Overheat at cruise only

[4 Reel]

Hi all
New problem here, at cruise RPM around 3400-3600 the temp climbs & the overheat buzzer sounds, at first i thought a blocked water intake @ the drive & reversed briefly & the temps came right down to normal. Now it does it consistently & if I back off to 1000- 1500 rpm it cools right down. Sound like a raw water pump? I am at the end of my second season since the pump was installed.1998 Merc 5.0 Alpha 1 Gen 2, raw water cooled. Thanks in advance

 

[Haut Medoc]

Re: Overheat at cruise only

Are you manifolds the original ones ?
They may be due for replacement........
Scale blocking the outlet dumps can restrict water flow....
Two years old is not old for an impeller, but, if you run in alot of sand or ever dry start your engine, it would be suspect.....
The first thing I would do is examine the impeller to remove it from the equation.......JK

 

[4 Reel]

Re: Overheat at cruise only

I knew I forgot something, manifolds & risers were installed in the spring

 

[Don S]

Re: Overheat at cruise only

Did this suddenly start or gradually get worse over the season.

 

[4 Reel]

Re: Overheat at cruise only

It happened the last 3 trips out, but only once & after quick reversing it cooled right down, & ran at normal temp the rest of the day so i figured intake blockage (lots of eelgrass here), but i supose that is not the case now it overheats every time now when i get up on plane. I checked the raw water intake & beside a barnacle or two (scraped off) it is clean. I also pulled all 4 hoses going into the T stat housing & they were clear. Both exhaust elbows were much hotter than normal.

 

[Don S]

Re: Overheat at cruise only

Time to check the impeller.

 

[4 Reel]

Re: Overheat at cruise only

As i suspected, thanks haut,Don

 

[4 Reel]

Re: Overheat at cruise only

Just an FYI update here, the overheat at cruise turned out to be barnacles (lots of them) inside the raw water pickup cavity of the drive. Cleaned them out & all is fine now, and i did spray the drive with the $30 can of Interlux drive antifouling paint in the spring.