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So, What the Heck is an OBD? 

     No, it doesn't refer to your Old Bald Dad. It's On-Board Diagnostics. Federally mandated On-Board Diagnostics, no less. That's right, brought to you by the Gu'mmint. A big change in OBD systems happened in 1996 when manufacturers changed from OBD I to OBD II. What's the difference, you ask? (I knew you were going to ask that.) I, the difference is I. (Get it?---OBD II minus OBD I equals I, like one. Get it? It's a joke. Oh, never mind.) The earlier OBD I system was relatively simple and not very restrictive. OBD II is both much more complex and also accompanied by more stringent emissions regulations. The net effect of these is that when BMW redesigned the induction system to meet OBD II emission requirements, they had to increase displacement from 3 liter to 3.2 liter to keep from loosing horsepower. So the trick set-up is to do an OBD I conversion on the later engine. That is, remove the later restrictive intake manifold, all the OBD II sensors, the wiring, and everything else from the 1999 injection system, and replace it with the intake from any VANOS equipped OBD I system (1992-1995 325i or 525i). 

     There are a few hurdles to jump when going back to an OBD I system. Both systems have VANOS automatic cam timing systems on the front of the intake camshaft, but the electrical connectors will not interchange. While it may be possible to cut and splice the shielded wires to switch the connectors, we think it's more reliable to change over the VANOS units---and that's not a simple process. First, the oil filter housing is in the way and has to be removed. Then we have to secure the cams with a special fixture during the actual removal and replacement of the VANOS units in order to keep the cam timing correct. Finally, the timing chain has to be retensioned to complete the change over.

     Another little hurdle comes from a change in the temperature sensors in the cylinder head. The newer head has only one sensor; the earlier head has two. Fortunately the newer head still has the casting boss, so drilling and tapping an additional sensor hole just takes a few minutes of careful work.

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