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Have a 88 f350 that will run perfect on front tank until driving long enough to use 3/4 of the tank then will start to act like its running out of gas. When switched over to rear tank will run fine. After letting sit for awile will run if switched back to front tank. All filters,fuel pumps, coil, ignition module, and rebuilt distributor. Any ideas?
When's the last time you've changed the fuel filter? have you checked the strainer? If something is partially clogged it makes sense for the pressure of a full tank to assist in forcing the fuel through, and when the pressure's reduced it can't supply the fuel at the pressure it needs to. It does sounds like a pressure loss along the way, which is what I think Conanski was going for.
Sorry, the strainer is on the pump in your model. My apologies. Can you run the truck on the front tank and take a fuel pressure reading and let us know what you get?
You could... but let's think outside the box here...
1. did you replace the fuel pump relays when you replaced the fuel pump?
2. what about the lift pump (they still have those with dual tanks I believe)?
If either of those is acting up it makes sense, those relays are mechanical and have very finite lifetimes...I'd check and try to find any bare/damaged wires to any of the fuel system pumps and relays.