The Loose Nut: Gearhead Miata Ramblings
by Kevin Morrison

On first-generation Miatas, a common place for an oil leak to develop is on the back side of the engine, at the crank-angle sensor. This sensor sits on the back end of the cams (intake cam on '90-'93, exhaust cam on '94-'97) and tells the ECU where the crankshaft is in its rotation. The ECU uses this information to time the spark plug ignition. The sensor looks like a mushroom and has a shaft that engages into a recess on the back of the cam cover, with a rotor with "ears" or "dogs" that engage a slot on the end of the camshaft. On the shaft of the mushroom there is a rubber o-ring that prevents engine oil from seeping out. Over time this o-ring gets hard and no longer seals well enough to prevent leaks. It doesn't seem like much could leak out here, but I've seen a car leak as much as a quart of oil every 1000-2000 miles.

Some unscrupulous shops (none around here that I know of) have reportedly charged people upwards of $200 to replace this $2 part. Any car over about 5 years old (which all early Miatas now are) should probably have this o-ring replaced. Save yourself some money and do it yourself. I'll just map out the steps to replace that o-ring as logically as I can. It's pretty simple, but can be frustrating due to the limited access to the back of the engine. Of course you will need to obtain a new o-ring. You can get one from a dealership or mail order it from one of the many Miata aftermarket vendors. A good auto parts store may be able to cross-reference a generic replacement for you.

1) You have to remove the crank angle sensor. It is held in place by a single long-headed 12mm bolt. You will need a 12mm box-end wrench with a slightly angled head. The longer the better. A 12-point wrench will make it a lot easier than a 6-point because of the limited space. If you don't have a wrench of this description, Sears sells a Craftsman wrench that is perfect. It's the one with the polished finish and costs about $8 or $9.

2)Before you touch anything, use a sharp instrument, like a file, to mark the position of the CAS. You must return it to the same position or your timing will be off. If you know how and don't mind setting the timing afterward, using a timing light, don't worry about marking it.

2b) there's a wire harness plugged into the CAS. Unplug it and move it out of the way.

3) Ok, reach back there with your 12-point, 12mm, angled box-end wrench and get it on the bolt. It's very tight back there. Loosen the bolt. Be careful not to turn it the wrong way. It's "lefty loosy", but you're working backwards, so it's easy to go the wrong way. It is possible to strip this bolt and you really don't want to go there.

4) remove the bolt and put it somewhere safe. The CAS is now free and with a little wiggling, you should be able to pull it out the back of the cam cover. Be very careful here. You need to note the position of the drive dogs upon removing the CAS from the engine. It will only go back in one way. There is a small notch on one of the "ears". Note its position and remember which way it goes when you try to put it back in.

5) See the o-ring? It's probably hard as a rock, which is why it's leaking.

6) You'll notice that the CAS is in your hand, behind the engine, but you can't get it out because it's too big. That's ok, you don't really need to get it out. You can replace the o-ring while holding the CAS in your hand behind the engine. It's tight and awkward, but it can be done. If you really want the CAS out of there so you can work on it easier, you may be able to weasel it out. You may also have to loosen the coil pack and maybe some of the brake lines to move them out of the way. It's up to you whether you want to mess with all of that. It does make removing the old o-ring and installing the new one easier.

7) Get the old o-ring off. Put the new one on. This will be self-explanatory.

8) Now the fun part. Remember how the "ears" on the drive dog were oriented, put it that way, and slide the CAS back into the recess it came from. It will be very tight because of the new o-ring. Smear some motor oil on it to make it easier. The ears have to be perfectly lined up for them to go back in all the way, so just fiddle around with it until it "snicks" into place. This could take a few minutes. You may have to push really hard. Just when you think it's never going to go back in, it will.

8a) Have a beer or a nice fruit drink. You've earned it.

9) Put the bolt back in, line up the reference mark you made earlier, and then tighten the bolt down snug. Don't put monster torque on it. Plug the harness back in.

10) get a rag and clean up the old oil on the back of the engine below the CAS and check it again in a day or so to make sure the leak is fixed.

That's it. You just saved yourself $200. Go buy some lottery tickets, or send some money to me to celebrate.