealer.

Work through the tank vent, fuel line, quick-connect fittings, primer bulb and anti-siphon valve as described in the fuel-pump-test guide. A VST can only be as full as the lift pump can keep it, and the lift pump can only move what the boat's fuel system supplies.

This is the most important filter you can change yourself, because it protects everything downstream. Yamaha recommends replacing its 10-micron fuel/water separating filter every 50 hours or after a bad tank of gas. It suggests a thin film of clean oil on the gasket and pre-filling the new filter about three-quarters with fresh, stabilized fuel so the system primes more easily. Don't drain and reinstall an old filter: Yamaha warns that trapped debris and water can get onto the clean side. See the fuel-water-separator guide for the full procedure.

Yamaha describes a primary on-engine filter, usually a see-through cup on the front, side or back of the powerhead with a small water trap. Look at it with a flashlight:

• Water or debris in the cup: replace the external filter (step 4) and, if your owner's manual lists it as an owner task, the on-engine element. On Yamaha outboards there may be a red ring inside; Yamaha says never to remove it, and if it is floating, there is water in the trap.
• Clean: leave it, and move on.

Some models also have in-line filters between the lift pump and the VST, or between the VST and the fuel rail. Your manual shows which ones exist on your engine and which are dealer items.

After any filter change, prime the fuel system the way your owner's manual describes (squeezing the bulb until firm, or turning the key on and waiting while the electric pump runs). Check every fitting you touched for leaks before starting. Then run the engine with cooling water: in the water, in a test tank or on flush muffs (the built-in flush port is for engine-off flushing on most outboards, unless your manual allows running on it). If the symptom only shows up under load, a careful test run on the water is the real test.

Yamaha describes a filter attached to the electric fuel pump inside the VST that cleans the fuel one last time before it is pressurized for the rail, and recommends having an authorized dealer replace it every 300 hours, as long as the external and on-engine filters have been maintained. Treated fuel helps it last. Other brands have similar parts with their own intervals; check your maintenance schedule. Because the VST is part of the pressurized system and has a float, gaskets and sometimes a pressure regulator inside, opening it is a job for a trained technician.

If the symptoms continue after steps 1 to 6, a dealer will typically:

• Connect a diagnostic tool to read fault codes and live data.
• Measure fuel pressure at the rail against the factory spec at idle and under load.
• Inspect the VST: float and needle valve, pump inlet filter or screen, high-pressure pump, regulator and vent.
• Clean or replace the VST filter, pump or float parts as needed.
• Check the injectors. Yamaha notes that if debris reaches the injector screens, the injectors must be professionally removed and cleaned or replaced.

Ask for the fault codes and pressure readings in writing so you have them for the boat's records.

Boating Magazine, quoting a marine injector specialist, suggests professional injector cleaning and flow-testing at roughly 300 engine hours or every three to four years, and notes that home cleaning methods can't confirm the injectors are back to spec. Signs include hard starting, a rough idle and surging on the way to full throttle. Injector service is usually done together with VST service, since the same contamination affects both.

• Treat every tank with a marine fuel stabilizer as your manual recommends, and buy fuel where lots of boats and cars fill up.
• Run treated fuel through before storage, so the VST and rail hold stabilized fuel instead of fresh gas that will go stale. Follow your owner's manual's storage procedure for your engine; don't just leave old fuel in the system for months. Yamaha's layup advice adds three habits: change the 10-micron fuel filter before storage (water left in the canister can freeze), store with the tank about 7/8 full to limit condensation, and never plug or cap the tank vent.
• Keep water out: check the tank vent, fill cap O-ring and the water-separating filter bowl regularly.
• Use the fuel your manual specifies. Mercury states fuels with more than 10 percent ethanol are not acceptable for its engines and can void the warranty.
• Don't add extra fine filters to the system. Mercury warns that another filter is another restriction that can starve the engine.

Note the date and engine hours of every filter change, fuel problem and dealer visit. A pre-purchase engine surveyor will often take a fuel sample straight from the injector rail, as Power & Motoryacht describes, and good records make a used outboard easier to sell.