Mercury 8 / 9.9 FourStroke (carbureted, 209 cc) service guide

Quick answers

How much oil does a Mercury 8 / 9.9 FourStroke take?

About 800 ml (27 fl oz). No oil filter.

What engine oil does a Mercury 8 / 9.9 FourStroke use?

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

What gear oil does a Mercury 8 / 9.9 FourStroke use, and how much?

Premium Gear Lubricant or GL5 SAE 80-90; Premium or High Performance. Quantity: Standard (2.08:1) 320 ml (10.8 fl oz); BigFoot (2.42:1) 370 ml (12.5 fl oz).

What spark plug does a Mercury 8 / 9.9 FourStroke use?

NGK DCPR6E.

How often should a Mercury 8 / 9.9 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 8 / 9.9 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 8 / 9.9 FourStroke?

In gear 900 r/min, neutral 950 r/min.

What is the propeller nut torque on a Mercury 8 / 9.9 FourStroke?

11.5 N·m (8.5 lb·ft) plus a cotter pin.

Maintenance specs

Engine oil	10W-30 all temperatures; 25W-40 above 4 °C (40 °F)
Engine oil quantity	About 800 ml (27 fl oz). No oil filter
Oil drain plug (M14 x 1.5)	24 N·m (17.7 lb·ft)
Gear lubricant	Premium Gear Lubricant or GL5 SAE 80-90; Premium or High Performance
Gearcase capacity	Standard (2.08:1) 320 ml (10.8 fl oz); BigFoot (2.42:1) 370 ml (12.5 fl oz)
Gearcase oil plugs and vent screw	6.7 N·m (60 lb·in)
Spark plug	NGK DCPR6E
Idle speed	In gear 900 r/min, neutral 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 50 kPa (7.2 psi); WOT 95 kPa (13.7 psi)
Anode / trim tab screw	5.6 N·m (50 lb·in); engine anode cover screws 6 N·m (53 lb·in)
Propeller nut	11.5 N·m (8.5 lb·ft) plus a cotter pin
Valve clearance	IN 0.13 to 0.17 mm (0.005 to 0.007 in); EX 0.18 to 0.22 mm (0.007 to 0.009 in); 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 8/9.9 FourStroke and 9.9 BigFoot service manual, 90-892248R01. Specs can change by model year and serial number, so always confirm your exact model from the serial tag on the clamp bracket.