No, I mean on fuel injection engines, the ECU controls the high pressure pump and only turns the high pressure pump on when the engine is running, or cranking.

Many ECUs (can't comment on the merc ones, but the ones I have built myself or maintained, wired up etc) - the moment you start cranking the engine, both the fuel pump and the O2 turn on.
So the lambda starts cold and warms up gently with the engine running - takes maybe a minute for the lambda to warm up, but if the engine is cold, the ECU ignores the readings anyway until it's warm.

This means it's not sitting there hot with the ignition on before you start the engine, and nor is it ever off is the engine is running (another thing which can kill them - if the engine is running, the O2 should be hot).
So I would never have it manual - no point. Have it running off the high pressure fuel pump feed, it means the O2 sensor will be on all the times it needs to be on.

This is a typical ECU wiring diagram - and is exactly what I run on my boat. O2, injectors & fuel pump all running off the (high pressure) fuel pump relay.

Finally, I don't think you "could" do a marine one - the precision of how they work is very "precise". An LSU 4.9 Lambda is probably better than 4.2, they last better apparently. But in reality, a saltwater moisture environment isn't ever going to let them last forever.
Fortunately, compared to a tank of fuel, they're not expensive if you have to, say, replace each season.

Check, thank you for the good layout diagram.

MadMat,

Are you running MegaSquirt?
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I've been following them since 2004. A friend bought the latest in 2020 ish, and had a circuit board issue...the board would not take data from the O2 sensor. (as I remember it).

Megasquirt would not warranty the board or even assist in the diagnostics or repair.
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Needles to say...my intent to use Megasquirts evaporated with that experience.

Curious how you've made out if you are running them...as per the photo diagram you sent.

Yes, megasquirt. For some reason I wanted a really custom setup converting an old school hilborn mechanical injection to EFI with all the electrronics as hidden as possible (see 3rd photos - injectors are all pretty well hidden. It was a tight fit)
Classic Megasquirt is a kit of parts and it's up to you build, test & diagnose it how you want - you need to be really comfortable building electronic circuits & reading & understanding circuit diagrams. You get what you pay for and it's also very cheap compared to pre-built and fully supported ECUs.
And also quite satisfying to see the pile of bits turn in to a functioning ECU.

On my setup I wanted distributorless, running a pair of 4 output coils in wasted spark mode - so you can see on the 2nd pic I added 4x coil drivers, the transistors on the RHS of the heat sink (there's also one mounted underside).
I also run a Innovate LC-2 Lambda setup on it which I use for setting up the fuel map. I've not bothered with a gauge like the beginning of this thread, but I have bluetooth on the ECU so I can monitor AFR with my laptop at any time without needing to even plug anything in.

MadMat,

Very nice setup. Are those ping pong balls sealing up the intake horns?
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Nice setup of the Megasquirt kit.

I had another look atthe Megasquirt site, its been a while. Seems they have a lot of options for the kits and assembled boards.

Relatively cheap, I'll have to dig further in. Hard to beat the twin 750DPs on the blowers for reliability. First step is the Daytona Sensors Digi ignition box.

But you've piqued my interest for sure. Cheap enough efi computer to have a spare on board.
​​teres

Again..nice clean setup. Thanks for sharing.

Yeah. the tennis balls are just so that when I don't have the proper cover over the horns (which is a bit bigger), if I'm doing some work on it, the last thing I want to hear is the tinkle of a screw dropping down the horn, past the butterfly and "hopefully" ending up on the back side of an intake valve.

750DP - I hear you. But it also frustrates me how inaccurate the old holley's are. Can't remember now, but don't you either end up having to have a midrange lean spot (nasty), or end up with it too rich at idle, or WOT? I can't quite remember.
I also have a nice 4-barrel throttle body somewhere - so if this setup hadn't worked, I still didn't want to go carbs, so I could have done a "conventional" 4 barrel intake with a throttle body on top. That was the backup plan, but this setup seems to work well enough. And makes quite a nice howl through the intakes when you open it up.

I think some of the modern prebuilt megasquirt options are pretty good to, and you can get them with fully supplier support, compared to the earlier kits where you were much more on your own.

While I'm no carb guru, the modern Holley style carbs have a lot of tunability. Add the wide band O2, and you now have buddy with real time monitoring.

For me, I have height restrictions and just fit the 420 blowers and twin carbs. No room for a taller intake to fit injectors, unless I went with a front/rear throttle body feed for the blowers. Or do the horizontal injector plate system with 4150 TBs.
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i'm only running about 650hp each, so maybe a 489 stroker, will get me most of the way there NA, and I can drops the 150lbs of blower and iron heads each side.

My friend that had the MS3 complete pre-made kit had a poor experience, but I'm more into the solder your own. I'll really have to re-think future plans. The Holley helm display is nice for sure.

Have you seen anyone use something like that for the MS3?

Yeah, I'm generally OK with carbs, although not up to date with the latest generation.

I agree that if he bought a pre-built one, then the supplier should have supported him. Both of mine have been solder your own jobs. Who did he buy it from? Both of my kits came from diyautotune, but I've never needed any support from them.
The above is an MS2 V3 - I didn't want to go full sequential, coil on plug etc, so didn't need the MS3 features. For anything I need, distributorless wasted spark ignition and batch fire injectors is fine. So far, anyway.

DIYautotune

MS3 Pro.

It was odd...it worked with the Jimstim simulator. But when fired it would dump fuel and stall out. The advice from DIY I think was keep it running until the O2 gives signal.

The easy solution would be DIY getting the problem unit and sending a new one....then DIY has an example issue board to use as R&D and prevent issues in the future.

He hired a few pro level tuner guys to help out and diagnose. No joy.

MS3 is more of a product style than a DIY​​​.

Thanks again for harvesting my MS interest. I kind of just archived it....now that there are so many other options.