Mercury · Outboard

Mercury 15 / 20 hp FourStroke

The Mercury 15 and 20 hp FourStroke is a portable two-cylinder four-stroke for tenders, RIBs and small fishing boats; in Europe it is also sold as a Mariner, and the 15 hp version as a ProKicker trolling motor. Since 2017 (serial 0R833820 on) both ratings use the same 333 cc engine with electronic fuel injection and an oil filter, built in Japan by Tohatsu’s joint venture with Mercury and identical to the Tohatsu MFS15E and MFS20E, so Tohatsu’s own part numbers are listed as alternatives. Its predecessor, built from about 2008 to 2017 (serial 0R235949 to 0R833819), is a 351 cc carburettor engine: oil filter and spark plug are the same, but water pump, thermostat, timing belt and service kits differ, so check the serial number before ordering. The EFI engine comes with 15, 20 or 25 in shafts, tiller or remote steering, manual or electric start and manual or power tilt.

Manuals & documents
Wear parts & part numbers
Service intervals
Specifications
Good to know
Videos & how-tos
On the marketplace

Manuals & documents

The links go straight to Mercury. We host no manuals or drawings — the manufacturer’s copy is the current one.

Operation, maintenance and installation manual 15/20 EFI FourStroke (2019, 8M0165094) Operation manual servicelit.mercurymarine.com · PDF · EN
Operation, maintenance and installation manual 15/20 EFI FourStroke (2018, 8M0145520) Operation manual download.brunswick-marine.com · PDF · EN · DE · FR · ES
Installation, operation and maintenance manual 15/20 EFI FourStroke, first edition (2017, 8M0125264) Operation manual servicelit.mercurymarine.com · PDF · EN
Brunswick Marine in EMEA download centre: Mercury operation manuals by year and language (15/20 EFI under 2017 and 2018, carburettor model under 2010–2016) Manual portal download.brunswick-marine.com · EN
Mercury service manual, printed, ordered by serial number (OB-SM-035; for the 15/20 EFI: 8M0125265) Service manual publications.brunswick.com · EN
Mercury parts manual, printed, ordered by serial number (OB-PM) Parts catalogue publications.brunswick.com · EN
Operation and maintenance manual 15/20 FourStroke with carburettor (2012, 90-8M0055689) Operation manual download.brunswick-marine.com · PDF · EN
Operation, maintenance and installation manual 15/20 FourStroke with carburettor, last edition (2016, 8M0114071) Operation manual download.brunswick-marine.com · PDF · EN · DE
Tohatsu owner’s manual MFS 9.9E / 15E / 20E, EU edition (003-11144-CBF1) Operation manual tohatsu.com · PDF · EN · DE
Tohatsu maintenance kits with their contents, including kit 3RS-87500-1 for the MFS9.9E/15E/20E Parts catalogue tohatsu.com · EN
Tohatsu propeller chart (MFS15E/20E and MFS15D/20D) Parts catalogue tohatsu.com · EN

Wear parts & part numbers

Part	OEM number	Also fits	Note
Raw water pump impeller	`8M0135803`^2,3	Tohatsu `3LD-65021-0`¹	EFI engine (serial 0R833820 on); Mercury has it replaced every 300 h or 3 years. The Tohatsu number is the impeller in Tohatsu’s own kit for the MFS9.9E/15E/20E.
Raw water pump impeller	`803748`⁴	—	Carburettor model (serial 0R235949 to 0R833819). One source (Crowley parts diagram), which marks it as no longer available; the same diagram shows guide plate gasket 8M0075040 and liner 803751.
Water pump repair kit	`8M0135821`^2,5,6	—	EFI engine and the 333 cc 9.9 EFI ProKicker (serial 1R008146 to 1R278792): pump case, impeller, liner, guide plate, gaskets, oil seal and key. It is the pump kit inside the 300 h kit.
Water pump repair kit	`8M0137849`^2,7,8	—	Heavy-duty version with chrome face plate and housing for sandy, abrasive water; EFI engine (serial 0R833820 on), instead of 8M0135821.
Water pump repair kit	`803750A06`⁴	—	Carburettor model (serial 0R235949 to 0R833819). One source (Crowley parts diagram), which marks it as no longer available.
Engine oil filter	`8M0162831`^9,10,11,12	Tohatsu `3BJ-07615-0`¹	Both generations; dealers also list it for the 8/9.9 hp (323 cc) and the 25/30 hp EFI. Older parts diagrams still show 822626K03. Oil the gasket and tighten 3/4 to 1 turn after it touches.
Fuel filter (engine)	`8M0134466`^13,14,15	—	EFI engine (9.9–30 hp EFI from serial 0R833820): complete water-separating filter with clear bowl under the cowl.
Fuel filter (engine)	`8537331`^13,10,12	—	Replacement element for the 8M0134466 filter, fitted with O-ring 8518151; oil the O-ring before pushing the element on. One source (Crowley parts diagram).
Fuel filter O-ring	`8518151`^13,10	—	O-ring for the filter element 8537331. One source (Crowley parts diagram).
Spark plug	`803507`^17,18,12	NGK `DCPR6E`^12,16Tohatsu `9701-1-1103`¹	Both generations, two per engine; Mercury’s number for the NGK DCPR6E in the parts diagrams. Gap 0.9 mm and 20 N·m per Mercury’s 2019 manual; Tohatsu and the carburettor manuals give 0.8–0.9 mm.
Thermostat	`8M0139434`^19,20,21	—	EFI engine (serial 0R833820 on), opens at about 52 °C (125 °F); also in the 300 h kit.
Thermostat	`855676002`²²	—	Carburettor model (serial 0R235949 to 0R833819). One source (Crowley parts diagram), which marks it as no longer available.
Thermostat gasket	`853702005`^19,10,22	—	Thermostat cap gasket, the same number on both generations; the 300 h kit includes thermostat gaskets. One source (Crowley parts diagrams).
Anode (lower unit)	`8M0137344`^2,23,24,12	—	Square anode on the starboard side just above the anti-ventilation plate; 9.9–20 hp EFI from serial 0R833820. Never paint it; the screw takes 6 N·m.
Anode (clamp bracket)	`823913Q`^25,26,10	—	Under the lower engine mount and, on manual-tilt models, on the transom brackets (up to three per engine); also in the 300 h kit. One source (Crowley parts diagrams).
Anode (clamp bracket)	`875208`^27,10	—	Power-tilt models: two anodes on the inside of the transom brackets; also in the 300 h kit. One source (Crowley parts diagram).
Anode (engine)	`8M0065073` · `8M0065071`^17,28	—	Powerhead anode in the cylinder-head water jacket and its plug (also sold as plug assembly 8M0135965); a dealer check item. One source (Crowley parts diagram).
Gear oil plug gaskets	`8M0082880`^2,29	Tohatsu `332-60006-1`¹	Blue sealing washers for the gearcase drain/fill and vent plugs, two at every gear lube change; dealers list them for the 15–20 hp EFI and the 8/9.9 hp FourStroke.
Drain plug gasket	`895146`^30,10	Tohatsu `3R3-07406-0`¹	Sealing washer under the engine oil drain plug 8M0111615; in both Mercury kits. One source (Crowley parts diagrams).
Timing belt	`8M0135591`^31,32,33	—	EFI engine from serial 0R833820 up to 1R162636 only; the sources do not name the belt for later engines. Mercury has it inspected every 300 h or 3 years (dealer).
Timing belt	`895120002`³⁴	—	Carburettor model (serial 0R235949 to 0R833819). One source (Crowley parts diagram).
Service kit	`8M0151469`^10,35,36,37	—	100-hour kit for the EFI engine (serial 0R833820 on) and the 333 cc 9.9 EFI ProKicker (serial 1R008146 to 1R278792): oil filter, fuel filter, oil drain seal and gearcase seals; engine oil and gear lube not included.
Service kit	`8M0151470`^10,38,39,40	Tohatsu `3RS-87500-1`¹	300-hour kit, same fit: the 100 h parts plus thermostat and gaskets, spark plugs, water pump kit and anodes. Tohatsu’s kit 3RS-87500-1 for the MFS9.9E/15E/20E is a different set: oil filter, impeller with liner, guide plate, gasket and key, plug gaskets, drain washer, two anodes and two spark plugs, but no fuel filter or thermostat.
Service kit	`8M0172123`^41,42	—	Quicksilver service kit for the 15/20 EFI (serial 0R833820 on), an alternative to Mercury’s kits; dealers list oil filter, fuel filter, gearcase seal, spark plugs and impeller, with small differences.
Oil change kit	`8M0081914`^12,43	—	Oil change kit for the 15/20 EFI (Mercury brand): engine oil and oil filter, as listed in the 2019 operation manual.
Service kit	`8M0120838`^44,45	—	Carburettor model: 300-hour kit for serial 0R235949 to 0R833819. Dealer content lists partly repeat the EFI kit text, so only the number and serial range are given here.
Service kit	`8M0172122`⁴⁶	—	Carburettor model: Quicksilver service kit, listed as “Serv Kit-15/20 Cb” next to the EFI kit 8M0172123. One source.
Oil change kit	`8M0081910`¹²	—	Carburettor model: Quicksilver oil change kit (oil and filter), from the oil change kit table in Mercury’s 2019 EFI manual.
Propeller	`897750A11`^48,49	Tohatsu `3BAB64518-1`⁴⁷	Black Max aluminium, 3 blades, 9.25 × 9 in, right-hand; dealers list it for the 15–20 hp FourStroke from 2008, so both generations; 14-tooth spline, Flo-Torq I hub 8M0054769. Choose the pitch that lets the engine reach 5700–6200 rpm at full throttle. Tohatsu’s 9.3 × 9 in is its standard propeller on the long-shaft MFS15E/20E.
Propeller	`897752A11`^50,51	Tohatsu `3BAB64521-1`⁴⁷	Black Max aluminium, 3 blades, 9.25 × 10 in, right-hand; same fit and hub. Tohatsu’s 9.3 × 10 in is its standard propeller on the short-shaft MFS15E/20E.
Propeller	`897754A11`^52,53	Tohatsu `3BAB64524-1`⁴⁷	Black Max aluminium, 3 blades, 9.25 × 11.5 in, right-hand, for light, fast boats; same fit and hub.
Propeller	`897748A11`^54,47	—	Black Max aluminium, 3 blades, 9.5 × 8 in, right-hand, for heavy loads; same fit. One source. Tohatsu’s nearest size for the MFS15E/20E is 9.2 × 8 in (3BAB64516-1), a smaller diameter.

Numbers checked against the sources below on 12 October 2026. Always confirm against your serial number and exact type before ordering.

Service intervals

Task	Interval
Oil level check	Before every outing: oil level, lanyard stop switch, transom mounting, fuel system for leaks, steering and propeller blades¹²
Fresh-water wash	After every use in salt, polluted or muddy water: flush through the flush fitting for 3–5 minutes with the engine idling in neutral and the propeller removed, so the thermostat opens¹²
Engine oil change	Every 100 h or yearly, with a new oil filter; more often with a lot of trolling¹²
Oil filter	Every 100 h or yearly¹²
Gear oil change	Every 100 h or yearly; when draining, look for metal particles and milky oil¹²
Fuel filter	Check the water-separating filter every 100 h or yearly; drain water as needed, replace the element if it is dirty¹²
Spark plug	Replace every 300 h or 3 years; in salt water also inspect every 100 h or yearly, with anti-seize on the threads¹²
Anode	Check every 100 h or yearly, more often in salt water; replace before it is used up, never paint it¹²
Thermostat	Inspect every 100 h or yearly for corrosion or a broken spring (dealer)¹²
Battery	Every 100 h or yearly¹²
Lubrication	Propeller shaft and driveshaft splines (dealer) every 100 h or yearly; all lubrication points and, on tiller models, the throttle and shift cables every 300 h or 3 years, more often in salt water¹²
Raw water impeller	Replace every 300 h or 3 years (dealer), sooner after overheating or when the water pressure drops¹²
Timing belt	Inspect every 300 h or 3 years (dealer)¹²
Valve clearance	Not in Mercury’s EFI schedule; Tohatsu: check every 200 h or yearly (dealer); carburettor model: every 100 h or yearly (dealer)^55,16
Full service	Every 300 h or 3 years: power tilt fluid, throttle and shift cable adjustment, wiring and connectors (partly dealer)¹²

Hours or years, whichever comes first, from Mercury’s operation manual 8M0165094 (2019) for the 15/20 EFI; “dealer” marks the jobs Mercury leaves to a dealer. Running-in: first hour up to 2000 rpm with varied throttle, second hour up to 3000 rpm with about one minute at full throttle every ten minutes, then for eight hours no more than five minutes at a time at full throttle. Tohatsu’s manual for the identical MFS9.9E/15E/20E differs: first oil and gear oil change after 20 h or 1 month, then every 100 h or 6 months; oil filter, valve clearance, timing belt, thermostat and impeller checked every 200 h or yearly; impeller replaced every 400 h or 2 years. The carburettor model’s manual (2012) replaces the spark plugs after the first 100 h and then only inspects them.

Specifications

Cylinders	2, four-stroke^55,16
Displacement	333 cc; carburettor model: 351 cc^56,55,16
Bore × stroke	61 × 57 mm; carburettor model: 61 × 60 mm^12,16
Power	11.0 kW (15 hp) or 14.7 kW (20 hp)¹²
Full-throttle range	5700–6200 rpm (JCI versions 5400–6100 rpm); carburettor model: 5000–6000 rpm (15 hp), 5400–6100 rpm (20 hp)^12,57
Idle speed	900–1000 rpm, in gear and in neutral¹²
Compression ratio	10.0:1¹²
Injection	Computer-controlled sequential multiport fuel injection; mechanical low-pressure and electric high-pressure fuel pump; water-separating fuel filter under the cowl¹²
Ignition	Computer-controlled CDI; 6° BTDC at idle, 28° BTDC at full throttle¹²
Fuel	Regular unleaded petrol, at least RON 91 (USA/Canada: 87 AKI); up to 10 % ethanol (E10)¹²
Spark plug	NGK DCPR6E (2), gap 0.9 mm, 20 N·m or finger-tight plus 1/4 turn; Tohatsu’s manual: gap 0.8–0.9 mm, 18 N·m; carburettor model: gap 0.8–0.9 mm^12,55,16
Engine oil capacity	approx. 1.0 l to the middle of the dipstick range (Tohatsu’s manual: 1.0 l, 1.2 l with a new filter); carburettor model: 1.0 l^12,55,16
Engine oil grade	NMMA FC-W SAE 10W-30 for all temperatures; FC-W 25W-40 above 4 °C¹²
Gear ratio	2.15:1 (28:13), both generations; dog clutch forward–neutral–reverse^12,55,16
Gear oil capacity	approx. 460 ml (Tohatsu’s manual: 465 ml); carburettor model: 370 ml^12,55,16
Gear oil grade	Mercury/Quicksilver Premium gear lube or GL-5 SAE 80W-90¹²
Cooling	Raw water with thermostat, impeller pump in the gearcase; water pressure 9 kPa at idle, 59 kPa at full throttle¹²
Thermostat opens at	about 52 °C (125 °F)²¹
Engine protection	Above 85 °C or below 26.3 kPa oil pressure the engine is held to 2800 rpm and the warning horn sounds¹²
Alternator	Single phase, 4.6 A at 900 rpm, 11.5 A at 5000 rpm¹²
Valve clearance (cold)	Intake 0.13–0.17 mm, exhaust 0.18–0.22 mm (Tohatsu’s manual; Mercury’s EFI manual gives no figure); carburettor model: the same values^55,16
Tightening torques	Oil drain plug 23.7 N·m; spark plugs 20 N·m; propeller nut 12 N·m, then on to the next cotter-pin hole; anode screws 6 N·m¹²
Shaft length	15 in (38 cm), 20 in (51 cm), 25 in (63.5 cm)¹²
Dry weight	From 44 kg (MRC) and 46.7 kg (MH) to 57.6 kg (ELHPT); ProKicker 55.3–58.9 kg¹²
Models	MH, MLH, MRC, EH, ELH, E, EL, EPT, ELPT, ELHPT; 15 ProKicker EXLPT and EXLHPT¹²
Tilt	Power trim −4° to 71°, manual −4° to 76°; power tilt fluid: Mercury Power Trim & Steering Fluid or ATF Dexron III¹²
Years built	EFI: from 2017 (serial 0R833820 on), made in Japan by the Tohatsu–Mercury joint venture; carburettor model: about 2008 to 2017 (serial 0R235949 to 0R833819)^56,3,44,58
Predecessor	15/20 FourStroke with carburettor, 351 cc (about 2008–2017); Tohatsu’s counterpart is the MFS9.9D/15D/20D^16,59

Good to know

Read the serial number on the transom bracket before buying parts: the carburettor model runs to 0R833819 and the EFI starts at 0R833820, and water pump, thermostat, timing belt and kits differ. On the EFI, timing belt 8M0135591 only fits up to 1R162636.^44,33
The EFI oil change is designed to stay clean: the oil filter has its own drain hose, stored on the front of the engine. Tilt the motor fully up for a few minutes so the filter empties, lead the hose into a container and then unscrew the filter. Drain the sump with the motor tilted, lower it for a minute and tilt it again to get the rest out. Do not use a crankcase suction pump.¹²
The water-separating fuel filter sits under the cowl in a clear bowl; a red ring floats at the water level when there is water in it. Pull the assembly off its bracket, open the drain fitting about 2.5 turns to empty it, and oil the O-ring of a new element before fitting it.¹²
If the warning horn sounds and the engine will not go above 2800 rpm, it is overheating or short of oil pressure. Idle in neutral and check the tell-tale stream: no stream means a blocked water intake or a worn impeller. Replace the impeller early if the engine has ever overheated.¹²
There are four external anodes: the square one above the anti-ventilation plate, one under the lower engine mount and two on the transom brackets. Keep the continuity wires intact, never paint an anode, and tighten the screws to only 6 N·m.¹²
Fill the gear lube (about 460 ml on the EFI) from the lower hole until it comes out at the vent, refit the vent plug before pulling the tube out, and always use new blue sealing washers (8M0082880). Milky or cream-coloured lube means water in the gearcase.¹²

Videos & how-tos

Links to YouTube, nothing embedded. The maker’s own channel comes first; the rest are channels we found clear and correct for this model.

Engine oil change on the Mercury 15/20 hp EFI FourStroke Video Mercury Marine · maker’s channel · EN · 2 min
Gear lube change on the Mercury 15/20 hp EFI FourStroke Video Mercury Marine · maker’s channel · EN · 1 min
Full service on a Mercury / Mariner F15/F20 EFI (Tohatsu MFS15E/20E): oil and filter, fuel filter, spark plugs, impeller and gear oil Video Marine Engine Servicing · EN · 26 min
Dealer handover video for the Mercury F15/F20 EFI: controls and getting started (German) Video Droste Wassersport OHG · DE · 29 min

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Sources

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