How to change center fuel tank
 

Put your truck up on ramps, or jack it up and use jack stands. Be sure to block the rear tires so the truck wont roll off ramps.
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Now drain the gas out of the tank. I know this is the rear tank, I drained both.
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Now support the tank, a trans or ATV jack works great, but a floor jack will work as well, just harder to balance.
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Now unhook the two straps. You may want to use penetrating oil on the bolts and a wire brush to clean the threads.
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Start lowering the tank.
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When you have enough room, reach up and unplug the fuel pump. Then disconnect the fuel lines.

You may need to lower the tank a bit more so you can reach the filler tube. Disconnect at the tank and leave the fill tube on the truck.

Now set your old tank next to the new tank, next to the old tank, just to make sure they are the same.
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Undo the retaining ring holding the fuel pump to the tank. It will likely fall apart. Then put the fuel pump in the new tank along with new seals and retaining ring.
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Place the tank under the truck and get it in to place. Then line up the fuller tube and tighten, then connect the fuel lines and the fuel pump. Place the plastic tank protector over the front end, put the protective mats so they end up between the tank and the truck. If not it will rub a hole in the tank.
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Line everything up, then flip tank straps in place and bolt them back up.
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Fill tank back up. Start truck and check for leaks.