my 4runners manifold is getting red hot

i bought a 1994 4runner 4wd and the manifolds on both sides are getting red hot. when i got the truck the check engine light was on and checked it, it was the knock sensor and had no other codes. Got the exhaust check and its not stoped up. And put $24 worth of gas in it and got about 75-100 miles if that, its like its just pouring it out. im asumming that raw gas is burning in the exhaust pipe but cant figure it out. :banghead: