Flushing question (start engine first? Or, water first?)

ThorSen

ThorSen

Active Member
Aug 17, 2017
219
Western North Carolina
Boat Info
2018 SPX 190 OUTBOARD 150
Engines
150 4-stroke Mercury outboard
I did several searches after reading and re-reading the Mercury 4-stroke 150hp Owners Manual and haven’t found an answer so, thought I’d post it here:

Q - When running the motor on the hard (on trailer and in order to warm the engine oil up for an oil change (in Neutral, of course) and after hooking a garden hose up the nifty hose connector they now include on the Mercury 4-stroke outboards, DOES IT MATTER WHETHER YOU START INJECTING WATER BEFORE THE ENGINE IS STARTED *OR* SHOULD THE ENGINE BE RUNNING BEFORE YOU TURN THE WATER HOSE?

(I ask as, with jet skis, it is absolutely CRITICAL that the engine be running BEFORE you ever start any water going into the machine.

When I go to warm up the engine oil on our outboard; however, either the engine will be running for a few seconds before I can hop off the boat and jog over to where the water spigot is OR the water will be running into the motors before I can clamber aboard to start the engine (in Nuetral, of course).

I just need to know which to start first, the engine or the water.

Thanks (d*mn, all this winterization talk is so d*mn depressing! )
 
Water first. If your Merc is anything like my Yamaha that nifty hose connection is for motor off, flushing only. I would put muffs on the lower unit.
 
Why not put a garden hose shutoff valve at the motor or in between two garden hoses, this way you can have the water shutoff located right next to the ignition.
 
scoflaw said:
Water first. If your Merc is anything like my Yamaha that nifty hose connection is for motor off, flushing only. I would put muffs on the lower unit.
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scofflaw, thanks, you’re probably right and geez, I had started to order a pair of muffs but canceled (back to Amazon, doh! :rolleyes: )

Sublimetime said:
Why not put a garden hose shutoff valve at the motor or in between two garden hoses, this way you can have the water shutoff located right next to the ignition.
Click to expand...
Excellent idea and we actually have something like that on our dock for the jet skis (as you actually have to start pwc engine first THEN water and you want to limit the time a pwc engine running without cooling to seconds) BUT, I was still uncertain as to which to start first (now I know, boat: start water then engine; pwc: start engine then water).

Thanks, guys! ;)
 
By design, jetskis and waverunners pump water into the water box to cool them. Water is pumped from the jet pump - metal prop with close tolerance to the pump bore. It forces water up into the engine. Depending on coolant hose routing, some engines are cooled first in the block/heads and then the water flows to the water box (series). Some are cooled in parallel - engine/heads and water box on two different loops. In effect, if the water is on too long during a flush routine, you could potentially back flow water into the exhaust which then works its way back up into the cylinder via the water box. That's why they tell you to start the engine first, so you have the assist of the exhaust pressure to clear the water box out. You can actually start and run a jetski for quite a bit of time without any water (maybe up to a minute?) without doing any damage. You'll see many folks do this after pulling the skis out of the water to blow out the water box.

Boat engines/outboards are a whole different animal. They have traditional rubber vane water pumps which require water as soon as the engines are started to keep them from melting. They just aren't plumbed the same.

Hope that helps.
 
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Doug! Your explanation - description helps tremendously - Thank YOU!

We figure if we take care of the boat (and WaveRunner! :) ) then, they will take care of us; and, we wanna get it right.

Have a great week and thanks again!
 
Earlier this year I wanted to start my I/O's on muffs before the marina racked the boat. I usually had help, but this time I was solo, so I first turn-over stbd to make sure sure'd fire. She did, so I immediately shut her off. I got out the hose and muffs, turned on the water and then snugged the muffs to the intake. Then I climbed into the boat and started the engine and revved to about 1k. About 20 seconds later, she stalled. I hit the starter and she didn't make a whole turn before suddenly stopping. I leaped down and pulled the hose off, knowing it's already too late. The weather was heating up, I had things to do and most of my tools were on the other boat, so I called the service manager over and explained to him what happened, and he agreed to pull the plugs and pickle the engine asap.

Sublimetime said:
Why not put a garden hose shutoff valve at the motor or in between two garden hoses, this way you can have the water shutoff located right next to the ignition.
Click to expand...


I kicked myself for 2 days, because I already had an inline valve. All I had to do was connect it.

The marina called me on Monday to explain that the engine was in fact hydrolocked, but with gas, not water. The carb was dumping fuel.

Even so, I'm never going to let water push into a non-running engine again. The valve idea is cheap peace of mind.
 
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There is no way for water to get into the cylinders, you could leave the hose on all day if you want. Now the jet ski is a different story
 
scoflaw said:
There is no way for water to get into the cylinders, you could leave the hose on all day if you want. Now the jet ski is a different story
Click to expand...

+1. I've flushed and worked on, literally, hundreds of boats. And many, many of them are "engines off" for a decent amount of time with the water still flowing. Any extra water would just go out of the exhaust. Most just squirts out past the flush attachment, though.

The only way I can see this happening is if there was a major blockage in the elbow or exhaust tube... but then there would already be an issue. Or, if the boat was at a severe bow-down angle.
 
Lazy Daze said:
Or, if the boat was at a severe bow-down angle.
Click to expand...

The exhausts of my 350's angle down nice and steep from the elbows, so I'm good there. I don't feel as confident about the components themselves. While they looked good at last inspection, they have spent most of their lives in saltwater and I'm always on the lookout for signs of failure. (stressing about it)

I guess you guys are right though - if water could go where it isn't supposed to on muffs, it could/would in the water too.
 
scoflaw, Lazy, Thank y’all kindly - “water on first okay” was my suspicion but, being as this will be the first time I’ve done maintenance on any boat motor, let alone outboard, I did not want to ASSume, ha-ha!

Rollercoaster, I’m glad your situation was resolved satisfactorily - I have decided to wait until I am not alone and for the Admiral to get back into town.

My plan now is to: 1) remove the prop and drain and replace the lower gear lube FIRST and; while the prop is still off, use that opportunity to, 2) put the muffs on and start the engine long enough for it to warm the engine oil up (but, not get “hot,” — the engine oil will drain much easier warm, etc.)

After I get the oil and filter changed I’ll inspect the low pressure fuel filter (I have a replacement if needed) and spark plugs and probably put some dialectic grease on the ceramic parts and anti-seize compound on the threads of the sparks, for good measure.

That should do it til the grandkids return and the towable tubes and knee boards come back out!

I’m not going to fog the cylinders as the Admiral likes taking the craft out on the frequent 60^F days we have in December and even into January.
 
Water on first if you have a rubber impeller. If you don't do that, plan on replacing that impeller.
 
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