Twitchy Tach on my 1.8 motor

It started jumping one day. At idle. it would bounce up. At a steady highway cruise, it would bounce around but sometimes would settle down. Sometimes, it would bounce up to twice the actual rpm.

In talking with a few others who have had this symptom, the car seems to run OK. In my case, I had to drive about 70 miles to get home when the problem first started and at one point my engine stumbled and quit when the tach really got erratic. However, it settled down and I did get home.

Look for the wire harness above  your alternator on the  left front side of the motor. The ground nut may be loose.  Take a 10 mm socket  and tighten it. Hopefully, this will solve it for you.

This was done on my 1994 miata. I've heard, but not personally verified, that the same problem has been seen on 95-97 miatas, and that the same fix has worked.

Tach jitter on a 90-93, in my opinion, is also a ground problem, but is a different ground, and not potentially destructive to the ignition coils as it is on a 1994. This ground isn't on a 1990-1993 miata anyway.

Why it works

If the ground nut is loose, the ignition coil circuit is noisy and it misfires. Tightening the ground gets rid of the noise. For further discussion, click here.

Why worry about it?

There are two coils. Normally, one coil sparks when two pistons in position and the other sparks when the other two pistons are in position. The misfire causes a coil to spark at the wrong time (when the other one is sparking). If this keeps up, the coils will eventually overheat and fail. This happened to me as my first response to this problem was to replace the coils. The twitching came back and the replacement coils burned up.

Credits

Summer 1998 issue of Miata Magazine. They had the same problem and wrote about the loose ground nut. They also burnt up their replacemnt coil.

I had figured out that the system was misfring, but didn't know to look at that specific ground.