How to Overhaul a Submerged Two-Stroke Outboard Engine

A submerged outboard engine is something all boat owners want to avoid. However, it happens—maybe you accidentally dropped the engine in the water, failed to tighten the clamp screws enough, or forgot to put in the drain plug and your boat took on water.

Luckily, one of the heartiest and most resilient marine engines is the two-stroke outboard. Even after being submerged, it’s possible to salvage this engine with a thorough overhaul. 

A successful overhaul depends on the type of water the engine was submerged in, how long it was submerged, and how quickly you can flush and overhaul it.  Two-stroke engines that are submerged in saltwater will begin to corrode quickly once they are exposed to air, often within a couple of hours. That makes it essential to move fast when salvaging an engine. If you can’t begin overhauling the engine immediately, “pickling it” in a barrel of freshwater until you can work on it can slow the corrosion.

Here’s a look at the basic steps in overhauling a submerged two-stroke outboard engine. Always be sure to consult your owner’s manual for tips specific to your engine make and model.

Rescue and Rinse

After recovering the engine, disconnecting the battery, and pulling the spark plugs, immediately rinse and flush the engine with clean freshwater. This is particularly important if the engine has been submerged in saltwater. You can dunk it in a big bucket of freshwater and/or thoroughly rinse and flush it with a garden hose. Make sure you rinse the outside of the engine, everything under the cowling, and especially inside the spark plug holes. Using water that has been fortified with a saltwater remover (such as Salt-Away) can work even better.

Empty Contaminated Fluids

When it comes to submersion, one of the biggest benefits of two-stroke engines is that they don’t have upper unit oil reservoirs, since the oil is instead mixed with the gasoline. However, it is still important to drain the fuel lines and carburetor, as well as the lower unit oil. These fluids may have been exposed to water during the submersion and need to be cleaned out and replaced. Remember to replace seals when replacing the lower unit oil, and make sure the carburetor and other fuel line components are completely clean and dry.

Fill Spark Plug Holes with Fogging Oil

It isn’t enough to flush the saltwater out of the spark plug holes, although doing so does protect against corrosion. As soon as possible, get a couple tablespoons of fogging oil into the holes, so that it can coat the inner surface of the engine (the cylinder walls and pistons) and protect it from corrosion and rust. If you don’t have fogging oil, two-stroke oil or diesel can work in a pinch. Make sure to get a decent amount in there, but remember that using too much will make it a lot harder to cycle the oil out and get the engine started later on.

Turn the Engine

Once there’s oil in the spark plug holes, the next step is to pull the flywheel repeatedly. This not only forces water out of the spark plug holes, but also moves the oil through the unit and coats the inner surface of the engine. The spark plugs should remain out during this process—the submerged spark plugs are ruined, so dispose of them. In addition, the spark plug holes should face downward to help force out the water.

Dry the Engine Block

After doing the critical work of flushing the engine with freshwater and getting oil moving through the cylinder and pistons, the next step is to dry the engine block and electrical components. Use compressed air to force any residual water out of crevices and hard-to-reach areas and dry everything faster. WD-40 can be used just about everywhere when it comes to getting rid of water, protecting surfaces, and re-lubing the engine, so don’t be afraid to spray it on the engine block surface and components. It can also be used in the spark plug holes to help move water out if you don’t have fogging oil.

Install New Spark Plugs

Only put in new spark plugs when the engine is completely overhauled and water is no longer coming out of the holes when the flywheel rope is pulled. Hold a rag in front of the spark plug holes while pulling the rope to see what type of fluids come out. The goal is pure oil, not a milky oil-water mix.

Start and Run

Once the engine has been overhauled and is completely dry, run it for a few minutes—ideally in freshwater, and ideally with a slightly over-rich gas mix (i.e., with a bit more marine two-stroke oil than is normally included in the gas.) This rich mixture will help to lubricate any internal surfaces you missed during the overhaul. It might take several pulls to fire up the engine, due to the rich mixture and the fact that the cylinder and pistons are coated in fogging oil or two-stroke oil. 

If the engine won’t start, remove the spark plugs and once again hold a rag in front of the holes while pulling the rope. A fine mist of mixed gas should come out. If you see mostly oil, there’s likely still too much of it in the cylinder, and you’ll need to pull the flywheel repeatedly with the spark plugs out until enough oil is expelled and the gasoline can ignite. Then replace the spark plugs and try again.

Head Back Out On the Ocean

When you feel confident about the engine, you can take it back out on the water.  If you’re not sure, have a marine mechanic inspect your overhaul. Keep your first trip near the shore, at least for the first few hours, just in case something goes wrong.