The crank-angle sensor is attached to the rear of the intake valve camshaft on a 1.6 engine, on the 1.8 it attaches to the rear of the exhaust camshaft.
It senses the position of the intake cam, from which the ECU can determine the position of the crankshaft. By rotating the the sensor relative to the intake camshaft the engines base ignition timing is modified. A worn CAS O Ring is often the cause of a small oil leak.
Changing the oil seal on the crank-angle sensor is remarkably easy. First disconnect a couple of electrical connectors (identified by the orange bits in the picture to left) in order to make some room to work.
Then mark the current position of the sensor with a permanent marker. After removing the single mounting bolt, the sensor itself simply pulls right out. The most difficult part of removing the sensor is manhandling it between valve cover and all the electrical lines around there.
The oil seal was definitely leaking.
A coating of fresh oil on the surfaces that mated to the valve cover, indicate a leaky o ring. At this point mark the position of the “dogs” that fit into the end of the camshaft. This makes re-installation much simpler.
Once the sensor is out of the car it is simple affair to pull the old oil seal off and put a new one on.
As this picture shows, the oil seal is a simple rubber O-ring. (Cheap from local Mazda dealer).
The thing that this picture doesn’t adequately show is that the old O-ring (on the right) was crushed flat on both the inner and outer diameters, making it sit exactly flush with the sensor/valve cover mating surfaces. Oil was free to sneak right by.
Reinstallation is simply a reversal of the removal steps. The most difficult thing is getting the “dogs” lined up with the slots they mated with on the camshaft. Luckily Mazda was smart enough to design things such that you don’t need to worry about getting the sensor mis-aligned by 180 degrees. It only goes in one way. When the unit is properly installed tighten the retaining bolt and connected all the electricals.
Use a high quality grease on the O-ring.