The Mercury 888725T1 VST EFI / DFI fuel pump, also commonly searched as 888251T02 and 855427A1, is a high-pressure electric fuel pump used inside the VST, or vapor separator tank, on many Mercury and Mariner outboards.
This pump is responsible for supplying clean, pressurized fuel to the engine’s fuel rail after fuel has been delivered into the vapor separator by the lift pump or low-pressure fuel system.
On Mercury EFI, DFI, Optimax, Pro XS, XS, Sport XS, Verado, and SportJet applications, the VST pump is not just a simple fuel transfer pump. It is a key part of the engine’s high-pressure fuel delivery system.
When the pump becomes weak, contaminated, or intermittent, the engine may still crank and even idle, but it can lose fuel pressure under load, fall flat at higher RPM, surge, stall, or set fuel-related faults.
Where This Pump Is Used
The 888725T1 fuel pump appears across a wide range of Mercury outboard and jet-drive applications. Common catalog fitments include many 75 HP, 80 HP, 90 HP, 100 HP, 115 HP, 125 HP, 135 HP, 150 HP, 175 HP, 200 HP, 225 HP, 250 HP, 275 HP, and 300 HP Mercury engines.
It is commonly associated with 3-cylinder 1.5L EFI, V6 2.5L EFI, 3.0L DFI / Optimax, 3.2L 300XS, and Verado 4-cylinder and 6-cylinder outboards. Fitment examples include many 75 EFI, 80 EFI, 90 EFI, 100 EFI, 115 EFI, 115 Pro XS, 135 HP V6 2.5L, 150 HP V6 2.5L, 175 HP V6 2.5L, 150 Pro XS, 175 Pro XS, 200 HP XS 2.5L, 200 HP 3.0L, 225 HP 3.0L, 200 DTS 3.0L, 225 DTS 3.0L, 200 Pro XS 3.0L, 225 Pro XS 3.0L, 250 Pro XS 3.0L, 225 Sport XS, 250 Sport XS, 250 XS, and 300XS 3.2L models.
The same pump family is also found in many Mercury Verado EFI 4-cylinder applications, including 135 HP, 150 HP, and 175 HP Verado, as well as 6-cylinder Verado engines such as 200 HP, 225 HP, 250 HP, 275 HP, and 300 HP Verado models.
Catalog listings also show use in 200 DFI SportJet, 200 DFI M2 Jet Drive, and certain Mercury jet-drive pump and powerhead configurations.
Because Mercury used this pump across many engine families and serial ranges, always verify fitment by engine serial number, VST style, connector type, and original fuel pump part number before ordering.
What the VST Pump Does
The vapor separator tank is designed to provide the high-pressure pump with a steady supply of liquid fuel while separating vapor and helping prevent fuel starvation. Fuel first enters the VST from the boat’s fuel system, then the electric pump inside the VST supplies the pressure needed by the injectors or fuel rail.
On EFI outboards, the VST pump directly supports injector fuel pressure. On DFI and Optimax engines, the fuel system works alongside the air-injection system, so correct fuel pressure is critical to proper combustion quality.
A weak VST pump on a Mercury Optimax, Pro XS, or 300XS can create symptoms that look like injector problems, ignition issues, or air-compressor faults, which is why proper fuel-pressure testing matters before replacing parts.
Common Failure Symptoms
A failing Mercury 855427A1 / 881705T1 / 888725T1 fuel pump may cause hard starting, no start, rough idle, poor throttle response, lean hesitation, surging, stalling, or RPM loss under load. Some engines will idle normally on the hose but fail when the boat is placed under real water load. That is a classic sign that the pump can no longer maintain fuel pressure when fuel demand increases.
Other warning signs include no pump sound at key-on, blown fuel-pump fuses, intermittent operation when hot, low rail pressure, contaminated fuel inside the VST, or a pump screen restricted by debris, varnish, or ethanol-related residue.
Why VST Cleaning Matters
Replacing the pump without cleaning the VST is one of the most common mistakes in marine fuel-system repair. If the old pump failed because of varnish, water, corrosion, tank debris, or deteriorated fuel hose material, the new pump can be damaged quickly if the contamination remains inside the vapor separator.
When servicing the VST, inspect the pump screen, internal hoses, seals, float needle, VST gasket, fuel-pressure regulator, and all filters. On older Mercury EFI and Optimax outboards, ethanol fuel, long storage periods, and deteriorated fuel lines are common causes of contamination. A clean VST gives the new pump the best chance of long service life.
Buckshot Racing #77 Tech Tip
Do not diagnose a Mercury high-pressure fuel pump by sound alone. A pump can run and still fail to build proper pressure. Before condemning the 888725T1 VST fuel pump, verify battery voltage, fuel-pump relay operation, fuses, grounds, harness connections, low-pressure fuel supply, and fuel pressure under load when possible.
On Optimax, Pro XS, XS, Sport XS, Verado, EFI, and SportJet engines, accurate fuel-pressure testing is the difference between a correct repair and expensive parts swapping. If fuel pressure drops as RPM increases, the problem may be the VST pump, but it can also be a restricted filter, weak lift pump, clogged pickup, anti-siphon valve issue, tank vent problem, or contaminated VST.
Part Number Cross References
This pump is commonly listed under Mercury 888725T1, 881705T1, and 855427A1. These numbers are frequently searched by owners working on Mercury EFI, DFI, Optimax, Pro XS, XS, Sport XS, Verado, and SportJet fuel systems.
For SEO and parts lookup, the best primary number to use is 888725T1, with 881705T1 and 855427A1 included as cross-reference and superseded-style search terms.
Final Thoughts
The Mercury 888725T1 VST EFI / DFI fuel pump is a small part with a major job. Whether it is installed in a 75 HP EFI, 115 Pro XS, 150 HP V6 2.5L, 200 HP Optimax, 225 Pro XS, 250 XS, 300XS 3.2L, Verado 200, Verado 300, or 200 DFI SportJet, stable fuel pressure is essential for clean starting, strong acceleration, and reliable high-RPM operation.
At Buckshot Racing #77, we recommend treating VST fuel-pump replacement as a complete fuel-system service, not just a pump swap. Clean the tank, inspect the filters, verify wiring, test pressure, and correct the root cause of the failure.
Done properly, replacing the 888725T1 / 881705T1 / 855427A1 pump can restore dependable fuel delivery and help protect the injectors, rails, and engine from lean-running damage.