Marine EFI Conversion


This is our family boat! We bought it used a few years ago and it has quite a history. According to the man we bought it from it was bought new and used very little. There was a nasty divorce and she kept the boat (and probably everything else too!). She didn't use it and let the engine freeze over the winter. The man we bought it from had replaced the engine.

It's a 1989 Four Winns 190. Propelled by an OMC Cobra stern drive which is powered by a Chevy 4.3l V6 engine.

The boat ran well but it didn't have a very good throttle response. It was hard to crank up and would stall often. You couldn't pull a skiier because of the erratic takeoff.

I knew there was only one thing to do- Inject it!

The original setup had a POINTS distributor, and a Rochester "2-Jet" carburetor. The manifold was cast iron. As with all marine inboard engines the exhaust manifolds have a water jacket. There is a "raw water pump" in the stern drive which brings in lake water which is circulated through the engine by the main water pump. After this water is hot it is discharged through the exhaust system along with the exhaust. This cools the exhaust and allows the exhaust system to be constructed of non-metal parts.

The "donor car" for the EFI system was an old Chevy Astro Van. It had the same 4.3V6 engine. I took the manifold and all associated components from the van. The TBI manifold is aluminum. There is an issue with corrosion using an aluminum manifold on a raw-water cooled engine so I had the water passages epoxy coated.

The original setup would run about 3500RPM and the boat would go about 40 MPH. Now, it will wrap out to 5000 RPM and the speedometer stops at 50 MPH but it is well past that. Could go faster with a different propeller pitch but it is more than good enough as-is! The throttle response is near perfect. This was one of my most rewarding EFI projects since it improved the boat so much.


The new TBI manifold was slightly different at the water outlet port. To make the marine water exchange housing work on it I had to fabricate an aluminum adapter plate. It simply rotated the bolt pattern a few degrees to clear the throttle body. The engine coolant temp sensor required to drill and tap a new hole in the front of the water passage in the manifold. Tried mounting the sensor it its original position but the water hose pressed on the connector and it was not a good setup.

To clear the water exchange housing, the alternator bracket had to be extended 3 inches and a longer belt installed.


The original distributor had no vacuum advance and it was a points-type system. To take full advantage of the ECM's spark control, I installed the distributor off of the van. It is marine-acceptable with a screw-on cap and explosion proof vent in the bottom. This was a direct bolt-in swap.

The original carb had a small-opening air filter. The TBI has a 4-barrel size top. Beleive it or not, the 4-barrel marine spark-arestor carb top was the hardest thing to find for this conversion!


The electrical box sits under the rear seat next to the engine. There are 2 sealed Deutsch connectors between the box and the rest of the engine. One connector is for the TBI and engine sensors, the other is power/ ground, fuel pump, ignition, and diagnosis.

This boat has a 15 gallon fuel tank but it is only about 6 inches deep. There are times when there is plenty of fuel in the tank but the fuel may only be a couple inches deep. There are no baffles in this tank. This design is required to fit in the space in the boat hull under the floorboard. As a result of this, the fuel inlet is exposed and the fuel pump will be constantly taking in air bubbles or even running dry as the boat moves.

To combat the air-entrainment in the fuel, I designed a dual-circuit fuel system. It has a low-pressure transfer pump drawing fuel from the tank and pulling it through the marine fuel filter/ water separator (red spin-on filter). Then it sends the fuel into the fuel accumulator. From here the fuel exits the accumulator at the top, allowing any air bubbles to rise and exit returning to the fuel tank via the return line.

The main pump draws fuel out of the bottom of the fuel accumulator below the level where the transfer fuel is coming in. This area will not contain air bubbles. The main pump then pushes it through the secondary fuel filter (this is a stock GM filter like any EFI car has) and through the TBI unit. Return from the TBI unt travels back to the fuel accumulator. Even if the fuel tank inlet is exposed and the transfer pump stops pumping, there is enough fuel in the accumulator to run the engine for several minutes until the tank inlet is once again submerged. This can happen during hard handling of the boat at high speeds.

The accumulator is a 6" long piece of 1" copper pipe with caps on each end, mounted vertically with fuel line connections brazed into the side of it. The transfer pump is a standard carburettor-type fuel pump. See picture. The main pump is an inline high pressure pump like that used on a mid-80's Ford. It has a special screen in the inlet fitting to prevent pump failures due to contamination. This was learned the hard way after a piece of copper from the fabrication of the accumulator locked up the first pump! I purchased a faucet aerator screen at Lowes and placed it in the pump fitting.

The main pump is under ECM control like in any EFI system. The transfer pump runs whenever the ignition is on. It takes a moment for it to draw fuel through the water separator each time the filter element is replaced.

The rude, crude and socially unacceptable drawing below may give an idea, IF you can read it....

All EFI-related electrical parts are mounted in the electrical box above. It is watertight with a gasketed lid and sealed Deutsch connectors. In this box are the main (ignition) relay, fuel pump relay, ECM, fuse block, and TVP unit (reverse battery/ overvoltage protector).

The ECM is a 1227137 running a modified version of the 4.3 calibration code "FJD." Using TunerCat, the calibration has been modified to run in open-loop only, no EGR, and the idle speed is set to fall to base idle at 120 degrees F. The raw-water cooling system does have a thermostat, but it would never reach the temperature the original calibration required to allow the idle to fall to base idle speed. This is important to allow shifting the stern-drive into gear smoothly. It is a "crash-box" transmission with no clutching and the engine has to be at low idle to avoid damage to the shifting "dogs."

This system is speed-density (uses a MAP sensor) and runs in open loop only with no oxygen sensor. The timing and fuel tables have been tweaked a little at idle but other than that the calibration is stock.



The weatherpack plug shown above is coming from the ECM harness outside of the electrical box near the boat engine cover. It connects to the box shown below. The cap in the above picture keeps dirt out of it when not in use. In this circuit are the ALDL data line, DIAG circuit, Check Engine lamp circuit and a +12V and Ground wire. The box breaks this out into a standard ALDL plug for using scan tools and computer diagnosis. The red lamp is the Check Engine lamp, and the switch will activate retreival of service codes over the Check Engine lamp (same as placing a jumper in the ALDL). The 12V power socket on the end of the box is for powering the scan tool. My Nova has the same connector and the diagnosis box works on this boat and that car.


Here is an excerpt from several e-mails I have received about this page - read this and maybe it will give some insight on this! Please feel free to e-mail me if you want. See front page for my e-mail address.