Re: Mercruiser 470 thermostat location??
I think you are reading too much into things happening that means little or nothing.
Smoke from antifreeze is normal around a spark plug and the exhaust manifold/head joint. It gets hot there and antifreeze steams when fried.
Second, hearing bubbles when you squeeze a hose doesn't mean anything.
You need to do some testing, not guessing.
A leakdown test on each cylinder with the cap off the reservoir will show a leak. You will see bubbles coming up thru the antifreeze.
There are Soooooo many things unknown about your problem that it's impossible to give you an absolute answer.
You need to get one of those non contact infared temp guns and check different places on the engine and manifold and see if you are actually getting an overheat, you need to do the leakdown or exhaust gas test I mentioned earlier.
You could have a plugged heat exchanger from old part of a deceased impeller, plugged exhaust riser, lots of things.
Sorry, no EZ, one fix fixes all, type fixes for your problem.
A Mercruiser service manual could walk you through all the steps needed to troubleshoot the problem.
Changing parts to find the problem is very expensive and usually doesn't work.