Jeep 258 vacuum diagram - Marina De Jesus Watching the voltage reading, slowly apply vacuum a little at a time. Vapor. 2 engine wiring diagram. There is also a similar-sized hose that runs from the air filter to the oil fill cap (V8) or the front of the valve cover (I6). 3. 2L Engine 1987-1990 Jeep Wrangler YJ w/ 4. The 258 I6 engine was always carbureted from the factory.