no fuel from tank; runs fine through filter

[kanizer]

I rerouted the fuel line coming from the tank to a jug of diesel ( so, jug to filter to IP ), and it runs like a champ...hook it back up to the fuel tank pipe, no fuel...

not sure how to diagnose whether this is an air leak in connections or if the tank/pipe is compromised/blocked?  this must be a common issue?  please help!!!

[Rabbit TD]

The first thing you need is a good clear {the kind you can see through} line from the filter to the pump and you will see bubbles coming through if it is sucking air.  Especially check the lines that come off the metal line that feeds the filter on the passenger side.  You will be amazed how much air these things will leak.  A cheap lift pump back near the tank will also help eliminate your problem and make it easier to find the air leak too as it will be slightly pressurized then alowing it to leak externaly and be  easier to find.  Tractor Supply has the line 1/4 in. clear line if you can't get it in an automotive store and you can get a lift pump at Advance Auto or Autozone for less than $50.  These pumps do absolutely nothing if the system is in good shape because the I/P doesn't need it as long as all the connections are free of air leaks and the input shaft isn't leaking.  They do definately help a car start though if you have air leaks and you almost deffinately do from the way it sounds.  Mine drove me nuts until I discovered this repair here on this site.

[Baron VonZeppelin]

What year and kind of vehicle is it ?

[kanizer]

80' NA caddy

I already have a clear line, and there is definitely air coming into the line.  I'm not sure what the lift pump does, and you please elaborate?  Also, is the leak typically at the connections ( clamps? ), or in the tube/hose itself?  How commonly does an air leak of this nature originate in the metal tubing from the tank, rather than the hose or connection?  Thanks for the help - so close to getting this thing running...

[Possum79]

The lift pump would push fuel up to the filter. As is right now the IP sucks fuel from the tank. IF you have an air leak the IP will suck air and not have full fuel and fire wrong or not at all. The slight vacuum from the ip doesnt alow fuel to leak out so its hard to see but if you put a lift pump on it it will slightly pressurize and you can see a leak.

Rumor has it if you have a water seperator towards the rear of the vehicle it will mostly likely cause air to enter. just route around it. I dont remember what kind of vehicles have it though so that may not help. Have you ever taken the sender unit out of the tank. Maybe a slight clog in its filter screen is causing air to get in.

EDIT Where abouts in california are you from?

[Baron VonZeppelin]

Okay, had a feeling you might say Caddy/Truck - or mk1 Jetta.

They have a sump cup and screen, and baffles in the tank - fuel doesn't just drop out the bottom/side like it would appear that it does.

If the tank has been dented much on underside - or rusted/corrosion on inside - those apparatuses will falter. It causes a lot of problems with fuel delivery at less than half tank.

May not be your problem, but it probably is something to do with those items. You would need to drop tank and inspect it inside.

Would also blow through both supply and return steel lines to make sure they flow, with compressed air, with fuel cap off.
Or a clean fresh piece of hose as a mouthpiece.

On next question, its usually the rubber hoses/connections that connect to steel lines where the steel lines come up into the engine bay - that give air problems. If the steel lines are fractured or ruptured they normally will also leak fuel as a tattle tale symptom.

A lift pump is an electrical fuel pump. Usually mounted in-line and around 4-6 psi on these IDI diesel machines.

The injection pump 'pulls' fuel from tank on supply side.
A weak pull can be compensated to degrees with an in-line electrical pump.

[Rabbit TD]

80' NA caddy

I already have a clear line, and there is definitely air coming into the line.  I'm not sure what the lift pump does, and you please elaborate?  Also, is the leak typically at the connections ( clamps? ), or in the tube/hose itself?  How commonly does an air leak of this nature originate in the metal tubing from the tank, rather than the hose or connection?  Thanks for the help - so close to getting this thing running...

A lift pump is an auxiliary {extra} electric fuel pump mounted in the rear as close to the tank as possible so it pushes the fuel forward under 4-7 psi. on the particular pump I have, a Mr. Gasket diesel rated one which is green in color.  I think it was around $50 at Advance Auto.  I have mine on a separate rocker switch on the dash.  Well you already know you have air getting in the pump from visualy seeing it.  Honestly about the only place they don't leak is the metal line itself unless it has been damaged in some way but any hose or connection can be the source and you won't see it because the lines are under vacum and not pressure and can be very hard to track down.  I found mine with the engine  shut off and the lift pump turned on putting a little pressure on the fuel lines and I saw fuel dripping around the line feeding the fuel filter.  It seems when you start moving these old 30 year old lines around like relocating the fuel filter you start having these problems.  The best thing you can do is just start at the back and change all the rubber lines especialy on an 80 and you have to make very sure you have the right size line to go over the metal tubing and use the band style clamps, not the worm screw type because they don't work that great on lines this small especialy if the rubber line is a little too big and it will let air in.  You don't have to worry to much about the return line as long as you don't see it leaking.  I would almost bet the problem is between the metal line on the passenger side where the rubber one connects to the fuel filter or a banjo fitting sealing washer somewhere.  You can splice those lines with the fittings as long as there is enough good  material left before the fitting to clamp it on a peice of metal tubing.  I think it was all 1/4 in tubing  from the metal line after the tank going forward but it was 3/8 coming off the tank to the first metal line on my Rabbit.  That's where I put the pump, between the tank and the metal line.  You can use clear line on every connection if you want to and see if it's good at every connection that way too.  I got my line at Tractor Supply and it's only line 39 cents a foot chemical rated but it's a little on the thin side for my liking but It's a quick cheap way to see where the leaks are or aren't.  They can suck air in anywhere but those old lines are  the most likey spot.  If you do go with a pump unhook the return line at the pump and block off the fitting on the pump with something that will hold pressure and then turn the lift  pump on with the key on to open the solenoid on the I/p and it will pressurize everything after the lift pump and may show your leak pretty quick that way from dripping.  You wouldn't have believed the air that was going in my pump, it ran fine but just got harder to start but starts with a bump now even when cold with the plugs hot.  I'd just change all the lines and have a good clear one feeding the I/P which you already have and I bet your problem goes away and you won't go off misdiagnosing something and end up thinking it's the I/P which happens a lot.  Your I/P can let air in too at the shaft seal but it will usualy show an external leak there also but with new lines you can at least verify that there is no air going in through the fuel line itself.  Like I said before as long as the lines are all airtight the lift pump doesn't do a damn thing.  Don't expect to see the fuel just go gushing through the lines though because the return fittng on the pump has a restrictor hole in iti to let the I/P build internal pressure.  The lift pump doesn't do a damn thing on a good airtight system but it does help diagnose a problem if it starts quicker when turned on till the clear line is actualy clear after it runs a little.  It's a cheap fix but sometimes a pain to get to the one that's the problem so just change them all for $15 or so