I've been lurking for ages, and this forum has been a huge help in working on my truck. Thanks to everybody who contributes their knowledge! I've searched for an answer to my current problem, but can't find anything that really helps, so here's a description of it, sorry i can't keep it shorter:

I'm traveling and towing an RV trailer. Couple weeks ago i noticed the fuel gauge dropping very fast while driving on the rear tank, and then a diesel puddle under the truck in the morning...i figure i know what's going on there...but when i looked, the rear tank was dry. No evidence of any leak whatsoever. I checked the FSV area, and also, no evidence of leak. The front tank, however, had rusted through and was dripping. I had just filled it up, so i spent the next couple days driving on only the front, to get it empty and stop losing diesel. I got 236 miles out of that tank (12+ mpg; it wasn't all towing). I've been driving on the rear tank only since then and getting abysmal mileage, 5-6 mpg. I don't trust the rear fuel level sender (the front one is already dead) so that is based on actual gallons pumped in and odometer readings.

I thought maybe the FSV was returning diesel back into the front tank while driving on the rear...but the front tank is still empty. After about 100 miles on the rear, i tried switching to the front and the engine died almost right away from sucking air.

There are no puddles under the truck (for the first time in forever). I've gotten under it at a shop and had someone rev the engine to 3k and looked for drips or sprays with a flashlight...nothing. No puddles inside the frame rails. The issue happens whether i'm towing or not.

About the truck:
1991 F-250 (7.3 IDI). Odometer says 2,290...i don't know if that's 102k, 202k, or more. PO used it as a farm truck on the east coast -- lots of rust and git-r-done type 'fixes' that i've been sorting out as i find. I did the electric fuel pump mod (Holley Red) about a month ago as part of a waste veg. oil conversion, and mileage on the rear tank was definitely fine afterwards. The WVO fuel system is totally isolated from diesel until immediately before the IP (switching valve is after diesel fuel filter). Fuel economy on veg. oil is in the 11-12mpg range. I have 2200 miles on it with diesel and 1820 miles on veg. oil since the conversion. The engine sounds happy no matter which tank i burn; front, rear, or veg. oil.

Any ideas about what my problem is? Or where i should look for all this mystery diesel that i'm buying but not burning? I'm totally out of ideas, so grateful for any thoughts at all. What have i missed?

 

What does the gauge say when you switch to the front tank?

So the two OEM tanks are still for dino-diesel only?

It's possible you have multiple issues. Maybe when it's switched to REAR, it's drawing from the rear tank and returning to the front, and in turn, if the front tank has a leak, it's all just leaking out as it returns. You might also have some fuel in the front tank, but if the pickup strainer (aka showerhead) has fallen off or broken, it will act like it's on empty when it hits aprx. 1/4 tank. Do you know about where the rust leak is in the front tank?

 

I was thinking the same thing ^^^^^. I'd seriously consider replacing the FSV.

Maybe before you replace the FSV, you run a bypass line isolating the rear supply and return lines versus the front supply and return lines. If it still leaks then, then IDK. But I suspect if you run a bypass for the rear supply and return lines, it won't leak up by the front tank.

 

That seems very low. Can you provide some more info about your truck? Like tranny, gear ratio, tire size, etc?

If you have a dually and hauling 8000 lbs, then 11mpg would seem about right. But if you have 3.55's and drive normal, then you should be getting at least 14-15mpg. Your timing might be off.

Also, what kind of grease are you running? How are you filtering it? What is your WVO-side filter and micron? What pump are you running on the WVO side? (I'm a fellow greaser, and I want to see if you have a fuel restriction that's pealing a few mpg away.)

 

Thanks for replying! Sorry again if i write too much, just want to give complete answers...

@ madpogue:
Yes, i left the stock diesel tanks and lines in service, no changes, except for the Holley pump and the veg. oil switchover valve. The Holley is on the frame rail just ahead of the FSV. The fuel gauge never worked on front tank since i've had the truck (needle stays below E). Until now i've just driven 150 miles on the front before switching to rear, to be safe. I tried jumping the pins on the FSV connector to see if it's the sender or the selector switch, and never got the needle to move...so it might just be the switch...but I was saving that troubleshooting for when the front tank rusted out, since i knew from these forums that was inevitable...

I did already read up on the showerhead issue -- i was hoping that would be it -- but i can't see how that would cause the problem. The front tank appears to give me full range, and on the rear the gallons i put in still correlate to gauge position, and it doesn't actually run out until the needle is at "E". Still, i figure i need to go ahead and replace the front, and while i'm at it check the rear and plan on doing the hose modification...

It took a week for me to drive those 236 miles on the front tank: i was stuck in a muddy campsite so just hung out in the woods instead of driving around. So yes that tank is leaking, but it's slow. Even if the rear were returning to the front, the front would spill out the filler neck before it could leak out the bottom, given the rate i'm losing it from the rear.

The leak is at the rear strap of the front tank, and likes to fill up the rock guard before spilling out. It leaked right up until the morning i ran it dry. I don't see a puddle in the rock guard so i'm pretty sure, after emptying the rear tank a couple times now, that it's not returning to the front. But you're totally right -- i won't really know unless i explicitly test for it.

@ genscripter:
The only "next step" i can think of is what you suggest, bypassing the FSV to troubleshoot. Actually, once i replace the front tank (and maybe sender) in a couple weeks (need to catch up with the tank i had shipped ahead), i have the option of just piping the lines to the front and giving up on this problem. But i really don't want to.

Rear axle is a single, 4.10. New rubber 4k miles ago, took it back to the nameplate size (LT235 / 85R16E). It's the E4OD, 4x4. The IP is a reman with 5000 miles on it now, installed by my old local shop that i have a lot of faith in. She likes to go 55 mph and i never bother to try for faster, especially on the long grades out west where i am now (yeah that's more like 15 mph). The trailer is, in fact, a 7500lb fifth-wheel so i'm pretty happy with 11mpg, especially given the altitude (7200 ft. currently). I'm asking a lot of this truck and i am really impressed by how well it does.

Short drives to town for groceries without the trailer are all on diesel 'cuz the veg isn't warm enough yet (i don't like to switch over 'till my coolant hits 180*F). So i expect my grease mpg to be a bit lower than diesel because it's basically all towing miles.

I collect grease as i travel so quality varies a lot. I try the Thai and Chinese joints first, but am currently full up on some of the clearest grease ever, from a fried-chicken joint -- go figure! I pump from the dumpster through 600/400/200/100 micron bucket strainers, then pour through a nested stack of sock filters (75/55/25/10/5/1 micron). It's a steel transfer tank in the truck -- yes i know, polymerization -- but grease rarely sits in there for more than a week before i burn it up. Grease pickup is a hotstick, then hose-on-hose to a heated filter head. Same water-separating filter as the diesel side, which i believe is 10 micron. Then a Walbro pump, FPHE, valve, IP. The return loops back to the filter. Grease temp is 10*F less than coolant within a minute of switching over.

I got the Walbro from greasecar and i do suspect it's undersized (my mistake in choosing it), because the engine wallows on very steep grades (9%) with the trailer but perks up if i switch back to diesel. I'm interested in a better-matched pump, but can't justify the expense/work right now.

 

I finally did catch up to the new fuel tank and got somewhere to install it. Dealing with the rusted hardware to get the old tank out was no fun. The showerhead was intact when i removed the old tank, and showed no signs of damage, so i left it in place. The fuel level sender problem was due to the strip of contacts being worn out at the base of the float arm. I didn't find any affordable replacements online so i just bent the little swiping arm to run on a different portion of the contacts that weren't worn out. So now i have a new front tank and a working fuel level gauge for both tanks (for a little while, at least).

I did not find evidence of any other issues while working. The rear tank is empty now and i'm burning a lot of veg oil so no chance to look for the problem continuing. However i have another long stretch of driving coming up so when i do, i can run on the rear and watch for fuel returning into the front.

My VO system could be partly to blame. I had a loose fitting creating a bubble in the line, so to clear it i'd purge the VO system with diesel. It's fixed now, but had gotten kinda bad. I don't think of purges as using much diesel, but each time i switch to VO and back, the whole fuel line's worth of diesel gets returned into the VO tank twice. That's maybe 0.17 gallons (3/8" line, 15 feet). But on an 18 gallon tank, it would take 52 purges to "waste" half the fuel and drop the mpg to the 5-6 range. And i definitely wasn't purging that much. I'm okay with it being my fault, but it doesn't seem like a "slam dunk" answer yet.

So we'll see...it's great to know the forum is here to help when the next weird thing happens!