496 mag ho running hot
 

I have a 2002 nordic rage 25 foot with a 496 mag ho and run it in the phoenix area lakes only fresh water.. water temp is fine at begining of the day but after running at higher rpms it seems to get hotter but not overheat and then when coming of plane to an idle it gets hotter and begans to overheat. but before it does i put it in nutreal and run it at 3000 rpms for a few seconds it cools back down to normal . any ideas of what it could be i just put in new sea pump and impeller. thanks

Impeller and the sea pump may have some grooves in it and need a rebuild kit.

Did it do this before the new seawater pump went on?
Could be sucking air if there was any bad connections in the plumbing before the S.W.pump. Make sure the winterisation system has no pressure in it. Check all the hoses (hoses may look ok on outside but can colapse inside), hose clips and fresh water flush if you have one. The heat exchanger could be partialy blocked easy to remove the ends and rod through the tubes. Also could be on fresh water side, check to make sure this is circulating properly. One easy check you could do is to put your had on all of the fresh water hoses and see if there are any cold areas in system. Further to this you could remove the thermostat and test it.
Hope this is some help.
Good luck. Be good to know how you get on, keep asking if you have no luck.
Si

525 and I would be weeeeeell done!! but it's the age old matter of available cash. Under the impression the 496 aint a bad lump if looked after...

That's very true but like everything else, there's been more than one that broke due to various reasons and suddenly they're junk.

Don't run it hot and keep good oil in it. I tore mine down after running the livin' crap out of it for lots of hours.

Wasn't anything wrong either. If you run anything at wot for extended periods you will break it. Just how it goes.

when i first got the boat in may in never did it . then after changing the impeller it would do it right away at first start up then would release the air valve and run it in nutreal at 3000 rpms and would cool off. so i had the sea pump replaced and now only does it at idle after running it for a while. thanks for the help man!

Can you fill in the actual temperatures that you are refering to ie "at the beginning of the day", "gets hotter but not overheat", "cools back down to normal" and "begins to overheat" after coming off plane to an idle?

start of day temp on the thermostate gage is 170-180 when driving over 4000 rpm it gets up to 185-190 and when slowing down to idle gets up to 200 something then alarms starts going off and then i put in nutreal before it oveheats because im afrair to see how far it will go like 240 and run it at 3000 rpms it cools back to 185-190. the actual water temp is low 80's and ambeante temp is high 90's

Little bit of advise don't push it into this situation (overheat alarm on) unless very briefly for diagnostic/testing. You are likely to be causing piston / cylinder damage. Certainly don't let it go up any further.

The 496 is a very stout engine indeed so when they break they cost dollars to repair and dont get your nickers in a knot.....there not junk.

Break the engine as this one is and cost out the repairs.

A grand for forged pistons because lets face it, there not and now is the time to do it. Price out the machining etc, gaskets and bearings...Head work??? the whole repair has to be done properly and not pieced together. If you are not able to do this yourself the money needs to be spent and that is not chump change.

Price out the parts from Merc and tally up the repair and you will see how quickly you can spend 7 to 10 grand on the rebuild (of course it all depends on what is hurt)

At the end of the day you have a rebuilt 496 engine...no arguments there......but If I was to do it again it would be a 500 efi hands down.
Pull the 496 in and put a 500 efi in its place.
B.S. on comparing the 2!!