The rear tank on my truck has always cut off much sooner than it should've which was annoying, but not bad. But then the gauge stopped working, which wasn't a bad thing until the front gauge stopped working.

Ordered the sending unit from LMC and it had the wrong connector, but instead of cutting the old connector for the new connector, I ordered the rear sending unit for the '89 model as it has the correct electrical connector and my truck was manufactured at the end of '89. Seated in perfectly, and seems a correct fit. The strainer was in little pieces all in the tank when I opened it up btw, hence why it cut off sooner than it should.

The sending line and the return line connectors on the unit were on the opposite sides as the original, so when I went to hook them up, the return line snapped because it was being strained too much. Did a quick patch with some fuel line and buttoned everything back up. Put in 5 gallons of fuel, started driving down the road and switched over to the rear tank, gauge showed a quarter tank as it should, then after about a mile, started sputtering like it had air in line, so I switched back to front tank to recover before it died. Did this for about 30 minutes and decided something wasn't right.

Took the tank back out and discovered the vent hose was crushed and torn, but didn't see anything else wrong. Replaced the vent hose and put everything back. This time I tried purging after switching to the rear tank, but get nothing but air. Purge with front tank, fuel. Purge with rear tank, nothing but air. I checked the lines where I can along the frame into the switch, but don't see any signs of broken line. Don't smell any diesel, so doesn't seem like it's leaking anywhere. Purged and purged and purged, but get nothing but air. I know the return line got lots of air in it, when repairing, but sending line shouldn't have got that much in it.

My next thought is to put more fuel in the tank just in case, but just don't want to have to siphon more fuel if I don't have to if I have to take it back out again.

Any suggestions?

 

even when new these trucks were known to run out of fuel at 1/4 tank.
i learned real quick back in 1988 to never let any of my trucks get lower than 1/2 tank.

 

Put 5 more gallons in, 10 total. Hooked up electric pump to fuel line to try and draw it out, but still nothing but air. At worse, it would stop pumping at 1/4 tank, 1/2 a tank should be more than enough. I'm guessing the fuel line is broken somewhere?

 

Or it's hooked to the return line ...

The bigger line (3/8") is Fuel pickup.
The smaller line (5"16") is Fuel return.

-Enjoy
fh : )_~

 

Disconnect at water/fuel separator (frame rail) and provide shop air at aft tank to clear line (I'd bet there's "debris" somewhere in the line or broken as you've mentioned). Not sure where you're getting your diesel fuel but I've noticed a lot more "debris" recently in both the on-road and off-road diesel fuels I use.

Also -- if your tank has the mechanical rollover valve on its vapor system, it can cause pressure issues as well.