Only a few people actually know about this 3.4 Liter Toyota engine problem in both Tacoma and 4 Runner where the valve cover bolts need to be re-tightened every 6 months because they constantly work themselves loose or else the valve cover constantly leaks oil even with new gaskets. The only solution is to have the valve cover/ crankcase bolt holes re-threaded and put new bolts. It's been this way for as many years as I can remember and is a fire hazard because the oil leaks onto the exhaust manifolds.