My exhaust manifold gets like that after a sustained amount of throttle is applied. Came to this conclusion after gassing the car with 1/4 throttle up a mountain road. Length of uphill is approximately 3 miles. In my opinion, these could be the convicts: Clogged cat converter. O2 sensor is acting up. Timing is off. But i just smogged the car this January. So the cat might be out of the possible suspects. If it was an O2 sensor, the car would be running rich, combusting the excess fuel in the manifold, or somewhere else in the exhaust system. And i would be able to smell it while idling. Would also idle rough. So would it be my timing? Thanks in advance guys.