Buddies boat has one of his 496 mag ho's that gets up to and stays around 190 at idle but comes back down to 160-170 when on plane. He has checked the impeller and it looks fine. He put a new thermostat in and cleaned out the exchanger. Still getting hot. Any ideas?? I told him to swap raw water pumps, but he doesn't think that's it.

I would look at the T just above the tstat, behind the heat exchange, then follow them to the elbow fitting at the exhaust manifold port and stbd. If flow is good, remove the rubber coupler that connects the elbow/riser to the down pipe and visually inspect that the water jackets are not corroded and restricted.

If the water cannot flow freely through the exhaust, the heat exchanger gets "heat soaked" due to the raw water staying in the exchanger for too long. This happens when there is a restriction in the exhaust.

How is the brass housing ends for wear. I have just installed the stainless repair kits in mine and a big difference in water flow. If the housings are worn it will have an issue moving water at idle but once on plane the water pressure from the drive overcomes the issue.

He did the stainless rebuild kits late last year. We will look into the exhaust thing. That makes sense.

What stainless rebuild? I might be interested in doing this.

Also check the transom assembly from the drive to the feed line to raw water pump. A good freind had this problem with his. The correction was to install a low water pickup and sea strainer and no more problems. It would get hot at idle.

1) How old is the Antifreeze? Is it still pink: or dark grey/metallic in color? AF is only good for 6-7 seasons, TOPS.
When it turns dark, it will silt up and not flow as well; and motor will heat up.

2) Does it have a fresh water flush connection? Some of the system that get installed have a ball/plunger/spring that get's gummed up and restricts flow. Usually, it runs fine (cool) at low RPMs and hot at high RPMs, but anything is possible.


What is the water pressure reading via the computer? (It's pretty much futile to troubleshoot these motors without a Rinda scanner or Diacom software; or Smartcraft visibility.)
Does he have water pressure gauges (mechanical)?
Is water flow even out of both tailpipes?
Is water flow even across all four tailpipes (comparing both engines) at idle?

Antifreeze was replace last season, along with the long block...

My scanner will not read his PCM for some reason, now I have to send it back to rinda.

No water pressure gauges.

he says water flow is even by visually inspecting across all four pipes.

http://metalartetc.com/shop/article_...3DSSMWP-001%26

Kit for the wear plate

sucking air somewhere maybe??