runs rough under 1/4 tank

[bajacalal]

So lately, I have been having an issue where, if I have less than 1/4 tank of diesel, or maybe even less than 3/8, I have problems that would suggest air is getting into the system. It will be difficult to start, it will run rough after starting and then will seem to have a random miss or idle poorly. I just recently adjusted the injection timing and it runs like a raped ape the when there is more than 3/8 of a tank of fuel. I try to keep it full but I feel like my tank only has 6 usable gallons now.

I'm assuming there is something happening like a cracked fuel pickup tube or the fuel return line is siphoning out the fuel but I've never taken the fuel pickup out so I have no idea how it works on these cars.

Could someone shed some light on this? Is this a common problem on these? Could it be algae or debris floating on the fuel? It does have 280,000 miles and I did delete the under-car fuel-water separator.

[damac]

I deleted the water seperator on my jetta as well and installed new lines from tank to the engine bay.  Your problem reminds me of my old f250 which I need to fix.  It had these plastic senders that broke off and took the bottom 1/4 of the tank away and let chunks get sucked up into the fuel pickup.

Don't know if our systems are the same but I have had the whole unit out of the tank in my jetta.  It goes from the metal lines sticking out which are fused into a big plastic casing and the bottom of this has a little plastic grill type deal that I guess catches chunks.  When I got my neglected car home last year I took it out to clean it, it had debris for sure, but it wasn't clogged.  Looked like a little mud was smeared on it.

Anything is possible I suppose but I wonder how that unit would get cracked at the metal lines.  Perhaps plug it if you can and run low pressure shop air under water and check for pinhole leaks.  I don't remember if you can actually get to the end of the metal tubes that sit at the bottom though.

If yours was good like mine, I would guess maybe there are other issues with your filter, pinhole in fuel lines or air vent clogging?

On mine I have actually driven on freeway and up/down mountain pass with it down at the bottom of the red just for a fun test with some extra fuel in the back   I wanted to test it to see if I could last going over a pass of 20 miles with no gas station.

I never did measure the actual fuel left at the bottom of the red either, just tried that test a few times.  And I actually have never had that rough, running out of fuel condition like on my truck from sucking my tank that low which I do pretty frequently, way under 1/4 tank.


Another though and I am not sure on this, what is the effect on the gauge vs. the actual fuel level if your sending unit/pickup wasn't orientated arrow to arrow?  Possible its positioned wrong and your fuel is lower than you think?

[fatmobile]

Might be a bad rubber hose on the fuel pickup.

[shwak23]

I know this is an old topic but I had this same problem. I replaced the tank and it was fixed.

I had a rigged up gasoline tank in my 1990 MKII Golf. The previous owner had converted the car to diesel. The tank had always been an annoyance because unlike my old 85 Golf I couldn't blast diesel into it at the truck stops. I had to hold the pump at half throttle. However I digress.

I assume my issue was either caused by the amount of dirt and grime clogging up every pore of the exterior of my tank or it was caused by the half assed sending unit. (a gasser one with the pump replaced by str8 rubber hose) Anyway that's my solution. I got my tank for $30 from a yard and it took me an hour to swap it. (Not including the time it took to siphon the 14 gallons of fuel, I had just jammed my tank with.)

[somolovitch3]

If the tank's insides are any thing like my 84 GTi, it might just be the fuel line (Fuel proof hose) that joins the outlet nipple (outside of the tank) to the fuel line on the inside that goes to the filter/surge device at the bottom of the tank.
If there is a problem with right hand turns, that is the problem. If the tank is more than 1/4 to 1/2 full, you don't suck air thru the gap. However, that rotting hose can leave/dump bits of rubber that can cause problems elsewhere. (at least it caused one fuel pump death on the DumBunny).
You might could see if you pull the sending unit and look thru that hole. I cheated and used a VERY small camera hooked to my laptop to look-see.

[R.O.R-2.0]

HAHAHAHAhahahaha...

DumBunny

LOVE IT!!

[ORCoaster]

You got a smile outta me with that name too. 

[bajacalal]

If the tank's insides are any thing like my 84 GTi, it might just be the fuel line (Fuel proof hose) that joins the outlet nipple (outside of the tank) to the fuel line on the inside that goes to the filter/surge device at the bottom of the tank.
If there is a problem with right hand turns, that is the problem. If the tank is more than 1/4 to 1/2 full, you don't suck air thru the gap. However, that rotting hose can leave/dump bits of rubber that can cause problems elsewhere. (at least it caused one fuel pump death on the DumBunny).
You might could see if you pull the sending unit and look thru that hole. I cheated and used a VERY small camera hooked to my laptop to look-see.

The problem was something I only experienced after the car had sat for at least a day without being driven. It would be difficult to start the engine, needing several attempts of cranking the starter for at least 5 seconds at a time, even in warm weather, and it would run rough for up to a couple minutes. Afterward, it would run great and would start again with no problems, even when allowed to cool down, and would operate just fine while driving around, even with an almost empty fuel tank.

For an update, I treated the tank with some chemical that is supposed to dissolve sludge and diesel nasties.

I also replaced the return hose coming off the injection pump, which seemed to solve the problem, at least it's been starting right up this morning and it was below 32F here last night. So I believe the problem was a leaking return line (it had tiny cracks all over it) which allowed fuel to drain back into the tank and air to enter the pump. I have no idea why this would happen as I thought there was a check valve in there, and I have no idea why it would only do it under 1/4 tank, though by the end it started doing this at 1/2 tank and above.

[ORCoaster]

I don't think they put a check valve on the return side.  Wouldn't need to if the line is sealed proper no air in, no bleed down, no problem.