EFI Fuel system setups using oil/water separator

[Frequent Flyer]

Ok, I know we beat this subject to the ground a while back but I have a simple question on the mercury recommended high pressure fuel system setup (gas tank to separator to high pressure pump to engine with return to separator). WHERE DOES THE AIR GO? And I really don't believe it goes back to the gas tank. I'm considering re-routing the return line directly to the gas tank to lose any air in the system. I unfortunately have the separator and pump mounted a few inches above the gas tank though I don't think suction head is a problem when considering the pump is capable of 50++ psi of discharge pressure. It's the air I'm worried about.

[daytona]

Cant tell you where the air goes but after seeing Ron Andersons dyno plumbed that way, I set mine up that way and have had no trouble. really simplifyed my setup as my eliminator tunnel has 2 separate tanks. 97 2.5 offshore. rick

[AlaskaStreamin]

Just got off the phone with Jamie Adams (Owner / racer / Builder of HydroDynamics Racing inc.) He's setting me up with my fuel system. He said turning even 8000, you should have a 1/2" pick-up tube and hose going to the filter and fuel pump. (They push fuel great, but don't suck well) He also said don't return the fuel to the filter due to air. He said the sending unit is the best place to put in the 1/4" return line but your idea is good also. He recommends a take apart 28 micron filter and to clean it every month. He said the final filters are junk because you can't see them clogging.

Yea, I'm watching out for you. Now,...how low can you go on that prop? ;o)..

[Mark C.]

I have my return line plumbed back to the separator. I have yet to be convinced that this is not a good idea.

Please explain where this air comes from that you are worried about and how it is introduced into the fuel lines.

Thanks,
Mark C.

[DaveR]

That's it Mark! dog-gone it, I refuse to change again!

[AlaskaStreamin]

You will get air upon first set-up, if you run the tank dry, when you drain out any water (if your filter has a drain), and when you change filters.

Since the fuel is constantly running through the fuel rail, one would think there woulden't be a problem. But, where would the air go?? Seems to me that it would stay trapped in the top of the filter.

I know returning to the filter is common and there hasn't been any reported problems,.....but,.....since I'm in the rigging stage, I'll return to the tank just because it sounds like the safer way to go. Maybe Greg, Jay, Sam, etc can give their professional opinion. My info came from Jamie Adams @ Hydro Dynamics Racing inc.

[Techno]

Although I think bigger is better for fuel flow I'd like to share what my boat got for fuel consumption the few times it did run. 2 miles per gallon. That is 4 gallons of gas sprayed into the engine in about 8-10 minutes. I don't think I had any fuel obstruction at this rate of fuel flow.
Mine is plumbed to the filter but that is something else to change in the future.

[DaveR]

This didn't seem to post the first time, and maybe everyone already knows this, but when I was checking into fuel pumps the Mallory tech guy had a table or formula that said 225 hp required 35 gallons per hour. From this he could calculate that the pump I was checking out would produce 35 GPH through 3/8 hose at 55 psi, which was way above the 39 psi I needed. Pretty cool. I should try and get the tables. I added something witty here the first time, but just can't type it again.

Hope this doesn't doublepost now

[Mark C.]

Yes, there is air when first set-up, but that just goes right out through the engine at initial startup. After that initial purge of the air, you're just bypassing extra fuel back through the system.

So, again, someone please explain to me how air is being introduced into the system just by draining back to the separator??

- Mark C.

[Frequent Flyer]

Ok guys,
I just came in from re-routing my return line to a newly installed tap at the top of the fuel sending unit (it used to be back to the separator). Now when I run the pump it sounds very smooth. Before, it wouldn't stop gurgling (when returned to the separator). As of where the air comes from, I agree with Ron. Mark C.; one thing I couldn't explain/figure out is why even after running the boat for one hour, the gurgling noise was still apparent (when the return was to the separator). I also removed my fuel bulb which was located right out of the tank (to help prime the previous closed loop system). One thing for sure, I feel a lot more comfortable knowing that if air got into the system it will immediately be expelled back to the tank (say during fuel slosh when running low on fuel).
Gotta love EFI's,
Now I can blast my CD player now that I don't have 2-fuel pumps running!
Mark S.

[espen]

Hi !
I have rigged my boat with catch tanks , looks like drysump
oil /air separators with the high pressure fuel pump lying under the tank with allways 1 foot of fuel feeding it.
First from tank to water /filter then to holly pump feeding
the catch tank with a return on top of it back to the tank.
The return in the tank are tee ed into a tube in the filling hose,but the best is to have the return out in the buttom of the tank .The high pressure pump is feeding a automotive efi filter and the return from the engine is to the top of the catch tank ,I would love to have a warning if the catch tank filling is low......
The injector dont know if its air or fuel passing.
The holly pumps are making lot of noise and I would like to change them for something better ,maybe a diagram pump ?
anyone know of a nice one?

[Delta]

I'm not sure if I understood you right,but for what its worth,Chuck Goodman at Hydrodynamics sells a pressure switch/warning light combo.You can set it,so if your psi drops only a few psi,the idiot light will come on.Hope this heps,Delta

[speed]

i have my return line into my tank. i tee my lines from both tanks into my fuel seperator. i do have an auto mobilr inline fuel filter just before my fuel pump. is this acceptable ? i only have about 20 minutes on my rebuild merc 1997 260 i was told to use double amount of oil for break in non synthetic. i was told to use yama lube for break in about 30 gallons then switch to merc race oil at 32 to 1. does this sound right ? i never thought i had to watch for so many things on this engine . any help on the right way to run this would be great.:confused:

[MTCM]

Speed,

This is an old thread. Just so you get your questions answered I replied by mail. Read it before you proceed with break-in...........