Alpha or Bravo: it changes where the water pump lives
First thing to know: which drive's on the back? On an Alpha, the raw-water pump sits down in the drive itself, fed through pickups on the gearcase, so a tired impeller means the drive comes off. On most Bravo installs the pump's on the engine, driven by the belt, and the drive just carries the water.
That one difference decides how you chase an overheat and what goes into an annual service. An Alpha that's run a season on the same impeller is overdue for a look, especially if it's been dragged through sand or weeds near shore. A Bravo that overheats? Start with the belt-driven pump and the engine-side hoses.
Hard shifting and a drive that clunks
Shift problems are one of the most common MerCruiser complaints. Stiff shifting, a loud clunk into gear, a drive that won't come out of gear at the dock: usually that's the shift cable, its adjustment, or the shift interrupter switch, the one that cuts the engine for a split second so the gears can let go. Replacing the drive to fix a cable is an expensive way to learn that.
Clunking can be the gears themselves, though, and the gear lube tells you which. Milky lube means water's gotten into the drive. Metal on the drain plug magnet means something's wearing inside. Either one is a reason to stop and look before the next outing, not after it.
Bellows, gimbal bearing and alignment
Rubber bellows seal the transom opening around the U-joints, the exhaust and the shift cable. Sun and cold age them, and a cracked bellows lets lake water into the boat or onto the U-joints. The gimbal bearing, the one the drive shaft runs through, starts to growl when it's on its way out.
Engine-to-drive alignment ties all of it together. Get the engine and the gimbal bearing out of line and the coupler and U-joints wear fast, with the bearing right behind them. Alignment's checked with the drive off. And with the drive off is the time to look at the bellows, the bearing and the U-joints together, not one at a time.
The engine side: a marine block, fresh water and risers
Behind the transom, a MerCruiser engine is a marinized V6 or V8, automotive-style. On a raw-water cooled engine the lake itself runs through the block, the exhaust manifolds and the risers. Fresh water's far kinder than salt. Manifolds and risers still wear inside with age, though, and a failing riser can let water back toward the engine.
Fuel's the other usual engine-side fault on a boat that sits half the year. Stale fuel and water in the tank turn up as a hard start or a rough idle long before they turn up as anything else. Rule them out before anybody blames a bigger part.
Winterizing a MerCruiser
A sterndrive holds water in places an outboard doesn't: the block, the manifolds, the risers, the hoses and the drive. Every drain point has to be opened and confirmed clear, or the system filled with the right antifreeze, before the first hard freeze. The drive gets fresh gear lube at the same time, and that's also when milky lube gets spotted.