Good point. These are aftermarket cooling kits, so I cannot recall exactly if the gasket had the conductive rivets in them. I think if I get the MEFI software for my computer, I can test that out easily enough, and correct as needed.

It seems that the different ECM programming for closed cooling is prevalent on the newer engines with the more sophisticated control modules. My engines have the MEFI-1 modules, which is somewhat Neanderthal in the evolution chain of engine controllers. so, I tend to agree that the only tweaking, if at all, would be in the Temperature table.

If you can see a difference in plug color between the 2 engines with today's gas you may have a good clue there. And may want to check your fuel pressure regulators and returns .

Ok im game here.....the ect is just a resistor that a 5 volt ref signal is sent thru....why would grounding be a issue the housing or engine itself is not in the loop its a pass thru voltage.

Yes, you are right . . .seems to be a different type of circuit than the gauge sender. Grounding would not be an issue. Similar to pic below . . .

So, on the 'things to do' list . . .

Check the Delphi and Mercruiser part numbers on the MEFI units
Get MEFIBurn Std. Edition and check the engine systems, see if there are any codes, etc.
Deal with any issues that I find . . .
Maybe clean the fuel injectors.
Wait until Springtime
Go boating

All mercruiser EFI and MPI use a 160 degree t stat. A lot of the closed cooled Mercruiser Horizon engines run and or can run 170 degree t stat. Also you need the t -stat gasket with the 4 rivets pressed into the gasket. This aids the ground path for the correct reading at the temp gauge plus warning horn.

Rich conditions could be a bad / faulty coolant temp sensor or the MAP sensor and or the MAT sensor and or their connection problems, or even a dirty flame arrestor and throttle body. Also a coolant temp sensor with heavy build up / scale / rust / hard water deposits and so on can make your engine run rich. Also a faulty fuel pressure regulator can make your engine run rich.

I would perform fuel pressure tests, and checking the fuel pressure regulator and compression tests before any parts changing. Also scan of the engine including a engine running with the scan tool. Also problems with the VST and the float level or needle / seat inside can make for a rich running engine. Just saying. I would also perform an engine vacuum test as well before any parts changing. You are looking for the internal condition of the engine - how good or maybe not so good.

Going from fresh water to closed cooling it is very wise to change t -stat, gaskets. and the coolant temp sensor all to new. Your engine needs a 160 degree t-stat or possible the 170.

If you change or if any of the internal vacuum of the engine has changed then the ECM needs a recalibration to maintain the correct parameters because the sensors will not read correctly / giving the ECM the incorrect inputs. Also if you changed any shape or form of the incoming air / intake / intake track air / heads / valves / headers / and so on for incoming and or breathing of the engine then the ECM will need a recal as well. FYI.

Anyways the 2 different ECM's was because of serial # dictates which engines had MEFI I and the later Serial #'s engines had Mefi 2 installed. Not much difference between MEFI 1 & 2 but some differences. If needed I could go thru all that as well for these marine apps. OEM trained since 1991 and still currently.

Scanning MEFI 1 engines is almost useless not much there to look at. MEFI II was better but still not much to exactly pin point a lot problems but some.

Some added info MEFI 1 & 2 ECM look the same. Also you are correct you need ECM part # 807268T3 in which should be MEFI 1. You should be able to use the stock tune if your engines are 100 % totally stock and the set up. Also make sure your engines are internally solid as well before anything. next focus on the VST system along with all the fuel filters and your fuel pressure testing along with the fuel pressure regulator. Next is all the other info I posted above. There is also a fuel filter in the VST. Lastly weak ignition components including spark plugs can make for rich conditions.

Not to contradict but the filter is actually a screen, right. I have opened mine a couple times to check and clean the screen. As you say the screen does filter

Also have to be real careful with the screen, if it breaks you cannot find another, need a new VST

^^^^ No problem - its a round caged filter screen called fuel strainer thru mercruiser. It is NLA correct info from you on both. I did have a source on them and might even have a couple still in stock. The last time I seen one for sale had a retail price on just that alone was 76 bucks. Crazy right.

Mercruiser calls for a fuel pump kit with the various parts, fuel pump and the caged screen filter for retail around 750 or close to that price. That filter is important in the VST as a lot of times it needs replacing because lack of maintenance overtime,

What is the part number of the 170 degree merc thermostat