36Tango

I have a 383 fuel injected Mercury Scorpion) with electric fuel pump. Fuel goes through the filter, through the fuel pump, and then through the regulator on the way to the engine (fuel bladder is in the closed bow). After the regulator, there is a "Y" that returns fuel to just before the fuel filter. The filter and fuel pump sits horizontal on the stringer at about the same level. The regulator is a tad bit lower.

When I get up to about 4000 rpms, I start to get some "cutout". It is worse if I am in rough water where the attitude of the boat changes. It can also begin to cutout if I lift the trim too high, putting the nose just a bit higher in the air. At 3000 there is no "cutout" unless I hit some rough water, or raise the bow with trim. The fuel pressure remains at 37-38 pounds.

When it happens, I get out of the throttle, come to a stop, and raise the hatch. I disconnect the fuel line (there is about 3 feet of braided line between the regulator and the intake) on top of the intake just before the pressure gauge, I put the line in a bucket and have Jr turn on the fuel pump. Just air comes out at first and then a steady stream of gas. Reconnect and good to go.

Before taking the fitting loose, the fuel pump runs and makes pressure, but you can tell by the sound there is no gas moving, it is just compressing air. After I bleed the system, the fuel pump changes tone after just a couple of seconds telling me that fuel is in the line again.

Where is the flaw in the fuel system? I was going to talk to my mechanic tomorrow, but some insight would be helpful. What could be happening in the plumbing to make this airlock? Should the regulator be higher than the pump? Should the filter, pump and regulator be in a straight line?

I did try loosening the gas cap to see if I had a blocked vent, and it did not help. I checked the fuel filter a couple of times and it is fine.

Thoughts?

stevesxm

well... the short answer is yes but the first question would be " has it ever worked correctly in this configuration before ?" never the less ,for me it sounds as if the fuel delivery to the the inlet of the pump is at capacity limit for any number of reasons... length of line, tank too far away, picking up air during low fuel/ agitation issue... who knows ... any number of things but the solution is easy in concept if not a bit of work in execution.
you create a fuel accumulator in the engine bay anywhere above the pump. you make or buy a 1 or 2 quart "tank" ( ATL sells them) that you pump your primary tank into the top of with a conventional hi volume /low pressure electric pump like a carter... that " accumulator" gravity feeds the electric pump for the injection. the accumulator needs a return line to the tank so it doesn't pressurize and while you are at it , you run the return line from the injection into that return as well.

so... fuel is pumped at hi vol/low press from the tank to the accumulator. that 2 quarts gravity feeds the hi press injection pump. the injection return line goes to the accumulator return line which goes back to the tank. what that does is that no matter what happens to the boat as far as g forces or anything else, the injection always has 2 quarts of relatively cool , non aereated fuel to draw on . this system solves the dreaded vapor lock issue on the mpi motors as well.

REDCHECKMATE

check the fuel pickup tube in the tank

36Tango

The boat has an ATL bladder mounted in the bow approximately 12 feet in front of the motor.

I talked to my mechanic this morning and he questions whether we have enough fuel pump on it. He says that since the pump is so far away from the tank, it needs to be well oversized in order to draw the fuel that far. He tell me that once the pump cavitates just a bit, it will be airlocked. He suggested replacing the current pump with one of those big $$$ Aeromotive pumps.

Any thoughts?

GPM

I agree with Redcheckmate, check the pick up, see how close it is to the bottom of the tank. Pump size won't matter if it's picking up air.

stevesxm

he is right and he is wrong. his analysis is correct but for the wrong reason. efi hi press pumps do a great job of pushing fuel and making pressure but are not very good at all at creating vaccum to pull fuel. the really need a positive head or at the very least not to have to make a lot of lift. it is not unique to you... they all do it. a more expensive efi pump isn't going to help much. the carter i mentioned is cheap and move 5 times the volume you need. i suppose you could always just use that at the cell to move fuel forward but without the accumulator /return you would be pressurizing the inlet side of the efi pump....

i don't know... seems to me a pretty clear cut problem... you need to not make the efi pump do the work of lifting the fuel and pulling it forward... how you do that is just a matter of how much work you want to do. and the efi return line really needs to go back to the cell... don't put hot aereated fuel right back in the inlet ... i know merc does it but thats a mistake.

the guys talking about the pickup have a point but i used ATL bladders by the hundreds... great stuff... and the chances of the pick up falling off or being misplaced are zero... but having said that, i did have a number of times, fuel contamination with water that would form a slime and coat that fine fine little screen they use at the bottom of the pick up and cause starvation. i presume that it has a 16 bolt access panel in the top... just unbolt that and pull the assy out and see what in there. should be pretty easy assuming you can get to it. and while we are on that subject... you could always just put a submersable pump in that cell to act as your lift pump... that hardware is off the shelf for those cells. every efi race car on the planet has at least two of them in there... often 4

36Tango

Can you give me a link to this pump? Does it get wired together with the fuel pump in the bilge to come on at the same time?

stevesxm

no link. they are generic . the ones i used to use were for all manner of gm cars but napa has a billion different types... i might give ATL a call. they certainly didn't have anything dedicated when i was working with them but that was 15 years ago and they might now. stand by and let me run a search

this is what they look like... http://www.jegs.com/i/JEGS-Performan...ductId=1167182 this one is 125 bucks and it probably isn't anything special... but this is what it and all others like it look like. i would wire it directly to a separate switch on the dash but thats just me and i don't know what the CG says about that. i certainly don't know what your installation looks like but , i suppose you could just use the in tank pump to replace your standard pump... i mean once you confirm the volume /pressure numbers then the rest is plumbing... gm uses a pump just like this to run the 400 plus hp corvettes... so capacity and location ought not be an issue...

36Tango

I appreciate the help. This bladder is in an extremely tight space, so I want to be as prepared as possible. I will need to send my son up there tonight to try to get a picture of the top of the bladder. On the ATL site, they also recommend a particular fuel gauge that I may need to add also. It makes total sense to take the return fuel back to the tank. I'll try to get a pic and post it in just a bit.

Thanks!

36Tango

Here are a few pics.

One is of the pump and the regulator. Both are mounted on the stringer with the regulator being slightly lower. The return line is on the bottom side of the regulator. We have it set at 37-38 pounds.

Another pic shows the Y where the return comes back in, the filter, and the pump. The incoming fuel line comes on from above.

Last pic shows the boat. The bladder is in the front, so it takes climbing over the dash to get to it. This is why I need to have some skinny friends!