1.6 Ground Wire Modification
Written and conceived by Bill Cardell (Flyin' Miata)
Another mod that I very highly recommend (for 1.6 cars only), is to sever the two wires coming from the 2C and 2D pins on the ecu connector, solder them to a new length of 16 gauge wire and lead it through the firewall. Do not get fancy and add 2A and 2B to this connection, as these are power grounds. Use a ring connector and put it under the bolt that holds the ground strap that goes from the driver's side rear of the cylinder head to the body. Please also clean and solder (or replace) the ground strap. This will give better signal to the oxygen sensor. You can see the mod below. Cut the two wires (black/green) which are second from the right end in the larger of the two connectors. The cut wire going into the harness can be left. If you put a four wire 02 sensor in your car, you want to ground the gray wire to the same bolt on the back of the head that the strap and your new ecu ground go to.
Another ground that can give trouble, especially if you have had your trans out to do the clutch, would be the ground strap going to the power plant frame, back near the differential. Clean this one and put a star washer under it for insurance.