To All:
My son recently purchased a Nissen pick-up fitted with a modest but thoroughly modern 2.5L inline four cylinder fuel injected engine having variable valve timing and dual overhead cams actuating 16 valves. On a
900+ mile trip the vehicle averaged better than 25mpg.As with all new vehicles sold in the States since 1996 the pick-up is fitted with version 2 of the On-Board Diagnostic system. (ie, OBD-II) This reflects the fact that like all modern internal combustion engines, its management is governed by a several embedded microprocessors, each dedicated to specific tasks. The electro-mechanical devices controlled by those computers, such as the fuel injectors and various actuators, as well as the numerous sensors monitored by the computers, are themselves embedded within the assembled engine, making manual diagnosis inconvenient if not impossible. Which is why we have an On-Board Diagnostic system. In effect, the system uses a computer to monitor itself. Should any part of the system fail it turns on a warning light and in most cases, falls back upon a less sophisticated method of control, allowing the vehicle to run well enough to make it back home.
By periodically interrogating the OBD system and creating an archive of the results you can see various components slowly degrade over time, allowing you to schedule their replacement well before any failure can occur. Of course, to do that you need to be able to connect to the on-board computer with a PC or lap-top running suitable software, which is commonly available. Some software facilitates this by linking parts procurement to the diagnostics, allowing you to place an order by simply right-clicking the mouse.
All of the information needed to diagnose and repair the vehicle is contained in the factory service manual, a two-volume set running to more than 1,600 pages which costs about $250. As with most modern-day vehicles(*), the pick-up has been designed to reduce the amount of time needed to swap-out those parts needing periodic replacement. Fasteners and adjustments have been located so that everything involved in a particular repair procedure is located within arms-reach of the mechanic.
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By comparison, in the mechanical sense an air cooled Volkswagen is about as complicated as a two-cell flashlight, with all of the Preventative Maintenance items clearly spelled out in the Owner's Manual and/or the Factory Service Manual. But when something finally wears out -- when repairs are finally needed -- the only diagnostic computer is the one between the mechanic's ears.
-Bob Hoover