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Mercury 9.9 / 15 / 18 two-stroke (247 / 294 cc) service guide

Oil, gear oil, spark plugs and the service schedule for the Mercury 9.9 / 15 / 18 two-stroke (247 / 294 cc) (9.9 to 18 hp, two-stroke), from the factory service manual, plus how we service it in Hawaiʻi.

Models covered: ME 9.9 MH and ME 9.9 MLH (7.3 kW, 9.9 hp), ME 15 MH and ME 15 MLH (11.0 kW, 15 hp) and ME 18 MH SeaPro (13.2 kW, 18 hp) two-stroke twins: 2 cylinders, premix, thermostat cooled. 9.9 and 15: 0.247 L (15.072 cid), 55 x 52 mm. 18: 0.294 L (17.94 cid), 60 x 52 mm. inside, parts change at 0N159950 and above (ground wire and head bolt kit), 0N233234 and below / 0N233235 and above (15/18 cylinder block), 0N179314 and up (guide plate gasket) and 0N260037 and above (9.9 forward gear shims, recoil starter).

Quick answers

What two-stroke oil does a Mercury 9.9 / 15 / 18 two-stroke use?
NMMA certified TC-W3 2-cycle oil, premixed with the gasoline.
What is the fuel to oil ratio for a Mercury 9.9 / 15 / 18 two-stroke?
50:1 gasoline/oil (2% oil) after the first tank of fuel. Break-in and reconditioned powerhead (first tank of fuel): 25:1 (4% oil). Fuel 87 (R+M)/2 minimum.
What gear oil does a Mercury 9.9 / 15 / 18 two-stroke use, and how much?
Mercury/Quicksilver gear oil API GL-5, SAE #80-90 (Premium Gear Lubricant or GL-5 SAE 80-90 weight). Quantity: Approximately 370 mL (12.5 oz) in the general specifications and the gear oil change (required volume 370 ml); Fill at the lower plug hole until oil flows out of the upper plug hole.
What spark plug does a Mercury 9.9 / 15 / 18 two-stroke use?
NGK B7HS-10 / BR7HS-10 or Champion L82C / RL82C, qty 2; hex 20.8 mm, thread 14 mm.
How often should a Mercury 9.9 / 15 / 18 two-stroke be serviced?
The factory schedule lists: Every 100 hours or once a year; Every 300 hours or 3 years. In Hawaiʻi's salt water we do a full yearly service every 200 hours or 1 year, whichever comes first, and a new engine needs its first (break-in) service.
When should the water pump impeller be replaced on a Mercury 9.9 / 15 / 18 two-stroke?
The factory schedule says to replace it at 300 hours. In Hawaiʻi's warm salt water we replace it every 200 hours or 1 year, whichever comes first, because a worn impeller is the most common cause of overheating.
What is the idle speed of a Mercury 9.9 / 15 / 18 two-stroke?
950 RPM in neutral, 800 RPM in gear, engine at normal operating temperature; pilot adjusting screw 2 ± 3/4 turns.
What is the propeller nut torque on a Mercury 9.9 / 15 / 18 two-stroke?
25 N·m (18 lb-ft), then the cotter pin; if the hole does not line up, tighten farther. The ring on the front thrust hub faces the gearcase.

Maintenance specs

Two-stroke engine oilNMMA certified TC-W3 2-cycle oil, premixed with the gasoline
Fuel to oil ratio50:1 gasoline/oil (2% oil) after the first tank of fuel. Break-in and reconditioned powerhead (first tank of fuel): 25:1 (4% oil). Fuel 87 (R+M)/2 minimum
Gear oilMercury/Quicksilver gear oil API GL-5, SAE #80-90 (Premium Gear Lubricant or GL-5 SAE 80-90 weight)
Gear oil quantityApproximately 370 mL (12.5 oz) in the general specifications and the gear oil change (required volume 370 ml); Fill at the lower plug hole until oil flows out of the upper plug hole
Gear oil upper and lower drain plugsInstall the upper plug first, then the lower plug
Spark plugNGK B7HS-10 / BR7HS-10 or Champion L82C / RL82C, qty 2; hex 20.8 mm, thread 14 mm
Spark plug torque27 N·m (20 lb-ft), or finger tight and then 1/4 turn
Idle speed950 RPM in neutral, 800 RPM in gear, engine at normal operating temperature; pilot adjusting screw 2 ± 3/4 turns
ThermostatOpens at 52 °C (125.6 °F); replace if it does not open at that temperature or does not close slowly and smoothly; new cap gasket with Perfect Seal on both sides
AnodesAnode/trim tab under the anti-ventilation plate: replace before it is completely eroded, never paint; attaching screw 6 N·m (53 lb-in); the water pickup/strainer screw beside it is drive tight. An anode and screw are also in the cylinder block water jacket under the exhaust cover
Propeller nut25 N·m (18 lb-ft), then the cotter pin; if the hole does not line up, tighten farther. The ring on the front thrust hub faces the gearcase
Power trim and tiltNone: manual trim, 6 positions

Service schedule

The factory maintenance intervals, with what is replaced and what is checked at each. Rows marked Hawaiʻi are our shop's yearly service for salt water use.

  1. Every 100 hours or once a year

    Replace: gear case oil, gear case plug gaskets, propeller cotter pin

    Check: thermostat, anodes

  2. Every 200 hours or 1 year (our Hawai‘i yearly service) Hawai‘i

    Replace: gear case oil, gear case plug gaskets, propeller cotter pin, water pump impeller

    Check: thermostat, anodes

  3. Every 300 hours or 3 years

    Replace: spark plugs, gear case oil, gear case plug gaskets, propeller cotter pin, water pump impeller

    Check: thermostat, anodes

We replace the water pump impeller at every service from 200 hours up, which can be sooner than the factory schedule, because of Hawaiʻi’s warm salt water.

Source: Mercury 9.9/15/18 Two Stroke service manual, 90-8M0131802, December 2019. Specs can change by model year and serial number, so always confirm your exact model from the serial tag on the clamp bracket.

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