To my knowledge, Ford never made a flex fuel 4.0L. They only made flex fuel engines in the rangers with the 3.0L. The 4.0L have "4.0l" written across on top of the intake manifold. I'm not sure if the 3.0L do or not. The 8th digit of the vin will tell you what engine you have. If its an "x" then you have the 4.0L. I believe a "u" is used to designate the 3.0L.

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1996 Ford Ranger STX 4x4 4.0L
1983 Honda Shadow VT750C