I have twin Merc 6.2s in my Formula. My left motor overheated the other day, found a shredded impeller to be the problem. Replaced the part, flushed out the broken pieces, and put everything back together. Engine seems to be running fine, but still running warmer then the right motor. Took it out today and water temp gets borderline too hot at full throttle causing the temp hi light to flicker on the gauge. Also note that left motor turns more RPM when throttles are matched. Any ideas what is causing this? My engines have only 117 hours.

When it got hot the thermostat spring could have lost its tension.Replace both thermostats and see what happens.You could also check the power steering coolers for impeller parts
Also how scored was the see water pump yours should be brass was the end plate scored really bad? As for the RPM how much and is it noticable at WOT or just mid range

How difficult are the thermostats to replace on these motors? My impeller kit included a replacement end plate. The difference in RPM is mostly during midrange, a little at WOT. Only noticed it because the RPM was pretty close between both motors before I had this issue. Does belt tension affect RPM at all?

Easy ther are two bolts holding the housing (10mm) I think I usually disonnect all hoses before I take housing off.You will need three parts the thermostat,gasket and the plastic ring you could reuse the plastic ring but the usually break tring to get them out so I don't even try.
As for the rpm mine are a little off at idle (always were) could belt tension do it,maybe the are turning a lot of stuff.Unless its more than 150 RPM I would not worry about it. Did you do both impellers?because the new one is harder to turn that might be it also.
http://www.mercruiserparts.com/Show_...at+and+Housing

You may have two gaskets one on each side of the thermostats

how hot is your motor running? temperature wise?

You probably have some impeller pieces still stuck at the front of the oil cooler.

skip the thermostat BIG Daddy and get the impellor pieces out of the cooler and she'll run fine 4 ya .did 2 of them up at the river this weekend and did'nt touch any thermostats

X2 on this one. If all you did was "overheat" it - as opposed to "smoked the living crap out of it", your thermostats should be fine. Most times we find a few extra pieces of the old impeller in places folks didn't look. That's why it helps to try to approximately re-assemble the smoked impeller to see if you got all of the parts.

6.2s don't have oil coolers unless he added them.As for the thermostat I had some see grass block my pickup and engine went up to 220 for a second or two.I changed the impeller as it had run dry.They are cheap and easy so why take a chance.That motor ran 180-185 after that.I took the thermostat out and tested it it was bad.Put a new one in problem fixed.But hey you can do whatever you want its your boat.

Good point. I missed that.