Overheating at idle and troll

it's not ideal but it's OK and unavoidable with the way the system was designed

New thermostat appears to be a bypass type so the large drop in temp is to be expected.

check under cam to see how much wear , if it runs cool with old impeller and over heats with new impeller ,it's the housing , new g21 housing is $150 , you can see some wear on this housing   http://www.ebay.com/itm/Chrysler-Sherwood-Raw-Water-Pump-Impeller-Housing-G7-10615-OMC-V8-/141901619419?hash=item2109fee0db:g:gkgAAOSwe7BWvSIg&vxp=mtr

Yes that is what the new one is doing. There was not one in there when I pened it up which is why the temp didn't even register.

Did u put the new one in already?

check your water pump housing for wear , old impeller will swell , new impeller is the correct size , put impeller in housing , should be above lip , no way to fix , buy new housing

Little gasket then Thermostat goes into the recess (turn yours over). Big Gasket goes next. Make sure you get ALL the old gasket off first or you'll be leaking water.

***************!   I have company coming over for the day to play on the lake. I finally got the new thermostat. DIM wasn't sure it was going to be the correct one because of the style switch that year. The plan was to open it up and see if we have a match. I opened up what I think is the right location and there isn't one there? Am I at the correct spot and what order and orientation do the thermostat and gaskets go in? Do I have it correct, see pic?

Oh, I see!
And regret if my attempt at a funny failed!

I think I'm more wary of barefooting with a too-cold block.

So the pistons run hotter from sustained load, running a colder block seems to exacerbate the already substantial differences in expansion.

Yes, more crystallization of salt occurs, That appears to be the main motivation for the recommendations.   I could find some references in Mercruiser docs, harder to find in PCM. But the chemistry and metallurgy is the same.

Higher temps from the 160 will produce more power and better efficiency; heavily debated but it follows prevailing wisdom.
Also, as any man worth his salt knows , hot slugs in cold jugs makes for scuffing and more wear.

I've always understood it that way too, but you can usually trust what GottaSki posts. I'd like to learn his reasoning actually.

I installed one of the Timmy T's last weekend. Very easy install and much easier to hook up the hose for running on the driveway. Nice Job on that idea Tim.
My 95 did have 1" hose so it was slightly different than your picture which had a 3/4 fitting in it but it was your idea and a very good one at that. Thanks for sharing.    All installed and I think it was under $20 bucks. Mark

The carbureted Marine thermostat for boats running raw water pumps is a 140 degree thermostat. The later fuel injected boats switched to a 160 degree thermostat.
Most passenger cars run a 195 degree so don't use one of them.
I have been told that Marine engines use a lower temp to avoid rapid corrosion inside the engine block.
Now that you have found the correct new impeller buy 2. Keep one in your boat as a back up.   It will save you in the future.

The root cause of my issue is that I received the incorrect Impeller for replacement. it was not as thick (for a Ford) and did not seal against the cover plate so it did not provide any pressure. The pressure I got was when I was at speed and it cam eform the hull inlet. I will be getting a replacement soon.
I put the old one back in and I run so cool that it doesn't register ont he gauge. i think this means my thermostat is stuck open. can I get a thermostat at an auto parts store (Chevy 350 1997) or should I order one for a marine application? best place to get it?
In my diagnosis process I removed the spring from the O'matic thing just to to make sure that wasn't the reason I wasn't getting cooling at idle. So basically it's a Timmy T with a free moving check valve now.
Thanks,
Adam

Here's a thread on the "Timmy T" Trust us it works even though water will come out the hull pick up.

BTW, just ignore CQ molesting poor Tim!!