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post #1 of 15 Old 04-10-2013 Thread Starter
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Exhaust Manifold Flooding with Cooling Water

I've read several cautions about excessive pre-start cranking potentially causing the exhaust manifold to flood with water, and I'm hoping someone can confirm my understanding of the physics behind this.

My engine is a raw water cooled Yanmar 2QM15, with the cooling water discharging into a mixing elbow, which connects to an exhaust hose, which runs through a water-lift muffler, through a loop under the deck, and then out the transom. The mixing elbow is about 18 inches above the waterline, but below the top of the loop.

My assumption is that prior to cranking, the level of water in the exhaust hose matches the water line. But during cranking (before the engine starts and pushes the water out the exhaust system), the cooling water collects in the hose until the water level eventually reaches the manifold, when it can run into the block.

Do I have the physics right here? How much cranking would it likely take to fill the hose? 30 seconds? 1 minute? (My engine takes about 15 seconds to start when it's cold). And if excessive water accumulates in the hose, is there a simple way to drain it out? (I guess I could temporarily drop the loop to allow gravity drain).
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post #2 of 15 Old 04-10-2013
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Re: Exhaust Manifold Flooding with Cooling Water

You have it right.. how long it would take depends on the length of the exhaust run, the volume of the waterlift, and how much water your RWC pump actually moves through the system. 30 seconds seems a reasonable average time at which point your should probably be a little concerned about it.

In point of fact the water will continue to be pushed out while you're cranking, but it's when you stop cranking the water buildup (could be up to the height of the loop just before the exhaust) will run back due to gravity and equalization, possibly finding an open exhaust valve through which to fill your cylinder. Trying to compress incompressible water leads to 'hydrolock'.. ie the motor can't turn through the compression stroke if water's in the cylinder.

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Re: Exhaust Manifold Flooding with Cooling Water

How long it takes to fill, depends on the installation. The water pump capacity, the hose lengths, the muffler volume...too much stuff.

Bottom line is that if your engine doesn't fire up right away, you want to shut down the raw water intake and then scurry back and open it again immediately after starting.

There are probably no drains on your muffler, etc. so if you want to drain things, you'll have to take off a hose.

Why is it done this way? Because its cheap.
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post #4 of 15 Old 04-10-2013
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Re: Exhaust Manifold Flooding with Cooling Water

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Originally Posted by hellosailor View Post

Bottom line is that if your engine doesn't fire up right away, you want to shut down the raw water intake and then scurry back and open it again immediately after starting.
hello's bottom line should be "hello's law"...if your raw water cooled engine is slow to start then shut the raw water intake until it does start and then open the intake.

Don't mess around with this. Water in a cylinder of a cranking or running engine can instantly ruin the engine beyond economical repair.
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Re: Exhaust Manifold Flooding with Cooling Water

I've been thinking about putting a SpeedSeal plate on the water pump anyway -- this conversation makes me think I should get the SpeedSeal Life (which includes a teflon plate to reduce impeller wear and allow a limited amount of dry running), and then just implement a starting procedure where I open the intake shutoff after the engine starts, rather than before, as a matter of routine.

Welcome to Speedseal Technical Details
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post #6 of 15 Old 04-12-2013 Thread Starter
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Re: Exhaust Manifold Flooding with Cooling Water

So I called to order the SpeedSeal (the US number still connects you to the UK). Turns out that my 2QM15 had two different pumps over the years, one of which they sell a plate for, and the other they do not. So I've got to do some measuring.

But that made me remember a discussion I had with Sen-Dure (previous makers of a fresh water conversion kit for this engine -- see separate thread for details), in which it was recommended that a fresh water pump (if added) be electrical, and mounted to the bulkhead.

I don't think I'll be making the fresh water conversion, but I got to wondering -- why not convert my raw water pump to electrical? Along with making the impeller much more accessible, it would also give me the ability to turn the pump on and off from the engine panel. That way, the cranking/flooding issue is eliminated, because I wouldn't turn the pump on until after the engine starts. Might offer other advantages for engine flushing, etc.

Downside would be lower system reliability, since engine functionality would depend on battery or alternator.

Anyone ever heard of doing this? Any downsides I'm missing?
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Re: Exhaust Manifold Flooding with Cooling Water

Rusty, the pro and con of electric-vs-mechanical pumps is an old story. Fuel pumps, oiul pumps, water pumps, air pumps, power steering mechanisms...

If you lose electricity for any reason, you lose the pump. So a direct mechanical pump is usually considered more reliable and cheaper. If you've got faith in your electricity, by all means, you can go electric. Just remember that if you lose electricity for any reason, you'll also have to shut down the engine or burn it out. And if the electric pump has a loose wire or blows a fuse...you have no warning and again, the engine is in danger.
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post #8 of 15 Old 04-17-2013
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Re: Exhaust Manifold Flooding with Cooling Water

I thought about the same thing. Have an electric pump as a back-up to the engines mechanical pump. Even thought about using a cheap bilge pump to pump the sea water through the engine. But then I thought about this: The engines mechanical pump flow rate is proportional to its speed. The faster the engine goes (rpm) the higher the mechanical pumps flow rate, and the more cooling water the engine needs (to disipate the higher heat at higher hp output). In would be imposible for an electric pump to do this (unless you want to install a Variable Speed Drive on it). With an electric pump you could flood the cylinders at low or no RPM.

So now I carry a new spare mechanical water pump- should take no more than 30 minutes to change out (and probably more like 15).
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Re: Exhaust Manifold Flooding with Cooling Water

" In would be imposible for an electric pump to do this "
You are forgetting, there is no need for the electrical pump to mimic a WEAKNESS of the mechanical pump. Regardless of the pump type there should be a thermostat, which is restricting the flow according to thermal needs.

So it doesn't matter how the flow rate from the pump varies, the thermostat has the only opinion that matters there.
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Re: Exhaust Manifold Flooding with Cooling Water

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Originally Posted by hellosailor View Post
" In would be imposible for an electric pump to do this "
You are forgetting, there is no need for the electrical pump to mimic a WEAKNESS of the mechanical pump. Regardless of the pump type there should be a thermostat, which is restricting the flow according to thermal needs.

So it doesn't matter how the flow rate from the pump varies, the thermostat has the only opinion that matters there.
Cooling the engine is not a problem, you are right, the thermostat will take care of that.

What I was addressing is overwelming the mixing pipe and flooding the exhaust manifold because the cylinders are not moving fast enough to expel the water injected by the electric cooling pump.
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