I have a 88 F-150 with a 302. Everytime I turn drive it after about ten minutes the cab starts to fill with exhaust. I have checked all the sensors to see if they are bad or rotted out and have looked and could find no exhaust leeking out of anywhere. All the bolts on the manifold seem to be in place and tight. My only other thought would be the heater core leaking but i thought i would check here to see if anyone had ideas.