I have a 96 5.2L 4x4 and I was wondering how you diagnose the leaky plenum pan gasket accurately. I have fixed it once but i think its leaking again. Burns/uses lots of oil(no leaks but can smell a little oil burning) Also I am getting absolutely horrible gas mileage like 14-15 maybe less on the highway and about 11 in the city. Any ideas?? Front o2 sensor has been replaced along with the tune up. Any ideas are GREATLY appreciated. I am on my last nerve and am ready to part with this truck if i cannot take care of these problems.