What would cause exhaust manifold to get red hot

Way too much fuel and it's burning in the exhaust manifold instead of the cylinders. It would be real helpful to know the engine size when talking about an engine problem. If it's a V-6 or V-8, if this is happening on both sides, pull the vacuum hose from the fuel pressure regulator to see if fuel is leaking through it. If it's only occurring on one side, suspect a leaking fuel injector.