High RPM overheat - Mercruiser 8.1

[Team PCBeach]

2003 Sundancer 360, got an overheat on PORT engine during a trip yesterday when running at 3200rpm. Shut the engine down and continued our trip on the STBD. After about 15 minutes restarted the PORT and ran the rest of the trip at 2400rpm. Temp rose to 176 and held steady. STBD was running about 166.

My first guess is a bad water pump impeller. The only thing giving me pause is it has only been about 18 months since I replaced both water pumps and impellers. Total run time since then is less than 50 hours. Very little use over the last 8 months and basically no use the last 6 months other than to start the engines every 2 months.

Risers and manifolds are coming up on 4 years old with about 100 hours of salt water use.

Is my guess at the impeller reasonable or should I be concerned about something more significant.

Thanks.

 

[scoflaw]

Restriction of some sort, v-drives or IO's?

 

[Team PCBeach]

V-Drives

 

[SeadawgVB]

What pressure is indicated on smartcraft? Not 100% accurate because the sensors often go bad, but that was one of the metrics I used to watch like a hawk while underway.
While running at 3500 rpm, my pressure indicated 30-32 psi on new impellors.

 

[Team PCBeach]

@SeadawgVB why weren't you with us, that is a great trip. We upgraded everything to VesselView as our Smartcraft display was dying. Used it watch engine temps, but didn't even think to check pressures. Might have to take a trip this weekend just to take a look.

 

[JimG]

Also with Horizon's, check you antifreeze levels. (I know, obvious, but I had to mention it)

 

[Team PCBeach]

@JimG thanks. I always check oil, antifreeze and belts before getting underway, but will double check to make sure I didn't get a leak.

 

[Team PCBeach]

Checked antifreeze and it was fine. Ran the engines at the dock and it looks like I might have a blockage since the PORT pressures seem high to me.
Pressures
Idle (700rpm): PORT-13psi, STBD-7psi
2000rpm: PORT-34psi, STBD-23psi
3000rpm: PORT-37psi, STBD 29psi

Seems like pretty significant pressure differences. A blockage would explain the higher pressure and the over heat as not enough water flowing. Any suggestions where to look for a potential blockage? Could it still be a bad impeller that threw a fin that is causing a blockage?

 

[Bill Curtis]

Yup, those numbers suggest a pretty good blockage.

Start by looking in the oil and PS coolers which are the 1st in line from the RWP. Those are the usual culprits.

PITA to get to, but disconnect the inlet hose and the bits are usually there. A bore scope is a god send when doing stuff like that so you can see what's going on. After ensuring they are clean, time to move on down stream.

 

[Team PCBeach]

Thanks for everyone's input on my issue. Turned out to be a clogged heat exchanger. Pulled the hose of the sea strainer and let the engine suck in 3 gal of Barnacle Buster then shut it down. Let it sit for 2 hours then fired it up. Significantly helped the problem, but pressure still running higher than STBD. Pulled the end cap off again and used a straw cleaning brush to break through on the tubs the Barnacle Buster didn't fully clean. Started her back up and pressure are the same in both engines! Pretty easy fix.