Help with marine 3.0 mercruiser tbi project

[4x4toys]

Hi, starting to gather info on a project I would like to complete this fall season.
I have a 2000 Bayliner Capri bowrider, it has the same year 3.0l mercruiser 4 cylinder. basically it is a stroked version of the 2.5l iron duke engine.
I would like to build a TBI system for it that would be as close as possible to Coast Guard Specs.

This engine already has the GM small cap EST distributor, one less thing to source.
I would like to find an under hood style ECM that will suit the requirements of my small displacement engine with a small cfm throttle body.
Also closed loop would be a plus, I have a good idea of where to mount the HO2 sensor so this should be possible.
So, This will be my first home made system.
Can't wait to get started. Thanks

[96lt4c4]

I did this to a 1990 Glastron 19 foot with a 305 in it. Used the 7427 PCM running in open loop. Never got around to trying to get the 02 sensors to work. I also installed a GM LT4 Roller cam and ported the heads. The block was a roller block with a flat tapet cam, I put all the OEM roller stuff in. The boat would fly, almost 60 MPH with just me in it. It was almost dangerous! Let me see If I can dig up some pictures. It was a cool project.

[EagleMark]

Dude, that is sweeet! You've been holding out on us...

What's it reving to now? Change the prop, you can go faster!

[96lt4c4]

Dude, that is sweeet! You've been holding out on us...

What's it reving to now? Change the prop, you can go faster!

LOL....Boat is long gone, wish I still had it though. I pulled everything back off before I sold it and put it back stock. I did change the prop out to get more top end but it would not pull as good out of the hole with a tube or skier.

[1project2many]

basically it is a stroked version of the 2.5l iron duke engine.

Haha... except that everything's just a little different. I spent a summer trying to chase down a set of connecting rods from a 3.0 for an engine build. What a lot of misinformation there is out there.

There is a 2.5 powered Lumina which used an underhood ecm. 90-91 was TBI although it used the "model 700" TBI which has a fairly small diameter bore. For the easiest swap I'd probably use that calibration as a starting point.

Depending on what the intake configuration is you might be able to use the manifold and TB from an 81-85 2.5 which uses a larger TB (model 400??). At a minimum i'd probably spend a little time looking for a marinized TB or consider doing things like making a brass TB shaft and installing stainless screws to slow corrosion. I've seen TB's rust up pretty good from a couple of years exposure to moisture in a basement or junkyard.

Fuel handling might be an issue. Many of the Mercruiser systems used a mechanical carb type pump to a reservoir. The reservoir has a float to regulate incoming fuel from the tank, and contains the high pressure pump along with a vapor purge line. Hot fuel returning from the engine is directed to the reservoir to prevent heating of the fuel in the tank. You don't want to deal with vapor from a large amount of fuel if possible. In other applications Merc used a "fuel module" or "fuel cooler" which routed fresh water around returning fuel for cooling. This might be the less expensive solution if you can't find that remote reservoir but then you'll need proper baffles in the tank along with an EFI capable pump which meets coast guard approval.

If you want to get a little more involved you can use one of the in-car ecms mounted in a box like pictured above, install a TB from a 2.8, 3.0 or even the 4.3, select the appropriate sized injectors, and run on a modified V8 calibration. Boats are fairly easy to tune as they idle and run WOT most of the time. What they don't do is experience rapid throttle changes and extreme loads (30 mph in 5th gear uphill for example).

Good luck, sounds like a fun project.

[EagleMark]

And be a good captain during the whole procedure and after. Before starting an inboard boat open engine cover because you want to check oil right? Wrong, it's because you want to stick your nose in the hull and smell for fuel! Then turn blower on for a few minutes, I used to stick my nose at blower vent to smell for fuel too.

Fuel vapors sink, no big deal on a car they just blow away on ground. In a boat they collect in the hull, under the floor boards! Ever seen a boats floors, seats, coolers, fishing equipment, coolers etc... all over the dock and water? = bad captain... no not me! But we have SO many lakes and boats up here and such a short boating season we have a couple blow up each year...