The Mercury 825815 EFI Air Plenum Gasket is used on compatible Mercury Marine, Mariner, and Mercury Racing 3.0 Liter V6 EFI two-stroke outboards to seal the air plenum to the intake assembly.
On these engines, the air plenum is part of the EFI induction system and must remain properly sealed so the engine receives a controlled, consistent air supply. A damaged or leaking plenum gasket can introduce unmetered air, disturb airflow through the intake system, and contribute to poor idle quality, hesitation, lean running, or inconsistent high-RPM performance. The Mercury OEM part number is 825815.
Fits Mercury & Mariner 3.0L EFI Applications
This gasket is used on selected Mercury and Mariner 3.0L EFI two-stroke engines, including performance and production applications such as 200 HP, 225 HP, 250 HP, 250XB, 300 Pro Max, Mariner 300 Super Magnum / SuperMag, and 300X.
These engines share related 3.0L EFI intake architecture, but exact fitment should still be verified by engine serial number, intake configuration, and original gasket pattern before installation.
The gasket is especially relevant to classic high-performance Mercury and Mariner EFI platforms where the plenum, throttle body, and intake system must remain tightly sealed for predictable fuel and air delivery.
Common High-Performance Applications
The Mercury 825815 plenum gasket is particularly useful for builders servicing engines such as the 250XB, 300 Pro Max, Mariner 300 Super Magnum and 300X.
These engines are commonly found on high-performance bass boats, offshore boats, tunnel hulls, lake-race boats, and other performance marine applications where consistent airflow and correct EFI calibration are especially important.
A small intake leak that may be barely noticeable at idle can become much more significant at higher airflow and RPM.
Why the Air Plenum Gasket Matters
The EFI plenum sits ahead of the reed and crankcase induction system and directs incoming air through the throttle body and intake assembly.
A proper seal at the plenum prevents unwanted air leaks around the mating surfaces. If the gasket is torn, compressed unevenly, heat-damaged, or reused after disassembly, the engine may develop an intake leak that changes the way air enters the system.
Possible symptoms can include rough idle, hanging idle, hesitation, inconsistent throttle response, lean running, or loss of performance under load.
Because these symptoms can also be caused by injectors, fuel pressure, TPS settings, reeds, wiring, or ECU issues, the gasket should be inspected as part of the complete EFI induction system rather than treated as the only possible cause.
Replace the Gasket During EFI Intake Service
Whenever the EFI air plenum is removed, the gasket should be inspected closely.
If the gasket shows signs of compression set, cracking, tearing, delamination, or surface damage, replacement is the correct approach.
For engine rebuilds, injector service, reed service, throttle-body work, or plenum removal, installing a fresh gasket is a low-cost way to restore the original sealing interface.
Buckshot Racing #77 Installation Tech Tip
Before installing the new Mercury 825815 gasket, clean both mating surfaces thoroughly and remove all old gasket material without gouging the aluminum.
Make sure the gasket aligns correctly with all intake openings, bolt holes, and sealing surfaces. Tighten the plenum fasteners evenly and to the correct Mercury specification so the gasket compresses uniformly.
After reassembly, inspect the surrounding EFI intake system for cracked hoses, loose fittings, damaged throttle-body seals, and other possible sources of unmetered air.
Final Fitment Note
The Mercury 825815 EFI Air Plenum Gasket is designed for compatible 3.0L Mercury, Mariner and Mercury Racing EFI engines, including selected 200, 225, 250, 250XB, 300 Pro Max, Mariner 300 Super Magnum / SuperMag and 300X applications.
Includes one (1) EFI Gasket; please order three (3) if for a complete motor.
Contact Mike Hill at +1-714-697-1716 for tech support.