I have been running my boat for the summer and the temps have been at about 150 degrees all summer. Last evening I ran up for dinner with some friends and the boat was running at about 180-190. The water temp this week was warm (about 78-80 degrees). Is it the warm water temps causing the problem or do I maybe have another issue to look into. These are 420hp 454's and they both are kind of doing the same thing. The temp held steady no matter what speed I ran.

Jon

Do you have the Merc 143 degree thermostats? My 454 370 HPs used to stay dead on no matter what the conditions. I wonder if you flow is restricted or maybe your impellors are weak. Are both engines the same? Any changes to the original Merc water system?

Those should hold temp no higher than 160* with the 140* tstats.

New water pumps this year and they are both doing the same thing. They just started this yesterday. That is why I was thinking the lake water was so warm causing this. Everything is totally stock.

Jon, change the controls, that is t-stats. be sure your flow is good and be sure your sea strainers are clean. say hi to Kurt.

My engine water temps never changed at all in my 525SC, no matter what the lake water temp was. Idle and cruise was around 145* and WOT was just under 160*. It had the same cooling system as the 420.

I will try and get some time today to take a look at it. I just find it so odd that this happened all of the sudden and with both engines.

Are those engines fresh water cooled? I dont think the water temp being maybe 10 degrees warmer then normal would account for a 30 degree variation in running temp. As others have said I would check your flow rate starting with the strainers and the impellers

Jim

They are raw water cooled. I am going to dive into the water today if I have time and check and see if the water pick ups got clogged with weeds then work my way from there.

78-80 degree water in Mass? must be a record!