I have a new mechanical fuel pump on my 390 it was working great but now it wont suck at low idle. I thought maybe the line got garbage in it but I blew back in to the tank and it was free. once I get the motor started and long as it is at high idle it will run but after I idle it down then I see the gas quite pumping into the filter. Any ideas? bad new pump? it has been working for about 2 weeks

 

Sure sounds like a bad new pump. I had to change one after a few weeks also,.

 

Does the engine die? Have you tested the fuel pressure with a gauge?

There is not much demand at idle...at all.

 

Yeah it has a year warranty but I lost the reciet. But I am thinking about going with an electric maybe the $98 pump and regulator combo from summit. I have been getting alot of rust flakes it looks like in my filter.I am thinking about putting a filter before the pump but dont know if a mechanical will suck too good through one.

 

Better than an electric. If you have a pump with the cannister on the bottom; that cannister is a filter.

 

When I was havin problems w/ trash in my tank, I had a filter right outside the tank, another where the line ends at the front of the frame, then the pump has a filter, and then another filter at the carb. You should at least have one before and after the pump until you get all the garbage out of the tank.

 

Take the gas cap off and try it. If it fixes it the vent in the cap is bad, causing a vacuum in the tank which keeps the pump from pulling fuel.

 

Are you sure you have the pump lever under the ecentric cam. The pump will pump either way but will have a 1/2 stroke if on top.

Check to see if the fuel line holds pressure when you turn the engine off, and the pressure is in the 4-6 psi range.

 

I dont have a way to check pressure but I can put my finger on the inlet and it will suck lightly. and I do have a canister on the pump but napa told me it doesnt come off. This weekend I plan on buying new fuel line from the tank to the pump just the hose the lines I will leave. they are really soft. A guy told me it could suck air and not leak fuel. I know when I pull the inlet line off the pump gas didnt run out it dribbled. That what made me think I had something in the line. But when I blew back into the tank gas ran like a river back out.

 

If your fuel lines are soft they can collaspe and restrict fuel to the carb. There are 2 check valve in the fuel pump. If any one of those fail, the suction and pressure is greatly reduced. Some times you can take air and behind fuel and clean them out, with out take the fuel pump apart.

 

I take it the engine continues to run at idle?

I suggested measuring fuel pressure and getting a real number on a gauge because I'm not convinced that anything is broke. Putting a clear filter between the pump and carb was the worst thing I did because it drove me nuts while doing just what you describe.

I learned that the demand at idle is so low that you can barely see any flow, but still have pressure. I also learned that Ford changed the pump design so that fuel would actually drain back to the tank, but keep the pump's prime. That second nifty feature came about sometime during the '70s as part of an emission control.

After replacing my pump and not having any change, I hit the books hard to dig up those tid-bits. I live with the fact that I have to crank the engine to get fuel to the carb after the truck has been sitting. I think of it as my poor man's pre-lube system. The only real fuel issues I had with the truck were directly related to where I bought gas.

Everyone here has good advice on what to check, and how, but until you get real numbers using standard test methods, it's not broke.

 

yeah it will run out of gas and die. with the line off of the carb it can turn over all day and never see a spit of fuel. I can take it off of the pump and same thing. So i know it wont do anything just by the cranking of the motor. The last time I started it a used ether to get it running then it started spiting fuel. But I dont want to keep doing it because ether isnt a good subsitute for gas lol!

 

OK!

I read it all again. I'm thinking this is going to end with you dropping the tank to clean the trash out and possibly replacing the fuel pick-up tube. I had one of those break in my hand as I was trying to run a wire through it...all rust.

That tank was so bad that I took off the filter sock and installed a clear view filter in line just outside the tank. If I took the filter apart, gas would flow freely from the tank. When the tube sucked enough rust to clog the filter, I took it apart and used that flow to rinse it off. Worked where-ever the car stalled. I don't recommend this approach as a permanent solution. I also recommend not smoking if you do try it.

Sorry if I seemed harsh in my last post.

 

yeah its a in cab tank so easy access. So does the tube running into the top of the tank have a filter on it? doesnt like like a very big hole just big enough for the tube. But if it is a bad line woudnt be that much to replace. I will try that first. I will replace the fuel line all the way from the tank to the carb. I will also drain the tank and try to clean it out with solvent or something. then I already got a new filter after the pump I will get another for before the tank and if still no suction then I will by a fuel pump. I am hoping I will get this parts truck and it might have a pump on it I can try before I buy a new one.

 

I had a fuel problem in my 70F-100. It would run fine, then at odd times leave me stranded on the road as if I were out of gas. I was in college, so I did the following in the school parking lot: I suspected a problem with the suction line that exited the top of the (behind seat) tank. I drained the fuel and removed the tank. You won't believe all the crap I found in there! A red rag, straw, several rocks, and some nuts and bolts. I don't know if that obvious fuel cap right behind the door is too tempting, or what. I cleaned all that out, but was still suspicious about the suction line. So as a temporary fix, I took a center punch and poked a hole in the lower left corner of the tank, about a half inch off the bottom. The center punch left a good recessed area around the hole. I put a piece of metal line through the hole and filled the recess with JB weld. My temporary fix is still in place 20 years later!! It's never leaked or caused any problems. In fact, it makes a reserve, as a couple of times I have ran out of gas, and was able to make the next station by swerving and sloshing the fuel. I made it over 20 miles once!

I think the problem was that the rag or the straw would sometimes get sucked up to the outlet line and block it. Then, a few hours later the suction would bleed off and release it, at which time it would run fine-until the next time. One thing I certainly did is put a locking gas cap on!!!