MerCruiser 5.0 running hot (bit of a story)

[Zorm]

Got a 2007 SeaRay 205 sport with the above engine. Took it out last year and after about a 20 min run, (I'm in Pensacola, Fl and run mostly in Brackish water) high temp alarm went off and gauge read over 220, shut her down and had the let the engine cool with hatch open. Was finally able to get it back to the ramp.

Got home and 1st thing I checked was thermostat which was gummed up, I ordered a new thermostat and the plastic housing that it sits in. I also ordered an impeller kit. Got em, and replaced everything. I then got some salt water flush, dropped the out drive into a large tote filled it with water and the flush, ran the engine for about 45 min at idle to hopefully flush out some of the crap.

Took her back out again and it seemed ok, ran good off the trailer, was idling fine loading folks in, ran her up to cruising speed with no issues. I then backed her down to idle for a little bit and she started to run hot, the alarm went off again and the gauge read above 220. I ran her back up to cruising speed to get her back to the doc and she cooled off, as we were waiting to get her on the trailer idling, she started to heat up more then usual again.

So, asking for thoughts on what the issue can be. I mostly work on Ford trucks and Jeeps but an engine is an engine right? Could this be a water pump problem or, recently someone said I could have a manifold / riser issue?

thoughts please as my grandson really wants me to take him out on the boat.,

 

[wellcraft-classic210]

An damaged impeller issue can definitely cause the issue your describing of temp increases at idle // you may want to check it --( They need a constant water supply when the engine us running or they tend to melt )

Its seems you can run at higher rpms which would indicate your not dealing with a blockage

Beyond that perhaps the experts can add some ideas

 

[Bondo]

I then got some salt water flush, dropped the out drive into a large tote filled it with water and the flush, ran the engine for about 45 min at idle to hopefully flush out some of the crap.

Ayuh,..... Welcome Aboard,...... The raw water pump is a flooded entry pump,......

With a tote, you can't get enough water in it to flood the pump,......

My guess is, you burnt up the new impeller,......

Did you replace the pump housing too,..??
If there's Any blemish in the housing, it needs replacin' too,......

 

[Zorm]

humm. so maybe replace the impeller again, with pump housing and instead of either hooking up a hose to the water intakes on the out drive, put the whole boat in the water to "seat" the impeller correctly?

 

[havoc_squad]

put the whole boat in the water to "seat" the impeller correctly?

Don't know where this reasoning is coming from, no this step is not necessary.

No service shop or DIYr does it this way to do first test/check unless they live right by the lake.

First run it with flusher attachment at 1500 rpm or less using a high enough output water supply faucet. Most houses are fine.

Get the engine up to operating temp and make sure it stays there for about 5 mins.

If you have a temperature IR gun, might be a good idea to check the exhaust manifolds temp.

I think the consensus is no higher than 160 degrees on engine block, or exhaust manifolds using raw water cooling system.

If you have terrible tap flow output, might need to be ran in the lake/river/ocean.

 

[QBhoy]

Sure sounds like an impeller issue.