I Can't Fill My Tank - '79 Rabbit

[libbydiesel]

I had the front of my '79 rabbit up on ramps for a couple months while I was working on other projects.  I don't recall how much fuel was in it when I parked it.  When I got it back together, the fuel gauge was reading in the red.  I went to fill it up and the pump kept clicking off.  I tried to fill it very slowly but could hear and see that the diesel was backing up in the filler.  After getting a gallon and a half into it I was tired of standing at the fuel pump trickling it in.  The gauge cam up above the red.  I work from home, so I do not drive much.  That gallon and a half lasted me a few weeks and I decided to try to fill it again yesterday as it was reading nearer the bottom of the red again.  It was the same deal.  The pump kept shutting off.  I got another 1.5 gallons into it and then filled a couple jerry cans, largely because the cost of diesel is so low right now and I figure it will climb as we get into the home heating oil season again. 

I figure one of two possibilities: either a blockage in the filler tube or the gauge is not reading correctly.  I lean toward it being a blockage as the level in the filler tube does drop if I fill very slowly.  I figure I'll drain the tank with enough empty containers on hand to hold all the fuel if it is the gauge.  I'll then drop the tank, clean it out, replace old supply/return hoses, etc...

Any other theories, advice, etc?   

[ORCoaster]

I would lean towards the sender being wonky.  So draining it down 5 gallons would get you enough comfort space to get the gauge out the top without having diesel flood your interior.  But if you can't get that much out then I might suspect some sort of gunk that slid up the back of the tank and is somehow now restricting inflow.  I can't remember if that tank is a top hose from the fuel fill or if it is lower down on the tank.  I can almost see the topside. 

So sender on fritz due to being sideways for too long.  Can't you measure the resistance coming off the sender and see if it comes close to the high or low end of the scale?  It might be your actual gauge the is fouling up too. 

Not much to source out on this one.  Remember to look for spare change under that back seat.  Seems like it is always worth a dime or so. 

[libbydiesel]

Thanks for the input.  The sender/gauge definitely act as though they are working.  Each time I added fuel it measured appropriately higher on the gauge.  Pulling the seat and sender is definitely an option and might give an easy view of the issue.  As you say, I also don't want to flood the interior with diesel.

The filler tube enters the bottom rear of the tank.   

[745 turbogreasel]

Heat the tank wall with a heat gun, and use your pyrometer to see where heat sinking by the mass of fuel begins

My Dodge went the opposite way,  my gauge was  going down, but at 500 miles in, I was only down half a tank.... then it ran dry.
Turns out  somebody had changed the   tank sender which has a  big rubber  ring that fell in, and it supported the float upward, but still allowing it to come down somewhat with time.

[RustyCaddy]

The fuel neck on the truck enters at the top of the tank and there is a gate in the tank at the neck.  Every so often the gate gets stuck shut (in my case from low conversion biodiesel 'napalm' gel).  Have to pull the neck back and clean the gate with acetone. Acts the same as what you are describing when trying to fill with the gate shut closed but dont know if your tank would have a gate since the neck feeds into the bottom of the tank?

[libbydiesel]

Here's a quick update.  I pulled the rear seat and fuel sender and the gauge appears to be reading correctly.  Fuel was quite low so I must conclude there is a blockage in the tank filler tube.  Despite efforts using a flashlight and mirror, I could not get a good visual.  I'll pull the tank in a few weeks after I get done with the current engine conversion I'm working on. 

[air-cooled or diesel]

man thats a bummer, for such a simple and reliable car, thats a difflcult problem. maybe with the sender out have a buddy add fuel thru filler and see whats going on at the other end,,if you can see. it has to be in the filler neck going down to tank, or possibly in the vent, although that doesnt seem as possible. have no idea of the mk1 fuel tank vent, in my mk2 its somewhat elaborate, some type of gravity vent with hoses, the famous vent button the cap activates, ?new cap?, and an expansion tank, thats located above the tank, so you have to drop tank to get to it.

[libbydiesel]

I don't think it's going to be that big of a deal.  The filler tube goes down to a section of flex hose that joins it to the tank.  I'm thinking that I can drain the tank, remove that flexible hose and the issue will probably become clear.  If possible I would like to avoid completely dropping the tank, though.  We'll see.  I'll update with more infor when I get around to fixing it. 

[ORCoaster]

When you find the hornets nest or mouse nest let us know. 

[libbydiesel]

Time for an update.  I took a piece of plastic fuel tube (the kind that is flexible, but hard - not hose) and fed that down into the filler tube.  I considered using my borescope to check it out, but I'm not sure if it will survive being submerged in diesel fuel.  When feeding in the plastic tube, there was a certain point that it felt a bit more resistance.  By kind of tapping it in while rotating the tube I could get it to go past that point.  When I pulled it back I could hear a sound that could certainly be a little metal door closing.  Assuming that it was a little metal flapper door, I used the plastic tube to 'exercise' it a few times and then went and filled up.  It filled up normally to the top of the gauge.  Sweet.

[ORCoaster]

Anti back wash flapper?  For all those times you pedal down and the turbo kicks in.  Prevents fuel loss out the filler neck? 

[libbydiesel]

The early gas vanagons, when the fuel tank is properly sealed, have the very irritating habit of puking fuel back at you after the pump clicks off.  The early diesels didn't do that, but I believe that is because the tanks were directly vented to atmosphere instead of vented through a charcoal canister.  The later gas vanagons also do not puke the fuel back at you because they have a ball check valve at the end of filler tube.  It's purely my imagination, but I picture some flapper door at the end of the filler tube or entrance to the tank, that performs a similar function.  All I can say for sure is that I'm glad to be able to fill up the tank again.