 A typical symptom of a porous head is usually a 'mayonnaise'-like substance forming somewhere inside the cooling system (usually, this can be found residing on the coolant reservoir cap).However, depending on the degree of porosity, symptoms of a porous head have a tendency to vary.Many "C20XE" operators have described the symptom as a curry-like residue or in more severe cases, a thick brown sludge which may overcome the entire cooling system.