Solved - engine alarm on Merc 496

LaRea

I finally solved a nagging problem with my Merc 496 Bravo III, so I'm posting details here for the archives.  So happy to end this saga!

The culprit was the raw water pickup hose (transom intake) from the Bravo III.  It's a 5/8" hose that enters the engine room via a thru-hull fitting in the transom.  The fitting had corroded, reducing the inside diameter of the hose to about 1/4" diameter at the fitting.  At low speeds, it ran great.  Above 3200 RPM, the engine would sound a constant-tone alarm and cut power.   

Diagnosing this problem was difficult because it never overheated and never threw any alarm codes on the engine computer.  The water temperatures were normal (150-160 degF).  I enlisted multiple mechanics to look at it, did lots of research, and had a long discussion on this forum (LINK HERE).  Everyone was guessing either a faulty sensor or a wiring problem.  Nobody ever suspected a cooling problem!  For months, I tried everything -- swapping sensors, replacing water pumps, you name it.  Nothing helped.  Last week, my mechanic had a moment of divine inspiration and checked the raw water pickup hose.  Bingo - problem solved.  

Checking the hose for corrosion is easy, and can be done with the boat in the water.  Replacing the hose requires hauling the boat to remove the outdrive and transom assembly. 

Not all boats rely solely on the Bravo III for cooling water.  On my previous boat, each engine had a separate thru-hull leading to a Y-connector.  The engine could draw cooling water from both the outdrive and the thru-hull.  I gather that setup is more common on high-speed boats, but maybe I should retrofit it on my 370.   

In case anyone wants the details, here they are.

2007 Rinker 370, Mercruiser 496 with Bravo 3 and Digital Throttle & Shift (DTS)

* Ran fine below 3200 RPM

* Above 3200 RPM, stbd engine would reduce power with constant-tone alarm

* Go to idle: alarm would stop within about 5 seconds

* Wait 20-30 seconds at idle:  could go back to 3200 RPM, but not above

* NO alarm codes on engine monitors

* Water temps normal and steady (150-160)

Thanks again to all who suggested possible solutions.

craigswardmtb

I don't suppose you have a pict of the hose/fitting location.  I'll check this on my 360 before she goes in the water in a few weeks.  Its finally feeling a bit springish here.  It may hit mid 50s today.  I hope to be bottom painting in next week.

LaRea

I didn't take pix, but here are a couple links for background. 

http://www.thehulltruth.com/boating-forum/426977-bravo-iii-r-r-raw-water-pickup-tube-w-pics.html#b

The fitting is directly behind the engine, just above the drive shaft.  It only takes a few minutes to take off the fitting and check it.  If it's bad, it can't really be repaired.  The only solution is to replace the hose.  

Dream_Inn

Nice find LaRea!  Thanks for posting the results!

Capt Ron

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Thanks for sharing

TikiHut2

+1 on that. Thanks for posting the solution to the mystery illness. Now to try and remember that post when it strikes again.

 Ironic how it seems so obvious after we find the solution. Man, that was a tricky one. My newish Smart Craft gauge has a water pressure reading that I need to start getting a baseline pressure at different rpm so I'll know if its a restriction/impeller problem in the making.

When I was having my water pump drama, one of the things my Merc mechanic friend asked me to do was hook up my muffs and leave the intake hose off of the water pump and make sure the flow to the pump body is completely unimpeded. Now I know why he suggested that and I'm sorry I neglected to mention it...... whats my name??, I'm losing it. Darned cheap rum.