Replacing the Miata’s Cam Angle Sensor O-ring

My 1993 Mazda Miata has been one of the most reliable vehicles I have ever owned. Since purchasing it in 1996 with 34k on the clock, I have put 75,000 miles on it and it has never left me stranded. Aside from maintenance (which to me includes normal wear-and-tear items like brakes, shocks, tires, and batteries), the only “repairs” I’ve made to the car in 27 years have been a clutch secondary cylinder, a heater core, a power antenna, and one headlight bulb. So when I discovered a minor engine oil leak a while back, I ignored it until recently. I finally decided to tackle the leak last week, which involved replacing the rubber o-ring on what Mazda calls the CAS, or Camshaft Angle Sensor.

I always take a “before” photo in case I mess up; the circled plug is 1 of 3 to be unplugged.

The CAS sits at the back of the cylinder head, and engages with the intake camshaft. This is how the ignition timing is changed: the CAS is adjustable, and by slightly rotating it in one direction or another, the ignition timing (in relation to the cams) is changed. It’s a DOHC engine, but it is not variable valve timing. Doing some research on the Internet, I found multiple sources identifying the CAS o-ring as the #1 cause of Miata engine oil leaks. The oil drips down the back of the engine onto the transmission, and from below, it can be difficult to pinpoint the cause. A quick check with a flashlight directly under the CAS confirmed this as the leak’s source.

Chat forums and YouTube videos are great places to find repair information, but in this case, there were two distinctly different approaches to the job. On one hand, the “book” method is to remove the valve cover, remove the C-shaped cap over the CAS, and lift it from the car. The alternate, and supposedly less time-consuming approach, is to leave the valve cover in place and only remove the CAS adjustment hold-down bolt. One can then wiggle the CAS out of the engine compartment, with the acknowledgement that re-engaging the “dogs” or teeth which fit into the back of the cam is tricky because it’s a blind operation. At least one commenter admitted that getting the new and unworn o-ring past the CAS hold-down cap can require severe muscular exertion. In either case, viewers were strongly advised to mark the position of the CAS so that re-timing the car would not be necessary. I marked it with a black Sharpie.

Plug wires tagged and about to be removed; CAS is circled in red

After watching 3 or 4 videos and then examining my car, I was not convinced that the savings in time was worth it. Removing the valve cover is not at all difficult, but it takes about 20 minutes. I actually had a more difficult time removing the 3 electrical plugs (which needed unplugging whether I followed method #1 or #2) because these plugs had not been touched in 30 years. But I eventually got them, and followed that by removing the spark plug wire set, the PCV hose, and the 11 bolts holding down the valve cover. I had ordered the CAS o-ring (63 cents) and a new valve cover gasket from Rock Auto; with tax and $9 shipping, I was into the job for twenty bucks in parts.

I had an ulterior motive for removing the valve cover: I had never done so, and I wanted to see how clean things were underneath, and they were clean indeed! Frequent oil changes, almost exclusively with Castrol oil, certainly has played a part. Going this route, the CAS was quickly free from the camshaft, but even with the valve cover out of the way, there was precious little clearance between it and the firewall, so I was doubly glad I went for the “longer” method.

Underside of valve cover, old gasket still in place.
Miata engine, 30 years old, 109,000 miles, looks clean to me


CAS on my workbench, the old o-ring was so dried out that it snapped like a piece of plastic. The new one slipped on very easily, helped by a little motor oil. Then the job was like it says in the repair manuals: reassembly is the reverse of disassembly.

CAS on my bench, old o-ring still in place

Again, I was glad to have the valve cover out of the way, as engaging the dogs into the cam was a cinch. I rotated the CAS to where I had marked it, locked it down, put the new valve cover gasket in place, and reinstalled everything which had been removed. The car started on first try, and while it was too wet today to go for a test drive, I plan to clean off the underside as best possible, then drive the car to confirm the leak is fixed.

Clear shot of CAS about to reengage with cam

I guess I need to add this repair to my list of “non-maintenance fixes” to my Miata!


All photographs copyright © 2023 Richard A. Reina. Photos may not be copied or reproduced without express written permission.