Mercury 15 / 20 FourStroke (carbureted) service guide

Quick answers

How much oil does a Mercury 15 / 20 FourStroke take?

About 1.0 L (1 US qt).

What engine oil does a Mercury 15 / 20 FourStroke use?

10W-30 all temperatures; 25W-40 above 4 °C (40 °F).

What gear oil does a Mercury 15 / 20 FourStroke use, and how much?

Premium Gear Lubricant or GL5 SAE 80-90; Premium or High Performance. Quantity: About 370 ml (12.5 fl oz), 2.15:1.

What spark plug does a Mercury 15 / 20 FourStroke use?

NGK DCPR6E.

How often should a Mercury 15 / 20 FourStroke 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 15 / 20 FourStroke?

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 15 / 20 FourStroke?

Neutral 900 to 1,000, in gear 850 to 950 r/min.

What is the propeller nut torque on a Mercury 15 / 20 FourStroke?

12 N·m (106 lb·in), then a new cotter pin.

Maintenance specs

Engine oil	10W-30 all temperatures; 25W-40 above 4 °C (40 °F)
Engine oil quantity	About 1.0 L (1 US qt)
Oil filter	Two instructions: 18 N·m (160 lb·in); 3/4 to 1 turn after the gasket touches. Filter fitting 34 N·m
Oil drain plug	23.7 N·m (17 lb·ft)
Gear lubricant	Premium Gear Lubricant or GL5 SAE 80-90; Premium or High Performance
Gearcase capacity	About 370 ml (12.5 fl oz), 2.15:1
Gearcase oil plugs (2) and flush plug	6.5 N·m (60 lb·in)
Spark plug	NGK DCPR6E
Spark plug torque	20 N·m (15 lb·ft)
Idle speed	Neutral 900 to 1,000, in gear 850 to 950 r/min
Thermostat	Starts to open 60 ± 1.5 °C (140 °F); fully open 75 ± 1.5 °C (167 °F); lift 3 mm (0.12 in)
Water pressure	Idle 21 kPa (3 psi); WOT 83 kPa (12 psi)
Anodes	Trim tab bolt M6 x 20 5.5 N·m (50 lb·in); engine anode bolt 6 N·m (53 lb·in); clamp bracket anode bolt 6 N·m (53 lb·in)
Propeller nut	12 N·m (106 lb·in), then a new cotter pin
Power tilt fluid	Power Trim and Steering Fluid or ATF Dexron III
Valve clearance, cold	IN 0.13 to 0.17 mm (0.005 to 0.007 in); EX 0.18 to 0.22 mm; locknut 7 N·m (62 lb·in)
Timing belt	Inspect every 100 hours

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: engine oil, oil drain plug gasket, spark plugs, gear case oil, gear case plug gaskets

   Check: fuel filter, thermostat, anodes

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

   Replace: engine oil, oil drain plug gasket, gear case oil, gear case plug gaskets, water pump impeller

   Check: spark plugs, fuel filter, thermostat, anodes

3. Every 300 hours or 3 years

   Replace: engine oil, oil drain plug gasket, gear case oil, gear case plug gaskets, water pump impeller

   Check: spark plugs, fuel filter, 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 15/20 FourStroke service manual, 90-899987. Specs can change by model year and serial number, so always confirm your exact model from the serial tag on the clamp bracket.