Water in the Lower Unit Keeps Coming Back - Seals and the Pressure Test Explained

With the engine upright and cold (not right after a run), crack the bottom drain screw, then loosen the top vent screw to let a little lube out into a clean clear container. Yamaha's guidance: milky color means water; a burnt look or smell means it is time for a change. Yamaha also warns that running the engine right before the check churns air into the lube, and those tiny bubbles can look milky too.

Other clues:

• Water is heavier than lube and settles at the bottom, so a few drops of clear water come out first if a lot has gotten in. If you are unsure, Mercury suggests pouring some of the lube into a glass jar and letting it settle: any water separates out at the bottom.
• Mercury's owner FAQ notes that lube can look slightly yellowish after running, and creamy where it mixed with assembly grease, without any water in it.
• A small amount of very fine metal paste on the drain screw magnet is normal wear. Black, burnt-smelling lube with larger metal chips means internal damage, per BoatUS.

If you find water, Mercury's FAQ says to have the lower unit inspected by an authorized dealer, who will open it up and check the oil seals and their running surfaces, the O-rings and the water pump gaskets as well as the gears and bearings. Read on to understand what that inspection involves and what you can check first.

The cheapest fix comes first. Check both screws for a missing, flattened, cut or doubled-up gasket, and look at the threads and the seat in the case for damage. If the old gaskets were reused, that alone may be your leak. Fit new gaskets every time, as Yamaha says, and keep a few spares in the boat.

Remove the prop and thrust washer (see the propeller guide). BoatUS describes how line wraps around the shaft just behind the forward thrust washer, winds tight, heats up and melts into a hard, sharp-edged ring that cuts the seal lips. Yamaha says to check for line or anything wrapped around the shaft before every trip.

Cut away every strand with a sharp knife or pick, being careful not to nick the shaft or the seal. A brittle plastic disc behind the thrust washer is melted line.

With a flashlight and magnifying glass, look at the prop shaft seal where the shaft leaves the carrier. BoatUS suggests checking the seal lips for cuts or tears. A seal that is torn, hard, or has lube weeping around it is suspect. Then look over the whole gearcase for cracks or fresh impact damage, especially the skeg and the leading edge, and around every screw.

Lube leaking out means water can get in. A seal can also look fine and still leak, which is what the pressure test is for.

A pressure test is how shops prove whether the gearcase is sealed. The idea:

• The gearcase is drained.
• An adapter with a gauge screws into the vent hole, and a small hand pump puts a low air pressure inside. The correct pressure for your gearcase is in the service manual; it is much less than tire pressure.
• If the gauge holds steady for the time the manual specifies, the case is sealed. If it drops, there is a leak.
• To find the leak, the tech sprays soapy water on every seal, screw and seam (or dunks the gearcase in water) and watches for bubbles.

Most shops also pull a vacuum on the gearcase. Seal lips are designed to hold against pressure from one side, so a seal can pass the pressure test and still leak when the gearcase pulls inward. That happens in real life: a hot gearcase that cools quickly in the water contracts the air inside and can draw water past a weak seal. If the vacuum holds for the specified time, the seals are good in both directions.

DIY: Pressure/vacuum testers are sold for outboard and sterndrive gearcases. If you buy one, use only the hand pump that comes with it, the adapter that fits your vent hole, and the pressure and vacuum values from your service manual. Never connect shop air.

Shop: A pressure and vacuum test is a routine, fairly quick job at a dealer, and it is the right choice if you will not reuse the tester or if the water came back after a seal job. Ask them to tell you exactly which seal or gasket failed.

• Drain/vent gaskets: easy DIY. New gaskets, clean seats, snug screws.
• Prop shaft seals and carrier O-ring: Boating Magazine rates this a 4 out of 5 skill level and about 8 hours on a MerCruiser Alpha One, with special pullers and seal installers, and torque specs from the service manual. Handy owners do it; many prefer a shop.
• Driveshaft or shift shaft seals: these sit at the top of the gearcase, so the gearcase must come off and the water pump comes apart. That is a good time to replace the impeller too (see the gearcase removal and water pump guides).
• Cracks or damaged bores: shop job. A cracked case may need welding or replacement.

After any seal work, have the gearcase pressure and vacuum tested again before filling. Then refill with the gear lube from your manual, from the bottom hole upward until it comes out of the top vent, and fit the screws with new gaskets (see the lower unit oil change guide).

Pull a small sample after the first few outings. Clean, clear lube means the fix worked. Any milkiness means the leak is still there.

• Look behind the prop for fishing line before every trip if you fish or run near anglers, as Yamaha suggests, and at least once a season pull the prop to look, as BoatUS suggests.
• Change the lube at the end of the season, before storage. BoatUS warns never to store a gearcase with old lube in it: any water inside sits on the bearings and gears and rusts them all winter.
• Stored outside in a cold climate? Yamaha suggests trimming the outboard fully down or slipping a tightly sealed plastic bag over the (dry) lower unit, so rain and condensation can't collect in it and freeze.