GregGold

I put a tbi 7.4 in my '88 SeaRay, replacing a 5.7 carb. The first season it ran good, til the alpha broke. After a year, I went with a Bravo III and it ran very well the first time out. The next time, it occasionally stalls, but not dies. More recently it runs rough at lower rpm. I suspect fuel pressure, but have not put a gauge on it, yet.

I have an inline, 15 psi borg warner dc fuel pump that I spent way too much for and I understand that dc pumps typically do not have much vacuum. I also recall working on a buds boat that has a one way valve in the fuel tank, requiring additional vacuum to get fuel. I wonder if my boat has one of these valves. (Is this standard equipment?)

Does it seem likely that my dc pump could be struggling to pull fuel? I also noticed the pump gets quite hot after a while. Could the sock be stopped up? The boat has at least 600 hours, if not more. I am considering a good mechanical pump to pull the fuel out and feed the dc pump. Also, do factory efi boats have in-tank pumps like cars?

Thanks,
Greg

cooltoys61

It would be a good idea to remove the sending unit or inspection plate (if there is one) and check the sock and for any debree that could be accumlating then falling off after shut down. I don't know much about the EFI but I don't think 15psi is nearly enough. Many FIs use 2 pumps , like you said. Others here will know.

SeaRay Jim

My 2000 7.4 MPI is being replaced so I may have a bunch of parts available including fuel pump and complete MPI (ECU needs to go back and get reprogrammed stock)

GregGold

CoolToys61,
The tbi system is a simple, dual injector setup, common on 90s vintage autos. It has a running psi of 13-14. This motor came out of my rv.

SeaRay Jim,
The MPI setup is quite tempting. What are you needing for it? What does it cost to adjust the ecu? Curious why it was moded, supercharger?

Thanks again

mcollinstn

all boats have an anti siphon valve on the fuel line. While it looks and functions like a one-way valve, that is not its purpose. Its purpose is to create a spring loaded restriction in the fuel line that will keep fuel from drooling out into the bilge in the event of a crack in the fuel line below the fuel level.

They do present a restriction for a fuel pump, but they usually do fine with them.
Excessive fuel pump heat is usually caused by lack of fuel flow thru the pump. If you have the pump deadheaded into a non recirculating regulator then the pump bogs down against the regulator and doesn't flow much fuel and overheats.

If you have a regulator that bleeds excess fuel off thru a return line, then the pump keeps a good flow of fuel thru it and that keeps it cool.

Waterfoul

I replaced my anti-siphon valve after having symptoms similar to what you describe... issue went away for several seasons. Then the symptoms came back and I simply replaced the valve with a nipple fitting. Issue again went away and has never resurfaced. As I understand it, this is not coast gaurd legal... but I inspect my fuel system regularly (and most other systems on my boat... when it's running....).

Vinny P

Greg,

Why do you think it is a fuel related problem? You say it runs rough at low rpm, but you didn't mention how it runs at speed. Generally, a fuel problem would be more pronounced at speed. When the engine is under a load, the fuel demand is much higher than at low rpm.
To the best of my knowledge, boats do not have a fuel pump in the tank. My stock 454 Magnum MPI has a pump mounted on the side of the block, where the motor mount would be if it were a car motor. It sucked the fuel up through the tank and anti-syphon valve just fine.
However, most aftermarket fuel pumps do not suck well at all. They are always best mounted low and as close to the tank as possible. If your 7.4 motor came out of an RV that had stock T.B.I., it most likely will not have the holes drilled or cast in the block for a mechanical pump. That leaves you with the option of using a fuel pump mounted on the sea water pump.

Waterfoul

And, a 15 PSI pump will NOT be delivering 15 psi at the carb.... especially if it's having to suck the fule any distance, thru the anti-siphon valve.

GregGold

I appreciate your input.

The TBI has an inline regulator and a return fuel line.

The highest I have run it is 4k rpm at 55 mph. It was still slowly climbing, but I felt for the guy on the tube, so I backed off. It seemed like it should have had more power. Another thing that is interesting, moving along at 25 - 30 and hammer it, it responds very strongly for a few seconds, then calm down.

I am suspecting decreasing fuel pressure as the first few times I had it out, I would put it on 2k and the boat would plane immediatly, every time, with 7 people. Lately, I put it on 2k and it will respond briefly, surge, then respond. Just seems like fuel. I plan to put a gauge on it.

I think, I will also look into the anti-siphon device and perhaps remove it.

Thanks again,
Greg

monsta

The existing fuel lines and pick up may not have adequate diameter, therefore restricting or reducing volume of fuel. Normally when upgrading power, I change the pick up and fuel system. I suggest doing so with the fuel system to help prevent possible future engine damage because of a lean condition. A big block should have -6 or better -8 if in stock form and larger for high hp application's. A good electric fuel pump will also work well, like a Mallory Marine EFI pump, or equivalent. Try to make sure the fuel water separator, fuel pump, and fuel line's are as low as possible, and only ascend to the engine, to avoid becoming air trapped or creating an exta strain on the fuel pump.. Good luck