I have 1999 2500 Cummins that has developed a fuel on the top of the tank while just setting in the driveway & not running ???? & the tank is almost empty. Have loosened fuel cap to relieve any pressure, but fuel is still leaking !!! How is this possible ? I don't understand how it could siphon UP out of an almost empty tank !!! I will be dropping the tank on Monday to fix the leak, but just wondering how this could happen.
I don't use the truck very often these days & have had very few issues with it, so don't post very often, but this site has been the best source of information I have found.
Thanks in advance, Stormy

 

I bet it is not siphoning up, but draining down.

My bet is you have an air leak somewhere between the tank and the motor and it is allowing air in the system and the RETURN line to draining out fuel on the way BACK not from the tank.
The fuel is leaking from the top of the sending unit, from a cracked fuel inlet nipple or a bad connection at the hose.

 

That sounds plausable, but after moving the truck there is a rather large spot -I'm guessing about 3 - 5 gallons. I have a dirt driveway, so no way to really tell. I've got help coming tomorrow to help in dropping the tank, & will check all fittings & hoses. I do have a Airdog raptor fuel pump installed if that would make any difference ?, but don't see how. I'm not that versed on diesel motors.

 

The Air Dog usually come with large diameter lines [holds lots of fuel] and several extra connections [extra spots to leak].

Let us know what you find.

 

Ok, so I dropped the tank, & found the leak is from the return line that is hooked to nothing ? When I installed this unit, I followed instructions to a T, but it never occured to me that there was no place for the return line & nothing in instructions as what to do with the return line. Pump was installed in 2016, & have had no problems till now. Could I have over pressure from the pump ? fuel pres. gauge shows 14 - 18 psi. I will be contacting Airdog tomorrow to seek their help, but thought I'd post here first to get some input, you guys are way smarter than me when it comes to diesel's. my truck has performed beautifully for the last 19 years, except when the # 53 block cracked 6,000 miles out of warranty, but that's another story !!!!
Thanks for the help, Stormy

 

It's hard to believe that you've been running for over four years without the return line from the VP44 connected - your fuel mileage would be horrendous and you'd have a puddle under the truck everywhere you stopped, and a trail following you. Does the Airdog have it's own return line? I have a '96 with the P7100 and stock block-mounted lift pump - don't know what an Airdog looks like...

Since you dropped the tank, can you post a pic of the return line at the tank? Maybe the return line fractured? And a pic of the return line on the frame rail?

 

Ok, so upon further inspection, the return line is connected, got return & supply lines confused, & fuel is leaking from original supply line that is disconnected from tank, but still connected to the old lift pump. The old lift pump is disconnected from everything & just sitting there on the side of the block, & I don't see how it could be getting any fuel to leak !!!! Can't see anything as starter is in the way, so will drop starter to cure another issue with the dipstick, & see what's going on with the lift pump & remove it if necessary. Won't be able to work on it till this evening, but will let you know what I find. Sorry, can't post any pic's as I only have a old flip phone & not much computer savy. I'm pushing 71 & you know what they say about "old dog's". Thanks texasprd, you've been a great help, will keep you posted, Stormy

 

OK, here goes. Got the lines sorted , not hard, only 2 !!!, but when you are in a dither, it's easy to confuse things. Turns out my tank had an extra outlet on top that was capped with a rubber cap, like you find on new radiators, with a clamp & was cracked to s--t !!!!! I'm assuming any pressure in tank would be expelled to the force of least resistance, which would would have been the cracked rubber cap !!!! At least I hope that was the problem, easy fix considering !!! Sorry it took so long to get back, but my back went out & was laid up for a couple days, SUCKS getting old, but beats the alternative. After priming the system, truck started right up, & seems fine. Is in garage, so will know if any leaks show up. Will take the truck out tomorrow, & make sure every thing is up to snuff. Thanks to all who chimed in on my issue, you were a great help, I pride myself on being able to figure out what's wrong, but sometimes we all just need a little help & guidance . OK, it's time for a couple of Whiskey's on the rocks & a good nights sleep. Thanks again, Stormy

 

Glad you seem to have it resolved!

Send us some of that whiskey.

 

So I drove truck to gas station, about 3 miles, &put in less than 1/2 a tank drove around for a bit, parked in the garage, that ****** the wife off, went inside & finished off that Whiskey, sorry about that, & next morning all was good !!!!! Now I have to figure out a way to get the 2-3 gallons of fuel out of the dirt ? was thinking of using a weed burner & shovel & rake ??? any other idea's that cost an arm & a leg ?? Also texasprd I see you the put larger wheel cylinders on you truck, did that help much ? That's about the only complaint I have on my truck, is braking.

 

It's good to aggravate the wife a little every day - make sure she gets 100% of her USDA recommended daily aggravation (that's the joke I have with my wife).

The easily burned aromatics from the fuel spill are probably gone by now. I guess you could try a burner and see if anything else will burn - otherwise, I'd either let rain deal with it or try the shovel and rake.

I went with the Dodge one-ton cylinders - didn't feel a need to go with the Chevy cylinders which are even larger. I don't tow much any more and I usually back off and coast a little up to stops when I can, so I really haven't noticed any seat-of-the-pants difference. I'm sure they improved the braking somewhat, but it hasn't really been noticeable as I usually don't stress the brakes much. If you do a search in this forum for "chevy wheel cylinders", you'll probably see some comments where members noted their experiences.