mercury outboard issues

[Jmstee]

Hi everyone

I have a 2003 5hp mercury outboard that was on my catalina 25. Half way through last season it started overheating and shutting down. It had a brand new impeller at the start of the season, I took it down and pulled everything apart used some compressed air and blew out the plumbing that keeps it cool and it worked for a couple weeks then started struggling again and died. Its old and not worth that much and getting it professionally serviced (they recommended pulling the power head at this point) will cost more than its worth. I pulled the power head myself today and there was so much salt/other crusties in there that i think it was obstruction the flow around the cylinder. I have always had flow from the tell but i never felt like it was enough. hopefully this will have fixed it. my question is while it is all open is there anything else I should look at? how do i know that in its overheating I didn't destroy piston rings or anything else? It all looks decent now that its cleaned but iv never pulled a motor apart to this degree.

I did a compression test in the summer and I felt like it was a little low but the motor runs great until it gets crazy hot and fizzles out. does anyone know what a small outboard like this should run for compression?

The other thing i did as a test when it was apart is put the foot in a bucket of water and i carefully attached my drill to the shaft and ran it. only a small amount of water was pumped up but i have redone all this before. does the drill just not spin it fast enough do you think? or is there some other issue? the impeller still looks new but i don't know what else to look at.

Hate outboards thanks for your help in advance.

[Williamp]

Hi,

Had the same issues on my 5hp yam last year, pulled the head and scraped out as much rubbish as I could, replaced impeller and the ran very gently in a bin full of central heating descaler. Eventually got a bit of water through and then it got progressively better each time I started it. It did take a good few days of messing about to achieve the desired result. Good luck....

[WSMFP]

White vinegar will eat that salt scale up. Take the thermostat out and pour some vinegar in there. Let it sit overnight and dump out the gunk. Repeat a couple times.

[Jmstee]

Thanks everyone i tore the whole thing down cleaned out a ton of buildup and got it all back together without any left over parts and only one lost not even important screw. I started it up today and it ran in idle in a small bucket without any issue. when it was apart i realized that the the tell tale water spout is right next to where the inlet to the powerhead is from the water pump after it comes up the shaft so I would always have great flow even if everything else was blocked. the right side seemed to stay cool but the left im just not convinced that it isn't going to overheat again. I plan to put it on my dingy as soon as we get a warm day and try it on a small lake rather than the ocean. Do you have any other suggestions of things that would cause it to heat up and be an issue? the impeller looks awesome and only had a few hours on it. every channel water flow area is clean. could something else be causing my issues? how do I know that my piston rings are ok or is there any chance that when it overheated in the past it damaged the cylinder that would cause an issue? (everything looked ok to my untrained eye when it was apart)

[a64pilot]

Your rings are fine if it makes good power and cranks well.
If it didn’t seize, it’s likely OK.
What often happens from a two stroke overheating badly other than seizure is the crankshaft seals leak, if they leak it won’t run well at all, if it runs.
If you fell the need you can get an inexpensive bore scope and borescope it to see the cylinder, or pull the head off.