How To Winterize a 4.3 Mercruiser? 8 Easy Steps

Mercruiser has a long history of producing 4.3L marine engines. These engines were also available with a variety of cooling and drain systems. You must winterize the motor before storing your boat for the winter. While boating, your Mercruiser 4.3 uses the water in the lake to cool the engine. This water is retained within the engine.

The water will freeze if it is not removed. The water expands as it freezes. The engine block will break as a result of this. A damaged block is a costly repair that may be avoided by taking a few minutes before winter arrives.

Is Mercruiser a good engine?

The MerCruiser 4.3L, according to my old gray cells, has been around since the 1960s, when it was placed in a front-wheel-drive Oldsmobile. The engine has been quite popular in both single and twin installations in maritime applications.

The 4.3L appears downright primitive when compared to low-emission petrol outboards with the same power. But, even with minimal upkeep, it has basic engineering that makes it difficult to kill. More importantly, the MPI version (which supports Mercury’s amazing SmartCraft technology) produces 15% more power than the original two-barrel carb version.

Mercruiser design

The MerCruiser MPI is a 90-degree V6 variant of the long-running 350 (5.7lt) V8 that produces 219.8 prop shaft horsepower at 4600 rpm and has a Wide Open Throttle range of 4400 to 4800rpm. The far oversquare bore and stroke dimensions of 101.6 x 88.4mm, along with hydraulic lifters actuated by pushrods from the center camshaft, make this one low-maintenance and long-lasting engine.

Freshwater cooling decreases cylinder wall condensation and adds 24kg to the dry engine/drive weight of 393kg, which isn’t terrible given that the cylinder heads and block are cast iron – no fancy alloys here!

A serpentine belt operates the MerCruiser 4.3L MPI’s 65 amp voltage controlled alternator, as well as the power steering and water cooling pumps.

The 4.3L comes with legs ranging from the old Alpha to the new Bravo 3, however the 3s are overkill for twin installations since they inhibit the classic one ahead/one behind strategy for spinning a hull in its own length. Cooling water intakes are often found in the drives of trailer boats, but the 4.3Ls I tested in a Sea Ray 315 Sundancer had through-hull intakes with strainers and seacocks. Just make sure they’re open before starting the engines!

Access to the oil filter and spark plugs is good in single installations, but getting to the inboard plugs in the 315 Sundancer, which had a beam of 2.87m, required a very flexible wrist and fingers. In a smaller cruiser, the price you pay for security!

After the first 20 hours, servicing intervals are every 100 hours or yearly; however, because most cruiser owners only log approximately 50 hours per year after that, I would replace the oil and filter every six months and use Quicksilver SAE 25W40 oil made for sterndrive engines. Standard 20W50 oils are incapable of handling the high loads that marine engines encounter. MerCruiser petrol engines require 87 AKI (Anti Knock Index in the US), which equates to 92 RON, whereas regular unleaded in Australia is 91 octane.


The freshwater-cooled review MerCruiser 4.3L MPIs used Alpha legs instead of Bravo 1 or 2, but they performed well in the 315 swinging 14.5 x 17in stainless steel Quicksilver Vengeance props, which were somewhat underpropped for heavy loads and drove through rather steep 1.62:1 reduction ratios. Despite the dog clutch drives, the engines started promptly hot or cold with minimal oil smoke, and shifting was quite smooth. The drives had to be turned off for any meaningful power behind the boat, although at roughly 1000rpm on the tachos, the hull could be spun in its own length in calm circumstances.

When the throttle levers were pulled down hard from 1,500 rpm, the engines swiftly got the 4.6-tonne 315 out of the hole, and the 4.3Ls had modest vibration levels throughout the entire rpm range, becoming noisy only when they approached WOT. The trim tabs were left on neutral, and the drives were tweaked as needed, like with all of my sterndrive cruiser testing.

No prop ventilation happened during tight bends at 3000rpm, and the engines maintained their rpm due to the MPI putting in extra fuel to compensate for the increased load.

According to Andrew Norton, author on Trade a Boat which is a website providing information and boat review, Twin 220 hp MerCruiser 4.3L MPI petrol V6 engines power the Sea Ray 315 Sundancer. Two-way runs averaged with SmartCraft instruments and Raymarine GPS.

610 (trolling) 2.5kts 3.3lt/h
1000 (fast idle) 4.4kts 4.9lt/h
1500 5.8kts 7.3lt/h
2000 (offshore troll) 7.6kts 12.9lt/h
2800 (min. plane) 13.2kts 26.2lt/h
3000 (offshore cruise) 19.5kts 28.7lt/h
4000 (max cruise) 30.1kts 41.0lt/h
5000 (WOT) 35.3kts 61.6lt/h

*Fuel burn is per engine.

How To Winterize a 4.3 Mercruiser

how to winterize a 4.3 mercruiser

There are 8 easy steps you can follow to winterize a 4.3 Mercruiser

Step 1

Open the fuel tank cap and add the appropriate amount of fuel stabilizer into the fuel tank. Refer to the label on the bottle for the proper amount.

Step 2

Remove the boat drain plug with an open-end wrench. It is located at the bottom-rear of the boat.

Step 3

Open the engine compartment and remove the nut holding the spark arrestor on the carburetor using an open end wrench. The spark arrestor is a round, metal air filter.

Step 4

Attach a garden hose to the motor flush tool and place it over the vents on the out drive. Turn the water supply on and allow the air bubbles to work out of the hose before starting the engine.

Step 5

Start the engine and allow it to run for five minutes.

Step 6

Spray fogging oil in and around the carburetor until the engine dies out. This will keep the carburetor clean during the winter.

Step 7

Turn off the water supply and remove the motor flush tool from the out drive.

Step 8

Remove the drain plugs on the bottom of the engine block using an open-end wrench. Refer to your owner’s manual for drain plug location, as it varies depending on the year of manufacture. Allow the water to drain from the block completely. Reinstall the drain plugs and tighten them using a wrench.

Lower the out drive as far to the ground as possible without hitting it by pressing the button on the throttle assembly. This will keep the gears in the out drive lubricated throughout the winter.


To winterize your 4.3 Mercruiser, follow 8 easy steps. To reduce the risk of costly repairs, remember to winterize your boat properly.

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