Trying to understand how running with one engine off could cause engine damage?

[DesertDweller]

I have read that you shouldn't go over 5MPH with an engine out as water will be forced into the engine. I'm a relative newbie to jetboats, but how does the cooling system water get into the cylinders? I understand that there is a water intake somewhere and that water may be forced into the cooling loop but, I just can't quite comprehend how this could damage the engine.

 

[Cobra Jet Steering LLC]

The pump provides water to cool the engine but the exhaust pushes the water out of the muffler so it does not build up in the water box, turn off engine and drag the pump through the water fast and you are forcing water into the pickup for the cooling line to the engine and it will fill the muffler "water box" with water , since the engine is not running the water works it's way up in the exhaust manifold and bad news, if you make sure the reverse is UP and not down you have a better chance of NOT pushing as much water into the cooling system and staying at slow speeds will usually avoid having enough power to get the water to fill the muffler.

 

[FSH 210 Sport]

The water intake is not an intake per se’, the water is pressure fed from the jet pump into the cooling system. There is a small square block on the side on of the jet pump that is the filter screen where the water goes into the cooling system.

The cooling system water not only cools the engine it also cools the exhaust and this is where the issues begin when towing. One or more of the exhaust valves on the engine will be open, so when you are towing over 5mph the water pressure is great enough to force its way into the cooling system and the exhaust cooling, and since the exhaust goes through water filled mufflers (water locks) there is pressure in there and will fill up the exhaust manifolds and water will get into the engine via those open exhaust valves. So now you have at least one cylinder that is filled with water, when you attempt to start that engine it will hydraulic and bend the rod or two and your motor is trash.

Make sense ?

 

[DesertDweller]

The water intake is not an intake per se’, the water is pressure fed from the jet pump into the cooling system. There is a small square block on the side on of the jet pump that is the filter screen where the water goes into the cooling system.

The cooling system water not only cools the engine it also cools the exhaust and this is where the issues begin when towing. One or more of the exhaust valves on the engine will be open, so when you are towing over 5mph the water pressure is great enough to force its way into the cooling system and the exhaust cooling, and since the exhaust goes through water filled mufflers (water locks) there is pressure in there and will fill up the exhaust manifolds and water will get into the engine via those open exhaust valves. So now you have at least one cylinder that is filled with water, when you attempt to start that engine it will hydraulic and bend the rod or two and your motor is trash.

Make sense ?

So even though the jet pump is not turning or pumping there is enough pressure at speed to turn the shaft/pump and the engine but, because there is no exhaust pressure the muffler fills and eventually backs up thru the exhaust manifold and the valves? Wouldn't that set up a venturi effect on the water in the muffler/ exhaust system to drain it out while underway? Is there any Yamaha reference in the manual to the possibility of this happening or our owners learning the hard way? Actually I just now see that in the manual under towing and "running on one engine". Glad this forum is here to make owners aware!

 

[FSH 210 Sport]

So even though the jet pump is not turning or pumping there is enough pressure at speed to turn the shaft/pump and the engine but, because there is no exhaust pressure the muffler fills and eventually backs up thru the exhaust manifold and the valves? Is there any Yamaha reference in the manual to the possibility of this happening or our owners learning the hard way?

The engine does not turn while off and being towed above 5 mph. Just the boat moving through the water above 5 mph is enough to push water up into the cooling system of the non running engine.

Yes, the warning is in your owners manual about towing the boat above 5 mph with out the engine running. You can find clamps made for pinching off the cooling water inlet line in the engine bay, do a search for that topic and several threads should come up about it.

 

[Cobra Jet Steering LLC]

It does not turn anything the water being forced into the pump body through the intake grate with the boat moving creates pressure to fee water to the cooling system. And with the engine not pushing it out the exhaust you have a problem

 

[fatboyroy]

Its also exact same reason when flushing engine you 1)Turn engine on 2) turn water on and stopping is reverse 1)turn water off 2)turn engine off. Just in case youre not aware

 

[DNUGENTUSMC]

Technically it's not the engine being off that causes the problem, it's if you go to start it after without pulling the plugs. Plugs in causes hydro-lock if there's water in the cylinder and the starter goes to pound the piston against it.

However, I just pull the plugs and crank it for 5-10 seconds.

 

[kjr6306]

Keep a pair of hose pliers in your onboard tool box. If you lose an engine clamp off the intake hose at the Y pipe and you won't have to worry about it. Idling a few miles back to the dock after losing an engine is NO fun. Ask me how I know.....

 

[Julian]

Keep a pair of hose pliers in your onboard tool box. If you lose an engine clamp off the intake hose at the Y pipe and you won't have to worry about it. Idling a few miles back to the dock after losing an engine is NO fun. Ask me how I know.....

This!

I have a set of hose clamp pliers (plastic ones from amazon) in my boat just in case I need them. Thankfully never used them!

 

[212s]

This!

I have a set of hose clamp pliers (plastic ones from amazon) in my boat just in case I need them. Thankfully never used them!

I bought a set of clamps for this reason too. I haven't needed it, but in case I do, I should be fine.

 

[Billtex2000]

Since we're on the subject;
Does anyone have a pic of where to put the clamp handy?
Just as a refresher.

 

[kjr6306]

This should help you....it's not 100% correct but @ 3:43 you will see the Y pipe. Mine are at the bottom of the rearmost bilge.

 

[Billtex2000]

So, we're clamping at this end?

 

[kjr6306]

Sure. Just follow the end of the hose toward the back of the boat. Blocking the large hose from water coming from the inlet screen prevents the water from making its way back up into the exhaust. It's more obvious when you look at in the boat. Take a piece of tape and label each one so you aren't second-guessing yourself if you ever have a problem.