MikeSXT25

496mag ho. With Dana exhaust
new raw water impeller
FResh water cooling. cooler runs cold to touch all the time

Seems at 4200 rpm +\- a couple hundred rpm I get a alarm and throttle goes away till I pull Throttle back then gets a surge of power and alarm goes off. On dash gauge water temp comes up to 180 ish at idle on hose or in water. And needle splits the 180-210 mark. At 4000 ish rpm (temp gauge accuracy unknown)all day runs fine under 4200 rpm.

AllDodge

iTrader: (1)

Assuming this is open cooled and Bravo drive
Place a piece of clear tubing between thermostat housing and seawater pump. Your looking for air bubbles. If you see bubbles then its either sucking air somewhere or its bravoits

MikeSXT25

No closed cooling system has engine coolant

AllDodge

iTrader: (1)

OK, use the clear tubing between pump and heat exchanger. Use a IR temp gun on thermostat housing to see if it compares with gauge reading.

May need to clean the tubes out in the HE

MikeSXT25

Tubes been cleaned.
Lots of water flow
HE cold to touch except back side near where thermostat is.
Can keep hands on risers. They are warm but not hot by any means

BUP

iTrader: (9)

you need an impeller and new housing. Also clean out and change some other parts in the cooling system.

On your DANA exhaust you have to make sure your using the temp sensors that was designed for the PCM 555 and they are operating as well - all that can be scanned to see all the parameters plus real temps and water pressure on the scan tool

Possible low water pressure or high if you have cooling water restrictions.

Also when was the last time your anti freeze changed ? All this matters for cooling a marine 496.

All Merc 496 OEM stock are closed cooled. Merc never did raw water cool for a stock black production 496.

How many hours on this engine WATER CIRC PUMP comes into play for changing around the 500 hour mark. Or sooner if poorly maintained. Meaning day one anti freeze and neglected / improper winterizations

Keith Atlanta

Yeah, BUP is right. Swap out the housings. You should have it scanned (surprised BUP didnt say this)

BUP

iTrader: (9)

I did say that --- " all that can be scan to see parameters and real running temps and WATER PRESSURE on the scan tool "

MikeSXT25

My vessel view is in today hopefully and I will run it again and get parameters from engine. When alarm sets.
i can make alarm sound and shut off with throttle. And when alarm goes off gets a surge of rpm and will run there till i push throttle up

MikeSXT25

Things I have found. 1 high water pressure is setting alarm. 2. Found some rocks in PS cooler and cleaned it out. Pressure still a touch high at high rpm. Seems like there should be less restrictions internally in the cooler. Pics to follow. Anyone seen this before. Seems a bit restricted

Clogged

Cleaned