Proper grounding of electrical components is of the utmost importance for electrical (DC) circuits.  Due to the age of the vehicles, electrical (ground) connections can get loose and corroded.  If these ground points are compromised, electrical circuits are compromised.  If electrical circuits are compromised, the engine doesn't run at it's best.

Unfortunately, the main ground for all ECU circuits run to a ground point at the top of the engine.  Accessing this area requires removing the stock top-mount intercooler and the upper intake manifold.  Once those components are removed, you'll see the ring terminal with pair of thicker black wires under a 12mm bolt position slightly on the driver's side under a bracket on top of the engine.  You can remove and clean the terminal to make sure this ground point is good.  You can also run an extra ground wire from this same point back to the battery negative terminal; the negative battery terminal is also the same reference as ground.  When adding extra ground wires, use at least 10 - 12 gauge wiring.

The main engine ground runs from one of the starter bolts to the bottom of the driver's side strut tower to the negative battery terminal.  When servicing the starter, make sure you secure this ground connected properly.  Without this main engine ground secure, all current will go through the auxiliary ground wire; this smaller ground wire cannot handle large current capacities such as engaging the starter!  If this auxiliary ground wire is still there, it's located at the back of the engine and terminates to a connector in the middle of the firewall.  Adding more engine grounds is a good idea.

Another place to add an extra ground wire is from the alternator to the negative battery terminal.  The alternator case is a negative reference point, and adding a ground wire to this will ensure optimum voltage output from the alternator itself.

Adding extra ground wires to the ECU ground leads can also enhance performance.  Please reference a service manual to check which wires are ground; splice additional 14 - 18 gauge wire into those leads and bolt the ends to a clean ground point.  Make sure you scrape off any paint for better metal-to-metal contact.

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