not getting fuel to the engine
Thread Starter
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Tuned
Joined: Aug 2014
Posts: 292
Likes: 6
From: Florida
not getting fuel to the engine
I have a '79 f-100 with a 351M. I know the truck will run if it gets gas because when I put gas in the carb it runs for a few seconds. The fuel line after the fuel pump was dry and I could suck gas (our what I thought was gas) from the inlet side. I replaced the fuel pump, put the outlet line in a soda bottle and turned it over ... nothing came out. I took the line off the gas tank and it was dry but the end of the pickup was in the gas. I tried to check the fuel line running all the way to the pump for a leak and didn't see one(no gas in it = no leak?). With all that bring said, is there a way to check the lines for a leak, or should I*just go ahead and replace all the fuel lines?
MSEE
Joined: Apr 2004
Posts: 10,386
Likes: 35
From: Austin, TX
Metal fuel lines don't typically leak unless they rust through or have been damaged somehow. However, rubber lines can leak for sure. If they crack and let in air, the fuel pump will lose its siphon. Sometimes a fuel line can look like it is not cracked when it sits in the position it has sat in for decades, but upon removal and bending of the line, it's very obvious it is full of cracks.
There should be one small length of rubber line about halfway down the frame, and one connecting the metal line to the sending unit at the tank. If they look old, replaced them.
How do you know the pickup was in the gas? Are you just assuming that? Without removing the tank and pulling the sending unit (not a quick task), I don't know how else you would visually confirm. Unless that's what you did. There's fuel in this vehicle, right? It's happened before.
There should be one small length of rubber line about halfway down the frame, and one connecting the metal line to the sending unit at the tank. If they look old, replaced them.
How do you know the pickup was in the gas? Are you just assuming that? Without removing the tank and pulling the sending unit (not a quick task), I don't know how else you would visually confirm. Unless that's what you did. There's fuel in this vehicle, right? It's happened before.
Moderator & parts slinger
Joined: Feb 2007
Posts: 50,186
Likes: 5,792
From: S/W Missouri, Polk county
Is the fuel pump arm under the eccentric cam? Was there pressure on pump body when you install it? You put it in, then the top bolt first and press down and it could/should be under pressure and then start the bottom bolt.
All depends where the eccentric cam is positioned when you shut down the truck.
Or with some low air pressure you can "back flush" so to speak the soft line that connects to the hard line than runs down the onside of the dvrs side frame rail then it goes back to a soft line that connect to a (valve it you have duel tanks) or to the sending unit.
Have a helper listen for bubbles when you back flush. Intake sending unit has a filter sock on the end it could be clogged?
Any inline fuel filters that are clogged?
All depends where the eccentric cam is positioned when you shut down the truck.
Or with some low air pressure you can "back flush" so to speak the soft line that connects to the hard line than runs down the onside of the dvrs side frame rail then it goes back to a soft line that connect to a (valve it you have duel tanks) or to the sending unit.
Have a helper listen for bubbles when you back flush. Intake sending unit has a filter sock on the end it could be clogged?
Any inline fuel filters that are clogged?
Thread Starter
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Tuned
Joined: Aug 2014
Posts: 292
Likes: 6
From: Florida
There is some rust on the hard lines, but it is not scaly. I currently do not have a bed on the truck (it rusted apart) so I removed the the pickup unit and visually inspected it. Side note with that, it does not have a screen. So a safe move is to replace all the rubber lines and try again?
MSEE
Joined: Apr 2004
Posts: 10,386
Likes: 35
From: Austin, TX
That'd be my next step. FYI on a 351M it shouldn't be possible to slip the pump arm over the eccentric; if the pump went in at all it should be fine. The reason to put the motor on TDC is to center the eccentric and relieve all the pressure off the pump arm. Otherwise the pump can't bolt on straight.
Moderator & parts slinger
Joined: Feb 2007
Posts: 50,186
Likes: 5,792
From: S/W Missouri, Polk county
I knew there was a way to make it easier, I just could not remember. Now I know how to make it easier, thanks to fmc400.
A filter sock will save you trash in the carb, if you do not have a few inline filters. How did the inside of the tank look? Here is a new sending unit filter.
Do not know if you can just order a filter for the end?
A filter sock will save you trash in the carb, if you do not have a few inline filters. How did the inside of the tank look? Here is a new sending unit filter.
Do not know if you can just order a filter for the end?
Postmaster
Joined: Nov 2011
Posts: 2,553
Likes: 10
Metal fuel lines don't typically leak unless they rust through or have been damaged somehow. However, rubber lines can leak for sure. If they crack and let in air, the fuel pump will lose its siphon. Sometimes a fuel line can look like it is not cracked when it sits in the position it has sat in for decades, but upon removal and bending of the line, it's very obvious it is full of cracks.
There should be one small length of rubber line about halfway down the frame, and one connecting the metal line to the sending unit at the tank. If they look old, replaced them.
How do you know the pickup was in the gas? Are you just assuming that? Without removing the tank and pulling the sending unit (not a quick task), I don't know how else you would visually confirm. Unless that's what you did. There's fuel in this vehicle, right? It's happened before.
There should be one small length of rubber line about halfway down the frame, and one connecting the metal line to the sending unit at the tank. If they look old, replaced them.
How do you know the pickup was in the gas? Are you just assuming that? Without removing the tank and pulling the sending unit (not a quick task), I don't know how else you would visually confirm. Unless that's what you did. There's fuel in this vehicle, right? It's happened before.
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Tuned
Joined: Aug 2014
Posts: 292
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From: Florida
Just to keep everybody updated. Had family in town from north Carolina today and taking the wife and kids to the zoo tomorrow. So I won't be able to get started on this till Wednesday. Will try and take pictures and week definitely let y'all know what happens.
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Thread Starter
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Tuned
Joined: Aug 2014
Posts: 292
Likes: 6
From: Florida
Sooo ... im lost. I replaced all the rubber fuel lines. Put the outlet side of the fuel pump in a bottle, nothing came out when the engine was turning over. I pulled the pickup out of the tank again and realised the JB weld was coming apart(forgot to mention I have a 38 gallon tank.) Cleaned it up and soldered the joint together. Gave the pckup the old CPR test, no leaks. Put everything back together, no fuel from the pump. Blew on the hose where it connects to the pickup and some moisture came out with a little bit of sediment. Had a buddy block the hose that would go to the inlet of the fuel pump and I CPR tested the line from where it connects to the pickup, no leaks. The only thing I can think of is I may have the wrong fuel pump. The old pump(this one did work at one time) has the inlet on the rear port of the pump, the new one has the inlet on the front port. Everything else about the two pumps are identical ... need help guys.
Postmaster
Joined: Nov 2011
Posts: 2,553
Likes: 10
At this point possible problems seem like 1) incorrect or inoperative fuel pump (it happens) 2) blockage in the hard lines 3) sneaky air leak. If you can, suggest blowing some compressed air from the tank end to pump (pump disconnected). If you get a free flow of air, re-do leak check. If all is OK, time to exchange the pump.
Thread Starter
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Tuned
Joined: Aug 2014
Posts: 292
Likes: 6
From: Florida
found the culprit!
I think I found the issue. Was frustrated enough I re-tested everything and found a small air leak in the souldering on the extended pickup. Tried to fix it and my sounding gun broke. Going to get that fixed Monday and see if the pin hole leak is the real cause. Lord knows im praying.
Believe Nothing
Joined: Jun 2004
Posts: 7,514
Likes: 398
From: W. Central FL.
I think I found the issue. Was frustrated enough I re-tested everything and found a small air leak in the souldering on the extended pickup. Tried to fix it and my sounding gun broke. Going to get that fixed Monday and see if the pin hole leak is the real cause. Lord knows im praying.
Any leak on the intake side will prevent fuel delivery.
Thread Starter
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Tuned
Joined: Aug 2014
Posts: 292
Likes: 6
From: Florida
pulling my hair out
Found the air leak in the fuel line. Fixed it and tried it again ... still no fuel from the pump. Pulled the fuel pickup everything seemed fine. Gently laid in back in the tank and brew into it, I heard bubbles. Re- installed it and blew into the pickup ... NO BUBBLES! I removed the pickup and realized the extension was bent just enough to keep it out of the 5 gallons in the tank. Now I'm wondering if this has been the problem all along. Hope everybody can learn from my mistake(s) 
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