Our recently purchased 1992 Chevy 3500 with 454 Idles rough when cold.Installed new MAP sensor after unhooking it fixed problem. New MAP sensor resulted in same simptom.
Does this have a TBI on it? Or is it a multi port fuel injection. Try snugging up the intake manifold bolts to start with, and run some fuel injection cleaner threw the gas.
It has TBI and I get where you are going. I will look for vacuum leaks.
Also since it is a TBI watch the 2 injectors spray. The fuel spray should be a nice cone shape with no droplets at all. Thats where the fuel injector cleaner comes in.
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