Fuel pump relays are a common problem. They sometimes hang up. Being a 1988, I would replace both relays located on the passenger side inner fender (The EEC and FUEL pump relays). Also, did you replace the fuel filter(s) when you did the fuel pumps? Ford ran a canister filter for the low pressure pump thru 1987, but my 88 Ranger S/C 4x4 had one in it from the factory. I know this because I bought the truck new. The canister filter housing is located on the frame rail behind a steel bracket. You'll see the fuel lines going to and from it. The high pressure pump filter is on the frame rail before the high pressure pump.
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