I have a 1991 Gmc sonoma 4.3
How nany miles on the old one? If allot and the body is good then it makes sense...
Its like why are they not firing? Have you done a compression test? If so and the cylinders are low in compression then yes, then get a re-manufactured engine.
Did you trace the two plug wires back to the coils.If both go to one coil then likely a faulty coil. Should be 3 coils together and 2 plug wires to each coil.
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