Fuel Pump Questions

[Argess]

I have a pressure guage on the fuel log just before the carb. When I shut the engine off, the pressure drops to zero in a few seconds. The fuel log also has a bronze filter. If I remove the cap from the filter, it is more or less dry. So, it appears the fuel in the lines siphons fairly quickly back through the fuel pump to the tank.

Is this normal for a mechanical fuel pump? Mine is a Carter, P/N M6905 I think.

My memory for these things isn't that great, but I think it used to take quite a bit longer....like a few minutes, or even much longer....for the pressure to drop once the engine was shut off. Do you think this may be a sign my fuel pump is failing?

Just curious as I am investigating what might be a fuel delivery probem. Pressure is fine at idle (7 psi). Pump inlet vacuum is also fine (17 in Hg). I have yet to do a volume/flow test.

[brettco]

if it starts fine then I wouldn't worry about it.

[elmariachi]

I have the Carter 6905 and a log-mounted pressure gauge on my side oiler with a 750 Demon, but the gauge is nothing more than an ornament because it never works. I stopped worrying about fuel pressure...the car always starts and has terrible gas mileage, so I know I am getting enough fuel.

[RICK LAKE]

Argess My ERA has a return line. Is yours not hooked up? Do you have a pressure regulator in the fuel system? If it does the pressure will bleed off in 3-5 seconds. If not it will bleed back through the fuel pump if there is no check valve in the system. You need to check the car out. Rick L.

[Argess]

No, I don't have a return line. Nor a pressure regulator. Don't think my ERA fuel tank even has a fitting for a return line. Sure would help if I am getting percolation though.

Anyway, seems to me that all a mech fuel pump is is a diaphram and two checkvalves. Mine isnt' leaking, so diaphram should be OK. But for fuel to flow back, both check-valves must be leaking, unless there is a purposebuilt bleed back valve in the pump, whihc I doubt.

My car starts on the fuel left in the carbs from the last run and the carbs manage to fill back up before they run out. I am getting some "lack of fuel symptoms" though and noticed that the guage drops really fast once I shut the engine off. I didn't think it used to do that, but as I can't remember for sure, I thought I'd ask.

I will do a volume test in a few days. If it fails, I'll dismantle the pump and see just how it works inside.

[Danr55]

Sounds like you have a constant flow system rather than a constant pressure system. One side of the fuel system (the carb) is always open. The other side only provides pressure when the engine is running. The pressure in the system drains off to the open side when the motor is shut off and there is not pressure from the pump. It would be normal to fuel pressure to drop to 0 when the motor is shut off.

[Argess]

Thank-you for the reply. I have confirmed the fuel drains back from the carb all the way to the fuel tank due to the siphon effect. Only thing is, it shoudn't if the check valves in the fuel pump are working correctly, or there is some feature of the pump I don't know about.

Wether or not leaking check valves will affect pump delivery volume is yet to be determined. My carbs are currently apart and waiting for gaskets, so I have to put the volume test off till everything is back together. I was going to do the test just by cranking, but all test specs seem to require the engine at idle, which is a bit faster rotation. Additionally, I want to rev the engine a bit and see if the delivery or the pressure drops, and I need the carbs back on for that.

EDIT: I dont' know where that "thumbs down" came from, but I didn't put it there.

[elmariachi]

FWIW, during the period of time when I was setting up my car before mounting the body, I had one of those timebomb glass inline filters installed in the line between the Carter 6905 and the log, and it never indicated that the fuel was draining back into the tank, even after sitting for a week or more. So maybe as you say there is something wrong inside your pump?

[Argess]

Well, looks like I was barking up the wrong tree. No doubt the check valves are dirty or their springs are weak, however I did the volume test....just cranking with ignition disconnected. Ford requires 1 pint in 24 seconds. I got 2,....yes a whole quart in 24 seconds (2 pints to a quart...right?). So I'd say that pretty much confirms the pump delivery is OK. I'll do the test again at higher rpms once I have things back together.

Speaking of rpms, I wonder what speed they rate the gph on these pumps. 120 gph should produce 2 gallons per minute....just a bit under 1 gallon for the 24 second test. Doesn't matter ...just curious about the pump manufacturer's test standard.

Thanks for all the replys. If anything else happens with the pump, I'll add to this post.