How to flush your outboard correctly and why it’s so important

Author: Stuart Buckingham   Date Posted:22 May 2017 

Blog How to flush your outboardOne of the biggest killers of the modern outboard is corrosion and often it's not what you see, but rather what you don't see that causes the biggest problems.

Outboards are raw water cooled and draw water directly from their environment to cool down. In fresh water, this is not a problem but in saltwater, it opens the engine up to salt build up and corrosion within the outboards internal cooling passages. The build up of salt over time can cause overheating issues or, worst case, corrosion that will eventually eat away at your engines block destroying your outboard.

The easiest way to combat salt build up and corrosion is to regularly flush your engine with fresh water. Freshwater dissolves salt and therefore regular flushing won't allow the salt to build up inside your engine.

Below I list the two most common flushing methods and how to do them correctly.

Ear Muff Method
'Ear Muffs' allow you to run your engine with fresh water using a standard garden hose to clean out both the block and water pump of unwanted sand and salt.

  1. 1.  Place ear muffs (grab yours here) over water intake grill on lower unit/gearbox making sure they fit firmly
  2. 2.  Connect garden hose to ear muffs making sure that your hose fitting clicks on
  3. 3.  Ensure outboard is trimmed down to be as vertical as possible
  4. 4.  Turn on hose and check your hose is connected to ear muffs correctly and there is good water pressure
  5. 5.  Start outboard, making sure that your outboard is pumping water from tell tail
  6. 6.  Run outboard for 2 to 5 minutes to ensure the thermostat opens and fresh water passes through all internal cooling galleries
  7. 7.  Once completed, switch off your outboard and then turn off your hose.

Drum/Tank Method
When you have a small HP outboard (under 25hp) the lower unit may be too small for aftermarket flushing devices to fit properly. The engine manufacturers may have custom flushing devices but these can sometimes be expensive and tricky to fit. Creating a homemade test tank may be the best solution. They're easy to make (Bunnings wheelie bins make great test tanks) and they'll save you wasting litres of water every time you want to flush your engine.

  1. 1.  Back boat up to drum or tank
  2. 2.  Lower outboard into water, ensuring the water level is around 2 inches above the cavitation plate
  3. 3.  Try to make sure your outboard is as vertical as possible
  4. 4.  Start engine, run in neutral for 2 to 5 minutes allowing the thermostat to open and fresh water to circulate throughout the internal cooling galleries
  5. 5.  When complete, switch off outboard and remove from tank

I really can't stress how important it is to flush your outboard after every use. If you keep to a regular flushing routine you are giving your outboard the best chance of having a long and healthy life.

To shop for ear muff outboard flushers, click here.

Comments (29)

yamaha 60 hp 4 stroke

By: on 10 November 2023
if i have the leg off to change impeller i also have thermostat completly removed housing off ect connect top flush garden hose water is coming from tell tale and bottom leg but not a drop coming out off thermostat opening does this mean a blockage somewhere

Outboard Spares Response
Hi there, Try blocking the water tube where the water pump connects to and wait for the water to fill up and you should then see water come out from the thermostat housing. If it doesn't come out then you most likely have a blockage somewhere.

Did I ruin my motor?

By: on 16 May 2023
I flushed my Yamaha 2 stroke motor in a 55 litre barrel for about 5-8 minutes. It was running and getting high end power fine before hand but now has no high end power. I’m worried carbon may have gotten into the pistons and scratched them causing the issue? There was black debris all over the prop which I had to wipe off annd the whole exhaust was very black. There is also black residue on the pull cord from after the rinse. Only other thing was it got dry started to check if it would start and ran for about 3-5 seconds. Anyone have any insight?!

Outboard Spares Response
Hi there, Start with a compression test and make sure they good. Whilst the sparkplugs are out make sure they are sparking. You might not be getting spark to all cylinders which will show in your exhaust as you stated. Also it could be running on a very rich fuel oil ratio. it needs to be 50:1


By: on 4 April 2023
How long should I run my 120hp 2stroke Merc outboard for the thermostat to kick in and can it damage the motor if no water comes out of the telltail after 5 mins Cheers Rob

Outboard Spares Response
Hi Rob, A thermostat will open approx after 3-5min of running from cold. If you are not getting water out of the tell tale it could be blocked or it isn't getting water which means most likely your waterpump is no good. I would suggest checking the waterpump regardless and fit a new one if there is no history of it. An engine can get damaged if it runs hot. 5 minutes with no water cooling it at all will be no good for it . If in doubt, do a compression test and make sure all cylinders are still within 10-15% of each other and above 110p.s.i.

Best way to flush a 3.3 Johnson outboard

By: on 17 January 2023
Have recently purchased a 3.3 hp Johnson. What is the best way to flush it? I’ve tried with muffs but can’t get water to come out the tell tale. Worried it will overheat. Is the water intake the single small hole to the front of the prop? Tell tale works fine when in the water. Tried a bucket of water but sprays water everywhere and empty’s the bucket. Thanks.

Outboard Spares Response
Hi, You'll need a drum to run this engine in. If it has neutral then just keep it in Neutral. If it has no neutral, then just keep the RPM as low as you can.

Oil coming from prop area

By: on 23 February 2022
I bought an old 5 hp Mercury outbaord and used for the first time yesterday. I did not flush it but noticed that there is oil leaking from the propeller area. Have I done any damage and is it ok to flush it in fresh water today?

Outboard Spares Response
Good Day, This is not uncommon on 2 stroke carburettored outboards. Unburnt oil/fuel can run down through the exhaust housing and dribble out around the propeller as outboards typically have through hub exhaust systems. If it is really black and sooty colored oil then I'd dare say this is what it is and it is nothing to worry about. The best way to double check in case it is gear oil is to drain the gear oil and check the color. If the color is totally different then you'll know it's not gear oil. If it is the same color, OR your gear oil is milky in color OR has obvious traces of water then you may have an oil seal issue. In this case just replace the oil seal, (then pressure test the gear box if you can get the tool) and refill the gearbox with fresh oil. Job done.

Water port flush

By: on 26 September 2021
What's the difference between the flush systems mentioned and the hose port method on myV6 Mercury. Do l need to run the motor and will I burn the water pump impeller.

Outboard Spares Response
HI, The hose port method is to be used without the engine running. This was designed for marina/docked boat use where the engines could not be flushed with an earmuff type flushing device easily. The earmuff style flushers are the most common and they are to be used with the engine running. Cheers

No earmuffs for old Mercury 7.5

By: on 20 October 2019
Hi. I’ve an early 80’s Merc with a single water port down the bottom. There are also two screws with the word flush on them up high near the engine. Are these for the earmuffs? Or should I insert a hose into the bottom? Thanks for your help.

Outboard Spares Response
Hi, from memory the old 7.5hp had a screw in flushing device that you could buy. I'm not even sure if they are available anymore. A drum of water is what most guys would flush these out in now. Make sure the water is well above the cavatation plate though so that the water-pump can prime easily. Cheers

Flushing motor

By: on 25 August 2019
Have a 4 stroke Yamaha motor 60 hp when flushing out board always taste water yesterday after using and when flushing could taste oil in water is this serious what could the problem be ?

Outboard Spares Response
Wow, that is next level engine care right there! I've never tasted the water, but I assume you are doing it to confirm the salt has been diluted from the engine. As for tasting oil. If you are running it on a flushing device and tasting from tell tail, you probably shouldn't be tasting oil. Could be worth checking engine oil color on dipstick to ensure you don't have a leak (water entering and exiting the sump). I'm no connoisseur of "flushing water" but my advice, when in doubt take it to a marine tech. Cheers

15 HP mariner

By: on 1 July 2019
I've got no water coming from telltale ,I've replaced impeller still can't get all the the gunk out of outlet, help please

Outboard Spares Response
Hi, It depends on what type of blockage you have. You could try blowing compressed air through the tell-tail (back blowing) which may assist in removing any debris that is causing the blockage. There are a couple of questions you'll need answer. Is the engine over-heating? Or is there plenty of water coming out the exhaust relief holes? If the engine is over-heating and you can't clear the blockage in the tell tail, it may be more in depth, like a blockage caused by internal corrosion or salt build up. If this is the case, depending on which Mariner 15hp you have (there is the old one produced by Yamaha, the USA Model and the Tohatsu based model) you may need to consider removing the side plates (to clear out any salt) or possibly lifting the powerhead if the blockage is caused by the water-tube grommet shrinking/collapsing due to corrosion. Hopefully I've been of some help. Good Luck.

Mercury 4 stroke flushing

By: on 26 May 2019
Can starting the engine while flushing using the direct connect hose damage the engine.

Outboard Spares Response
HI, to my knowledge you cannot run an engine on the direct flushing attachment. One reason is that the impeller in the lower unit would be running dry which would destroy it and your water-pump. So when flushing the engine with the direct hose attachment do not run the engine. Cheers

Flushed water into gearbox

By: on 27 February 2019
My mate borrowed my boat and whilst flushing motor he accidently used an attatchment that caused him to flush the gearbox and it spat out oil. Is this bad, or an easy fix. Will we need new seals? Its a 40hp Yamaha 2 stroke outboard motor. Tanks

Outboard Spares Response
Hi, I think your mate owes you a few beers or maybe a new gearbox. I would be draining the gearbox oil (undo the bottom and top drain plugs) which will dump any oil and water that's in the gearbox. Then I'd be stripping the gearbox ASAP before any rust begins to form on the gears etc (if it has not already). If you can't strip the gearbox right away then I'd refill it with fresh gear oil to ensure no air gets to the internals which will help to preserve the gears until you can strip it down. Once the gearbox has been stripped down then you'll be able to evaluate what you need. Time is precious so I'd be moving pretty quickly on this one. Cheers

Draining two stroke fuel from tank

By: on 26 February 2019
If I use my boat on fortnightly basis do i need to drain 2 stroke fuel from tank and diguard it.

Outboard Spares Response
Thanks for your question. The answer in short is "No". You really only need to disguard it if it is over 6 months old (approx) and the boat is kept in an area of high humidity or sub zero conditions. A good idea in these instances is to use a fuel stabiliser which helps to retain the fuels "light" components. It's also a good idea to keep the tank full rather than half empty. The less "air" in the tank will reduce the likelihood of moisture/condensation forming in the tank when sitting for long periods. I hope this helps. Cheers!

2010 yamaha 150 Four Stroke Flushing question?

By: on 19 February 2019
I am looking at buying a 2010 Parker 21 SE with a 2010 Yamaha 150 Four Stoke..... While looking over the boat all was well "until" the owner told me that the Boat was kept on a "lift" at the end of a Saltwater dock and it was NEVER FLUSHED...EVER!..... Again the Boat was ran in Saltwater and kept on a Lift outdoors in a Saltwater Environment.. Question.... How much damage has been done to the motor with approx 500 hours on it and "Never Flushed"!?

Outboard Spares Response
Hi, Thanks for you question. Wow! Ok, well if the engine was used everyday or every second day then it may be ok. The engines do have internal anodes on them to corrode before the aluminium does inside. However, if not used very regularly then I'd be concerned. This is me though. Some people may say it'll be ok, but I'd be concerned about internal corrosion and salt accumulation on an engine that is now 8 years old. Possibly get a technician to check it for you by pulling out the anodes in the head and inspecting them. They'll be able to tell pretty quickly if the engine is suffering from any corrosion outside of what would be expected of an engine that age. Cheers

225 Yamaha flushing fuel off or turn key off

By: on 29 November 2018
I have a 225 fuel injection Yamaha when flushing motor is it better to turn fuel off tell it runs out or turn it off after 5 minutes with the key .

Outboard Spares Response
Thanks for your question. Just run your engine as you normally would at idle while flushing your engine out. There's no real need to run the engine out of fuel. Cheers

Flush test

By: on 8 November 2018
Tasting the water from the tell tale does not indicate a flushed motor.The open thermostat indicates a flushed motor so the motor must be run until it is warmed up and the thermostat opens.The engine passages could still be constricted but if it is not overheating you are right to go.


By: on 3 November 2018
Ho do you know when the thermostat has opened

Outboard Spares Response
Thanks for your question. When water begins coming out the exhaust relief outlets on the exhaust housing that's when the thermostat has opened.

salt away

By: on 1 November 2018
a big rapp for the salt away, I use a cap full in a drum, then another quicker flush with straight fresh water, the makers also recommend doing this

Yamaha flushing coupling

By: on 1 November 2018
My Yam 60 4 stroke has the 3/4" flushing coupling. I understand these were partly supplied for moored motors to allow them to be flushed. I have been told that these flushes don't get to every part of the motor particularly the water pump. I always start the motor but only briefly while flushing to make sure the water pump gets flushed. When I did put a pump kit in the pump it was as clean as a whistle.


By: on 1 November 2018
Ive heard vinegar is a great salt away to wipe down clean salt from power head etc leg and all those difficult places a hose just can wash

Running the engine dry

By: on 1 November 2018
We have 3x 25hp for inflatable surf rescue boats, we have been told to pull the fuel line from the engine to while flushing it, to run it dry. Is there long term problems with doing it this way? What would be the pros vs Cons - cheers

Outboard Spares Response
Thanks for your question. There are varying opinions on running a 2 stroke pre-mixed engine dry. Many believe that doing this often can cause wear on the cylinders of the engine over time.. Two stroke engines require that are pre-mixed get their lubrication from the fuel mix. Therefore it is said that running them dry of fuel is starving them of lubrication. Obviously this is more a problem at High RPM rather than Low RPM. If you are wanting to remove the residual fuel in the carburetor it is better to drain the carburetor bowl using a spanner with a rag/cloth underneath to absorb the small amount of fuel that drains out. Additionally, if you are using the outboards regularly you probably don't need to worry about draining the fuel at all. Cheers

Flushing outboard motors

By: on 1 November 2018
I have been flushing religiously using both the ear muffs and a tank ever since new. Boaties should be aware that this is not a guarantee that the impeller will never need replacing. I always check my water tell tale before launching my boat but last time no water was seen. I had to replace the impeller which had sustained enough damage from sand, etc. to render it useless. Cost of the impeller was $25.00 (from Outboard Spares) taking about half an hour which I did myself. It always pays to check for water flow before leaving home or the ramp. Happy boating.


By: on 1 November 2018
Where does the salt go when flushing in a drum? back into the water and your engine, if flushing in a tank/drum the water must be replaced at least once, preferably twice during the process, you can add a silicone based additive to the final flush thus coating the internal surfaces of the motor, displacing water and providing an oxygen barrier, fabric softner provides a very good additive and cost about 60c per use.


By: on 1 November 2018
My Suzy has a flushing port to connect hose to, I then run about 20lts water thru without running motor. Is this method ok, my yamaha had same.

Outboard Spares Response
Thanks for getting in touch. Any type of flushing is better than not flushing the engine out at all. You are best to run the engine while flushing it out to allow the thermostat to open. I don't believe you can do this on the factory fittings, but I may be wrong. Your owners manual will confirm this. Cheers


By: on 29 May 2018
Hi , I just got an old half oil drum for this purpose but it still has a lot of oil residue it it, will a small amount of oil in the water hurt anything or do I need to thoroughly clean it out first ? Any tips on how to clean it ? thanks. Lynton

Outboard Spares Response
A little bit of oil shouldn't be anything to worry about but if you're worried a degreaser should clean it up pretty well before filling it with water.

Flushing outboards

By: on 25 July 2017
yea guys i do the taste test when flushing my motors after use as i can not see if the salt is gone , seems the best way to do it .


By: on 24 May 2017
we sometimes get away camping where there is no water to flush motor, sometimes we are away for 2 weeks, we cant flush motor till we get home, how much damage are we doing??

Outboard Spares Response
Your engine will be fine if you're using it almost everyday while you're away. The damage can start to take affect if the outboard was used and sat for a long period of time without flushing after use.

camping away

By: on 23 May 2017
I take a 20lt water tank with a 12v pump and fit my muffs to it and flush my motor every time It comes out of the water when away camping at the very least until I get home.

Outboard Spares Response
Sounds like a good system you've got and nice work saving water where you can.

Salt Away

By: on 23 May 2017
Is Salt Away a good option to run through my out board . If so how often should I run it through Regards Rob TRob R

Outboard Spares Response
Hey Rob, we actually haven't ever used Salt Away so we can't really comment on how well it works but people on our Facebook page have said good things about it. We tend to use plain old fresh water as soon as we can on our outboards and that works just fine as long as you flush it for long enough.

Taste the telltail water

By: on 23 May 2017
I let my outboards run with fresh water until I can't taste the salt in the telltail water anymore. Usually about 5-10 minutes.

Outboard Spares Response
Nice one! It definitely is a popular method of knowing you've flushed all the salt out.

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