Fuel supply issue (mechanical pump / filter location)
 

Hello all. I’ve been trying to sort out a fuel supply issue.

The boat is a naturally aspirated, BBC with a Baker 6 vane fuel pump. I have an Aeromotive bypass regulator and the pump puts out 15PSI. I have the regulator Set to bypass all but 6 PSI.

I’m using a FST fuel filter with -12 ORB ports. The fuel tank is in the belly of the boat and has a 1/2” fuel pick up tube (no sock) and a 3/8” NPT to -10 AN adapter. -10 AN line to the filter inlet and -10 line to the Baker pump. The output of the pump is -8 and it feeds both bowls with a -8 “T” on the rear bowl and feeding the bypass regulator.

The filter is roughly 6” above the top of the tank.

My issue is that as my fuel level in the tank drops, my fuel PSI becomes unstable. It has dropped to 3 PSI, leading me to idle the boat back to the dock. I thought maybe I had a ruptured diaphragm on the regulator and it was bypassing most of the fuel. I’ve disassembled it and it’s fine. I disassembled the Baker pump and it’s fine. I’ve removed the comp cams bronze tip pump pushrod and it shows perfect witness marks. I’ve checked all the lines for blockage and checked the pickup tube in the tank for a fault. All is well there.

I’m wondering if having the filter higher than the tank could be causing my issue. My thought is that, when full the pump has less vertical rise to siphon the fuel into the filter, and as the fuel level drops, I’m asking more of the pump on the vacuum side.

My other thought is that I should have the filter after the pump and before the carb. I called and talked to the rep at FST filters today and he says that the filter will flow in excess of 300 gallons an hour. It’s only 3 microns, but has a bunch of surface area. He thinks my issue is the location of the filter being 6” above the tank.

I’m wondering if I would be better off with the filter mounted on the front of the cylinder head, above the pump and near the carb feed. I would have a horizontal run from the tank to the pump, a 10” piece of -10 up to the filter and the -8 feeding the carb.

Any suggestions or thoughts would be greatly appreciated.

Rob

I’d bet small crack in fuel pick up or your return is foaming the fuel and at a certain level pick up is grabbing it.

I’ll look for holley’s vid of fuel foaming from returns. Will open every one’s eyes greatly.

try bypassing the filter and see if the problem goes away.

the pickup has been checked. No cracks. I thought the same thing. I can remove the fuel
line fitting from the tank and put a few pounds of pressure on the vent tube, and watch the fuel rise in the pickup tube without any air leakage.
my return dumps into the fuel fill near the fuel fill at the top of the fuel fill hose.

that’s tomorrow’s test.
I fed the filter from a 5 gallon gas can and all was well. The only variation with the gas can was that it was approximately 2 feet above the filter.

Here is one from Fuel Lab:

The one I am trying to find shows more on return placement snd effects than this. But this is still good vid.

Have you verified the fuel tank vent line isn't plugged?

yes

The vent hose doesnt drop below the vent outlet

At 3psi do you have a drivability issue ?
You had a drive ability issue and then noticed gauge or you just see gauge back down and thus you return to idle to play it safe ?

Is the fuel psi gauge in dash or engine mounted ? If engine mounted is it a liquid filled gauge or dry ?

it has dropped to around 3 PSI after running for a bit. No drivability issues. Most of my time is with the family in the boat and for short runs to the beach. I took it out with a full tank last Thursday and when I got to 5/8 tank, I started to see the PSI drop. The boat stalled leaving the restaurant dock (first time the boat has ever stalled) I restarted and Left for the short trip back the the home dock. During the trip back it stumbled when pushed past 2800 to 3000 RPM. I was just on plane trying to not ask too much of it. 3 PSI at that time.

i have an autometer electric fuel gauge. (With the correct 130 dollar transducer) my first step was to remove the transducer and install a non liquid filled mech gauge at the carb.

I’m leaning toward this being a fluid dynamics issue. My thought is that lifting the fuel up 6” above the tank siphon tube with a mechanical pump is at the threshold when the tank is full and as the fuel level drops, the vacuum from the mechanical pump isn’t strong enough to pull the fuel through.

still waiting for a call back from Baker fuel pumps for their input.

when I brought the boat home and installed the mech gauge, I couldn’t get good pressure. Thinking I had a tank siphon tube issue, I put a section of -10 on the filter inlet (usually fed directly from the tank) and put the free end in a can of gas. Instant fuel pressure. The gas can was 2’ above the filter.

I have a Airtex mechanical fuel pump on the water pump housing and my filter is probably 8"-10" above the tank with a NAPA 3352 10 micron filter/separator on the suction side. I have no problems with fuel pressure unless I'm idling at 600rpm. How old is the fuel pump? Vane pumps do wear out and suction is harder on them then pressure.

Baker pump with around 40 hours on it.

Exactly. Most boats, the filter/separator is atleast 6” above top of tank. And all Mercruisers are before the mechanical pump, and higher than them.

Thanks for all the question answering chromemoly.

I’ve never used a Baker pump, so I don’t know if they have a typical issue or not. I do question why the use of a high pressure pump in this app. ?

Oh wait, brain actually did something.

Did you have to clock the pump, snd if hou fid, did you know there is a specific procedure for doing so on mech pumps ? Not just unscrewing base, reorientation, then screwing back down ?

the 15 PSI pump
was used with a bypass regulator to avoid dead heading the system. This was recommended by Dale Cubic.

I didn’t clock the pump, but when I disassembled it to check for any issues, it appeared to be straight forward.

Some hsve mistaken me with him a few times. CFM :)
Great carb builder he is. :thumbs:

I have a call into him as well.
he’s a good guy!

Glad to see you found your problem.
A mechanical pump should have no issue lifting fuel 6” :)