I bought a 1999 Nissan sentra with 120,000 miles. After owning the car for a while a noticed an engine knock on start up and when I am on the highway stepping and letting off the gas. I thought it was the timing chain so I took the valve cover off to look at it. What I found was that all non moving parts had a quarter inch of black hard crusty crud on it. It looks like it was a sludge problem at some point. I tried an engine cleaning treatment with no results. I am thinking the engine needs to be rebuilt but it passed emission testing no problem. Could it be the oil pump gone bad, any ideas?