Metals should have a very dark (basically black) diffuse, as the look of pure metals is all in the reflection, metals do not diffuse light. It looks like your texture has a bright silver and yellow color in the diffuse, these should be in the specular map instead. The spec map should be pretty basic, pretty much a flat…
it already starts with the model/highpoly looking like its plastic then, your texture right now is basically just noise with one high spec and high gloss value, the secret lies in not having just one value maybe it helps you when you understand, that in metalness workflow, the diffuse is turned black and what you put in…
UE4 uses the metallic workflow so the reflection color gets put into the base color texture. The metallic workflow already assumes your diffuse is black based off the metallic map.