Boiling water in exhaust?

[Knot Waiting]

1991 Mercruiser 7.4 454 Mag / Corsa selectable thu-hull exhaust / Bravo 1

I took my boat out this past weekend for a final cruise to run some fuel stabilizer through it and fog the motor. However while in the process a brand new issue came to light. Here are the symptoms as follows:

1. Hard starting. Had to crank and prime for quite some time to get her to fire.
2. "popping" or "ticking" at idle (sounded like a slight mis-fire)
3. Large amount of steam from the right exhaust outlet
4. Boiling water sound coming from the exhaust outlets after the engine had been shut off (boat still on the lake)

Notable items:
1. The engine has about 750 freshwater hours on it and has been very well cared for.
2. Exhaust it original
3. Ignition components are new this season (cap, rotor, plugs, wires)


Any ideas as to what my problem could be? I'm leaning towards risers/manifolds. Thanks!

 

[SangerDX]

Re: Boiling water in exhaust?

1991 Mercruiser 7.4 454 Mag / Corsa selectable thu-hull exhaust / Bravo 1

4. Boiling water sound coming from the exhaust outlets after the engine had been shut off (boat still on the lake)
Any ideas as to what my problem could be? I'm leaning towards risers/manifolds. Thanks!

Is the impeller pumping good? Boiling water sounds like water has not been circulating

 

[Knot Waiting]

Re: Boiling water in exhaust?

That's another thought I had. The impeller is a few years old and I felt that the amount of water coming from the exhaust at idle was low.

 

[MarkSee]

Re: Boiling water in exhaust?

How's the temperature showing on that gauge? Any difference lately? Able to keep your hand on the elbows/risers without it getting so hot you have to take your hand off?

Mark

 

[Knot Waiting]

Re: Boiling water in exhaust?

How's the temperature showing on that gauge? Any difference lately? Able to keep your hand on the elbows/risers without it getting so hot you have to take your hand off?

Mark

Engine temp gauge has been right in the normal operating range (170 if I remember correctly) pretty much where it had been all summer. The riser in question was a tolerable temp when I leaned in to listen to the boiling water noise (put my forearm on it).


Note: The sound was most audible from the above water exhaust outlet by leaning over the swim platform

 

[CharlieB]

Re: Boiling water in exhaust?

Service the water pump at least every other year. Besides replacing the impeller take a close look at the housing , cam and cover to gauge the amount of wear.

Check any strainers and the inlet thru-hull for any object obstructing or reducing flow.

Edit; Also check the inlet side of the oil cooler, grasses can collect in here and restrict flow.

 

[alldodge]

Re: Boiling water in exhaust?

Check your engine circulating pump if your impeller turns out to be ok. Pull the belts off the pully and turn it quickley back and forth and listen for any sound