My motor sat unused for two seasons. Do I need anything beyond this checklist?

Probably yes. Two-year storage usually means stale fuel (definitely drain), possibly damaged fuel lines, and a higher chance of impeller failure. A full pre-launch service is worth the few hundred dollars. We've seen multiple cases where the owner skipped this and the first start did $3,000-$5,000 of damage.

Is it ok to run my Mercury on a hose with earmuffs for the first start?

Yes, with a properly-fitted flush attachment that delivers enough water flow. This is how dealers do first-starts for new motors. It's also how careful owners verify the cooling system before the boat goes in the water. Just confirm strong water flow at the muffs and the telltale before running.

My battery reads 11.8 volts. Can I jump-start it and go boating?

You can, but you shouldn't expect the battery to recover under boat use. Jumping a deeply-discharged battery to start the motor is a one-way trip: the alternator output isn't enough to fully recharge a deeply-discharged marine battery under typical use. Replace the battery, then go boating.

Does ethanol-free premium fuel solve all my fuel problems?

No, but it helps. Ethanol-free fuel doesn't phase-separate in storage and doesn't attract water. It's worth the extra cost if available, especially for boats that get used infrequently or stored for long periods. Stabil or Sta-Bil 360 also helps for any fuel.

Should I always change the impeller in spring?

Mercury's recommendation is every 3 years or 300 hours of use. In Ontario freshwater, many impellers last 4-5 years. If you have any doubt, $200-$400 to replace it is cheap insurance compared to overheating and seizing the powerhead.

What's the worst spring issue you've ever seen?

A customer who didn't check anything, didn't watch for telltale water, and ran for 10 minutes at cruise with a failed impeller. The motor overheated, the head warped, the powerhead seized. Total damage estimated at $7,500. The whole disaster was preventable with a 30-second look at the pee-hole.

Mercury Spring Run-Up Checklist

Quick answer Before you launch your Mercury outboard for the first time after winter storage, work through this five-minute checklist: check the battery voltage and connections, inspect the fuel for separation or stale fuel, look for damaged or dry-rotted fuel lines, confirm...

Mercury Premier Dealer·Family-owned since 1947·Mercury dealer since 1965·Gores Landing, ON·Quote builder available

Quick answer

Before you launch your Mercury outboard for the first time after winter storage, work through this five-minute checklist: check the battery voltage and connections, inspect the fuel for separation or stale fuel, look for damaged or dry-rotted fuel lines, confirm the propeller is tight and undamaged, check the lower unit oil for water contamination, and at the dock, watch for the telltale water stream from the cooling system within 30 seconds of starting. If anything looks wrong, do not run the motor under load. Most Ontario spring no-starts and overheat events are preventable with this exact checklist. It takes longer to read this paragraph than to actually do it.

Why spring run-up matters in Ontario

A Mercury outboard that sits from October to May goes through five months of cold, fuel breakdown, condensation, and pest activity. Most boats come out of storage just fine. The ones that don't usually fail in one of three ways: they won't start, they overheat within 5 minutes, or they run rough and stall. All three failure modes are predictable. All three are preventable.

We see roughly 40 spring run-up issues in a typical April-May at our service shop. About 30 of those would have been avoided if the owner had spent 10 minutes on a structured run-up before launching.

The checklist

Before you leave home

1. Battery voltage and terminals. With the motor off and disconnected from any chargers for at least an hour, measure the battery voltage with a multimeter. You want to see 12.4-12.7 volts on a healthy fully-charged battery. Below 12.2 volts means the battery is undercharged or failing. Cracked, swollen, or leaking batteries should not go into the boat: they should go to a recycler.

Clean the battery terminals if there's any white powder or corrosion, tighten the terminal bolts, and confirm the battery is secured in its tray. A battery that slides around in chop is a battery that loses contact at the worst time.

For more on battery selection and care, our Mercury Boat Battery Guide covers the full conversation.

2. Fuel inspection. Open the fuel tank and look at the fuel. Fresh gasoline looks pale yellow and smells like gasoline. Old fuel looks darker, often with a varnish smell. Phase-separated ethanol-blend fuel looks cloudy or has visible water at the bottom. If any of those describe your fuel, drain it before launching. We see ethanol-related fuel issues every spring and they account for a meaningful share of our spring "won't run right" calls.

3. Fuel lines. Visually inspect the fuel line from tank to motor. Look for cracking, dry rot, kinks, or rodent damage. Mice can chew through a fuel line in a single weekend if the boat sat unmonitored. Replace any line that looks suspect: $20-$40 in parts versus a $4,000 powerhead repair after fuel-air mixture damage.

4. Propeller condition. Spin the prop by hand (motor off, in neutral). It should spin freely but with some resistance from the gearcase. Look for dents, chunks taken out of blade edges, or fishing line wrapped around the shaft. Confirm the prop nut is tight (don't over-tighten, the nut should be hand-snug plus 1/4 turn with a wrench).

At the launch ramp

5. Trim and tilt operation. With the motor down, press the trim button up and down through the full range. The motor should pivot smoothly and quietly. Grinding, hesitation, or no movement at all means the trim hydraulic system needs attention.

6. Lower unit oil check. This is the one most owners skip. Remove the lower oil fill plug (the upper plug, near the top of the gearcase) and look at the oil. Healthy gear oil is dark green or brown and smooth. Milky white or chocolate-milk colored oil means water has gotten in, which means the seal needs replacing before you run the motor. Skipping this check and running a motor with water-contaminated gear oil is how you turn a $200 seal job into a $2,000 gearcase rebuild.

Starting the motor

7. Prime the fuel system. Squeeze the primer bulb until it's firm. This pushes fuel from the tank to the motor and confirms the line is clear.

8. First start. With the motor down in the water (always, never run a Mercury outboard out of water without a flush attachment), key on, choke or fast-idle as needed for cold starts, and crank.

A healthy motor should start within 3-5 seconds of cranking. If it cranks but won't start, stop after 10 seconds, wait 60 seconds, and try again. Repeatedly cranking a flooded motor can drain the battery and overheat the starter.

9. Telltale water stream. Within 30 seconds of the motor starting, you should see a steady stream of water exiting the small "pee hole" on the side of the motor. This stream confirms the water pump is working. If you don't see water flow within 30-45 seconds, shut the motor down. Running without water flow will overheat the motor in under 2 minutes and cause expensive damage.

A weak or intermittent water stream means the water pump impeller needs replacement, which is a routine $200-$400 service item we do dozens of times every spring.

10. Idle check. Let the motor idle for 2-3 minutes at the dock. Listen for unusual sounds (knocking, hissing, irregular firing) and watch for smoke beyond the normal first-start exhaust. Brief blue or white smoke at first start is normal as oil burns off. Continuous heavy smoke means something is wrong.

11. Test under light load. Idle out from the dock at no-wake speed. Run for 2-3 minutes at idle to confirm everything is steady. Then bump up to 2,500-3,000 RPM for another 2-3 minutes. Watch the gauges (if you have SmartCraft, watch RPM, fuel flow, water temp). Listen for new sounds.

You can build a live CAD quote for your repower online at Mercury Repower Centre.

12. Test under full load. Once you've confirmed everything looks normal at low and medium RPM, run the boat through the full RPM range. Trim through the range. The motor should accelerate cleanly to its target WOT (wide-open throttle) RPM.

If anything feels wrong during this sequence, stop. Don't push through. Most spring problems get worse, not better, when ignored.

Technician checking the cooling system telltale stream on a Mercury FourStroke outboard during spring run-up.

When something is wrong, the most likely culprits

Won't start at all: dead battery, bad ground, flooded engine, or a fuel system issue. In that order, check those four.

Starts but won't stay running: stale fuel, clogged carb or fuel injector (less common on modern Mercury FourStrokes with EFI), or a fuel filter that needs replacing.

Starts but runs rough: ethanol-related fuel issue, weak spark from old plugs, or a vacuum leak somewhere in the air intake.

Starts but no water flow: impeller failure. Stop the motor. Do not run. We've seen impellers go from "fine in November" to "failed in May" sitting through one winter, especially if the boat was stored with the lower unit slightly tilted up and the impeller compressed against the housing.

Overheats within 5 minutes: impeller, blocked water intake (mud, weeds, mouse nest), or a stuck thermostat. Same rule: stop the motor.

Our Mercury Outboard Won't Start Troubleshooting Guide and Mercury Overheating Guide cover these in more detail if you want to dig deeper.

What we see at HBW

The single most common spring issue we diagnose is a weak or dead battery. Number two is a fuel-related issue (stale fuel, ethanol separation, or fuel line problems). Number three is impeller failure. Together those three account for roughly 80 percent of our April-May service calls. All three are 5-10 minute checks the owner can do themselves.

If you want a structured walk-through of any step, email info@harrisboatworks.ca with your motor model and what's not behaving. For full spring commissioning service where we do the run-up plus a full inspection, our intake is at hbw.wiki/service.

For engine repairs, we only service Mercury and Mercruiser.

Sources

Mercury Marine seasonal maintenance procedures (dealer technical reference, 2026)
Transport Canada Pleasure Craft Safety Guide
HBW spring service intake records, 2020-2026

About the author

Reviewed by the Harris Boat Works service team. HBW handles Mercury and Mercruiser service from Gores Landing on Rice Lake. For engine repairs, we only service Mercury and Mercruiser. About Harris Boat Works.

Ready to price it out? Build a live CAD quote for your repower online at the Mercury Repower Centre.

Frequently Asked Questions

My motor sat unused for two seasons. Do I need anything beyond this checklist?: Probably yes. Two-year storage usually means stale fuel (definitely drain), possibly damaged fuel lines, and a higher chance of impeller failure. A full pre-launch service is worth the few hundred dollars. We've seen multiple cases where the owner skipped this and the first start did $3,000-$5,000 of damage.
Is it ok to run my Mercury on a hose with earmuffs for the first start?: Yes, with a properly-fitted flush attachment that delivers enough water flow. This is how dealers do first-starts for new motors. It's also how careful owners verify the cooling system before the boat goes in the water. Just confirm strong water flow at the muffs and the telltale before running.
My battery reads 11.8 volts. Can I jump-start it and go boating?: You can, but you shouldn't expect the battery to recover under boat use. Jumping a deeply-discharged battery to start the motor is a one-way trip: the alternator output isn't enough to fully recharge a deeply-discharged marine battery under typical use. Replace the battery, then go boating.
Does ethanol-free premium fuel solve all my fuel problems?: No, but it helps. Ethanol-free fuel doesn't phase-separate in storage and doesn't attract water. It's worth the extra cost if available, especially for boats that get used infrequently or stored for long periods. Stabil or Sta-Bil 360 also helps for any fuel.
Should I always change the impeller in spring?: Mercury's recommendation is every 3 years or 300 hours of use. In Ontario freshwater, many impellers last 4-5 years. If you have any doubt, $200-$400 to replace it is cheap insurance compared to overheating and seizing the powerhead.
What's the worst spring issue you've ever seen?: A customer who didn't check anything, didn't watch for telltale water, and ran for 10 minutes at cruise with a failed impeller. The motor overheated, the head warped, the powerhead seized. Total damage estimated at $7,500. The whole disaster was preventable with a 30-second look at the pee-hole.

Harris Boat Works · 5369 Harris Boat Works Rd, Gores Landing, ON · (905) 342-2153

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