I posted a previous thread regarding a rough idle problem after having my LP and IP replaced. After fighting it for many months, installing a FP gauge and a FASS, and still having problems, I finally found the culprit. A light bulb came on when my FP gauge started acting up when the fuel level dropped below 5/8 of a tank, but worked correctly with a full tank. Below 5/8 of a tank, the pressure stayed at 0 until after the engine was running for a few seconds, and then slowly came up to 13 lbs. Once running, the FP stayed at 13 lbs (+/- 0.5 lbs for idle and WOT). I checked the fuel volume, and found a significant amount of air was being pushed through the line when below 5/8 tank, but was clean when full – more air than the FASS was able to remove! I went over to jetjockey99’s house Friday night to install a new pick-up tube. I asked his son to take some photos since my hands were covered with diesel fuel – should have showed him how to work the camera first. I've loaded a few of them into my member gallery. Sorry for the long post, but here is what I found inside my tank…

The sending unit consists of a cylindrical two-piece cage with three plastic fuel lines curled up inside – the pick-up tube, return tube, and a spare line that is plugged on top of the sending unit (no obvious purpose – maybe for a saddle tank). The two halves are held together by 3 "shouldered" screws, which allow the bottom half to slide up and down inside the top. There are two screens and a check-valve on the bottom half of the cage – the check valve holds fuel inside the cage during starts, stops, and turns (acts like a sump). The fuel lines act like a spring causing the bottom half of the sending unit to stay tight on the bottom of the tank – not a lot of downward pressure, but enough to cause the top and bottom halves of the sending unit to compress and expand ¼ to ½ inch as the bottom of the tank moves up and down (rough roads, railroad tracks, etc.). The problem is that the sender is fastened to the outside of this cage… with the tips of the screws sticking into the inside of the cage. My pick-up tube had a hole rubbed into it by one of the screws – not a big hole, but enough to allow the fuel pump to suck some air once the fuel level dropped below the hole. I replaced the pick-up tube with ADT’s 1/2” stand pipe system – no more rough idle.

Several members have commented about problems with the pick-up tubes, and have posted on how they resolved their issues. After tearing my sending unit apart and seeing first-hand what was causing my problem, I’m a believer! I’m willing to bet that more people have this problem and don’t even know it. I have to believe that much air in the fuel line will resemble a LP cavitating.

Franklin

 

Is that the standard configuration for 2nd gen's? I wonder if I should tear into my truck for an inspection/upgrade!

 

I would like to know if it applies to the 2nd Gens.???

If so I will be looking into this soon...

 

Yup. It does.

 

Question...

1. Can this be done with the fuel tank in the frame?

2. If not... how hard is it to do?

 

I found it very easy to remove the bed of the truck to get at the module. The bed does not even have to come all the way off, just slide it back (I did it by myself) enough to access the module. The reason I do it this way is that I want to eyeball the connections with the truck running so I can determine if there are any leaks. I had to drop the tank to get at the return line between the tank and the framerail and founnd it difficult to drop the 1/2 full tank even with the bed off. If you are going to drop the tank, try and get it closer to empty. Hint: If you go with the bed removal method, take the rear bumper off, it makes things easier and use some kind of impact wrench with a long extension to get at the 6 bed bolts. I use two extensions for some wobble on the rear most bed bolts. Electric impacts are cheap if you don't own a compressor.

 

I dropped my tank with about 10 gallons of fuel still in it. If you have a floor jack and a block of wood (2x10x12 or bigger), you can slowly lower the tank down to the floor. Just make sure you dont place the jack directly below the sending unit, and don't put too much upward pressure on the tank (can cause damage to the unit). I used an impact wrench to remove the two support straps. If you are able to sit on the ground and stick your head up between the side of the box and the frame, you can see the fittings on top of the tank. There is enough room to reach all the fittings depending on how big your hands are. Word of warning: as you are are sticking your head up into that area, watch out for the spring's support bolt... I have a red welt on my forehead from learning this the hard way.

Basic steps:

1) spray the support strap nuts with penetrating oil
2) remove the fill hose and over-flow hose from the tank.
3) remove the output line and return line -- the "fast connect" fittings have two tabs that need to be pinched tight against the steel tube in order for the hose to be pulled off.
4) remove the electrical plug for the sending unit -- there was a red locking tab that had to be pushed sideways (toward the rear bumper) in order to release the plug.
5) place the jack gently on the bottom of the tank -- use a block large enough to support as much of the bottom as you can without getting in the way of the support straps. Do NOT place the jack directly below the sending unit.
6) remove the support straps -- they have nuts holding on one end and a "T" cut into the other end which slids into a slot on the frame. Completely remove them to make it easier to move the tank around.
7) with someone else helping, guide the tank as you lower it down -- make sure it doesn't tip over (unless you have a lot of "oil dry").
8) using two extra-large screwdrivers or prybars, tap on the ribs of the sending unit's locking lug (counter clockwise) to remove it. Before pulling the unit out, make sure you clear away all the dirt & sand so it doesn't drop into the tank.
9) installation is in the reverse order...

Franklin