Mercury 115 FourStroke
115 HP · Raw-water cooled, with a lower-unit water pump
The Mercury 115 FourStroke is a 115 hp petrol outboard built on Mercury's 2.1-litre four-stroke powerhead, and one of the most widely fitted engines on small centre-consoles, pontoons and aluminium fishing boats. Its maintenance runs on a 100-hour or annual cycle, with the water pump impeller on a separate 300-hour clock and a valve-clearance check at 400 hours — an item sterndrives and marine diesels in this library do not schedule for owners. Below are its published specifications, Mercury's inspection and maintenance schedule, and genuine service-part numbers.
Specifications
| Engine type | Four-stroke petrol outboard |
|---|---|
| Displacement | 2.1 L |
| Rated power | 85 kW (115 hp) |
| Full throttle rpm range | 5000–6000 rpm |
| Cold warm-up speed | 700–800 rpm |
| Engine oil | Mercury 10W-30 four-stroke marine engine oil |
| Gearcase lubricant | Mercury High Performance Gear Lubricant |
Maintenance schedule
Mercury's recommended service intervals for this engine. Most items are time-or-hours, whichever comes first — boats that sit still hit the calendar interval long before the hours.
Before each use
- Check the engine oil level
- Check that the lanyard stop switch stops the engine
- Visually inspect the fuel system for deterioration or leaks
- Check the outboard for tightness on the transom
- Check the steering system for binding or loose components
- Visually check the steering link rod fasteners for correct tightness
- Check the propeller blades for damage
After each use
- Flush out the cooling system if operating in salt or polluted water
- Wash off all salt deposits and flush the exhaust outlet of the propeller and gearcase with fresh water if operating in salt water
Every 100 hours of use or once yearly, whichever comes first
- Change the engine oil and replace the oil filter
- Drain and replace the gearcase lubricant
- Replace the spark plugs at the first 100 hours or first year; after that, inspect them every 100 hours or once yearly
- Check the engine fuel filter for contaminants
- Check the corrosion control anodes
- Lubricate all lubrication points — more frequently in salt water
- Lubricate the splines on the drive shaft
- Inspect the thermostat visually for corrosion and a broken spring
- Check the engine timing setup
- Check the power trim fluid
- Inspect the battery
- Check the control cable adjustments
- Inspect the timing belt
- Check the tightness of bolts, nuts and other fasteners
Every 300 hours of use or three years, whichever comes first
- Replace the water pump impeller — more often if overheating occurs or reduced water pressure is noted
Every 400 hours of use
- Check and adjust the valve clearance, if necessary
Before periods of storage
- Follow the storage procedure in the manual for your engine
Genuine service parts
Mercury part numbers for the routine service items on this engine. Several vary by build date or cooling type — check the note before ordering. Genuine parts are stocked by the Mercury dealer network; where a cross-reference is shown, an aftermarket equivalent is also available.
| Engine oil and filter change kit | 8M0107510 Mercury's own oil change kit for 2.1 L 75/90/115 hp EFI FourStroke outboards, serial number 2B094996 and up — five 1-litre bottles of Mercury 10W-30 four-stroke marine engine oil, an oil filter, an oil drain hose and instructions. NOTE the grade: these outboards take 10W-30, not the 25W-40 used by Mercury's petrol sterndrives. Check your serial number before ordering. |
|---|---|
| 100-hour maintenance kit | 8M0097854 Mercury's 100-hour service kit for 2.1 L 75-115 hp FourStroke outboards, serial number 2B094996 and higher — includes the oil filter, fuel filter and gear case drain seal. This is the cheapest way to get the right filters and the single-use drain seal together. Engines below that serial break take a different kit. |
| Water pump repair kit (impeller) | 8M0100526 Mercury's fitment list for this 15-piece kit explicitly includes 75-115 hp four-stroke outboards. It contains the impeller plus the gaskets, o-rings, plates, seals and key needed to do the job properly — the impeller alone is a false economy, because the wear plate and seals are what fail alongside it. On an outboard the pump is in the lower unit, so this is a gearcase-off job. |
| Gearcase lubricant | Varies by configuration Mercury High Performance Gear Lubricant is the specified lube for these gearcases, but Mercury has changed the part numbering over time and sells it in several container sizes, so no single number is given here. Order by product name and the quantity your gearcase takes, and replace the drain and vent screw seals at the same time — they are single-use and are included in the 100-hour kit above. |
| Spark plugs | Varies by configuration No spark plug part number or gap is stated for the 115 FourStroke in any Mercury document we could open — the specification table is published as an image in the accessible manual copies. Mercury's schedule is specific about the cadence, though: REPLACE at the first 100 hours or first year, then INSPECT every 100 hours or yearly thereafter. Get the plug type from the manual for your serial number or from a Mercury dealer; do not carry a number across from a sterndrive. |
| Corrosion control anodes | Varies by configuration Outboard anodes vary by model, year and mounting position — there are typically several on the engine, not one. Mercury's schedule is to CHECK them at every 100-hour or annual service rather than replace on a fixed cycle; replace when significantly eroded. Anode material should match your water: Mercury publishes magnesium for fresh water and aluminium for fresh or salt, and no longer publishes zinc drive kits. Identify your model and year before ordering. |
| Fuel filter | Varies by configuration The engine-mounted fuel filter is included in the 100-hour maintenance kit (8M0097854) above, which is the reliable way to get the correct one for your serial number. Mercury's schedule says to CHECK it for contaminants at each 100-hour service. A separate boat-mounted water-separating filter, if your installation has one, is a different part and is not covered by that kit. |
| Timing belt | Varies by configuration Mercury's schedule has you INSPECT the timing belt at every 100-hour or annual service but publishes no owner replacement interval for it in this manual, and no standalone part number we could confirm. Treat it as an inspection item, and take a worn or cracked belt to a dealer rather than guessing a replacement part. |
| Power trim fluid | Varies by configuration Checked at every 100-hour or annual service. Mercury specifies its own Power Trim and Steering Fluid for these systems; automatic transmission fluid is the commonly cited substitute but is not what Mercury publishes. Order by product name rather than by a number carried across from another engine. |
What owners watch
Valve clearance is a real scheduled item on this engine
If you have come from a sterndrive or a marine diesel you may have been told valve adjustment is not an owner-interval job. On this outboard it is: Mercury publishes a 400-hour interval to check and adjust valve clearance. It is far enough out that most recreational owners will never reach it, but a hard-working engine will, and it is the item most likely to be skipped entirely because nothing else in the library schedules it.
The impeller is in the lower unit, and the kit matters
Replacing the water pump impeller means dropping the gearcase, so it is not a dockside job. Mercury schedules it at 300 hours or three years, and sooner if the engine runs hot or the telltale flow weakens. Buy the full water pump repair kit rather than a bare impeller — the wear plate, seals and gaskets degrade alongside the rubber, and reusing them is how a fresh impeller still ends up overheating.
Spark plugs are replaced once early, then inspected
Mercury asks for new plugs at the first 100 hours or first year, and after that inspection rather than automatic replacement at each service. Owners tend to do the opposite — skip the first change, then replace plugs every year out of habit. Following the published pattern costs less and catches the early-life issues the first change exists for.
Salt water changes the schedule, not just the wash-down
Flushing the cooling system after every use in salt or polluted water is a scheduled item, not an optional courtesy, and Mercury also asks for more frequent lubrication of the grease points in salt water. The anodes are the visible proof of how the engine is faring: they are checked at every service and are the cheapest part on the engine to replace, which makes ignoring them an expensive habit.
Manufacturer documentation
The figures above are summarised for quick reference. For the full operator's manual and illustrated parts list, go straight to Mercury — they keep the current revision, so you always get the right version for your engine.
Mercury serves the correct Operation, Maintenance & Installation manual against your engine serial number. Several service parts for this engine change at serial number 2B094996 — pull the manual for YOUR serial rather than relying on a generic revision.
Never miss a Mercury 115 FourStroke service
Keeply builds your engine's maintenance schedule, tracks the hours, and reminds you before a service is due. Ask First Mate "what's overdue?" and get an answer from your own records. Free to start, no credit card.
The inspection and maintenance schedule is transcribed verbatim from Mercury's Operation, Maintenance and Installation manual for the 75/90/115 EFI FourStroke. Rated power, the full-throttle rpm range and the cold warm-up speed are from the 'Specifications 75–115 FourStroke' table in Mercury's 75-115 FourStroke manual. Displacement and oil grade are from Mercury's own product listings for the 2.1 L 75-115 hp FourStroke service kits. Part numbers and fitment text are from mercurymarine.com product pages. The remainder of the specification table — cylinder configuration, bore and stroke, gear ratio, idle speed, oil and gearcase capacities, spark plug type and gap, alternator output and dry weight — is published only as an image in the manual copies reachable to us and is therefore deliberately absent rather than guessed. Figures are for reference only; always confirm against the manual for your engine's serial number before ordering parts or carrying out service.