do i need a valve cover ground wire?

[BABY J]

^^^ beat me to it... also...

The longer a wire, the more resistance it has. When a current flows through a resistance, a voltage drop is produced. Because of this, the ground reference at the source is no longer the same as that at the chassis of the vehicle. This ground potential differential can lead to noise (electronic), improper operation of the source (alternator / battery). If your voltage source is operating improperly... what do you think that does for the ECU - harness -- lights --- I could go on. ALSO... if there is electrical noise... knock sensors, MAFs, TPS, and many other sensors can go ape shit.

[speedminded]

^^^ beat me to it... also...

The longer a wire, the more resistance it has. When a current flows through a resistance, a voltage drop is produced. Because of this, the ground reference at the source is no longer the same as that at the chassis of the vehicle. This ground potential differential can lead to noise (electronic), improper operation of the source (alternator / battery). If your voltage source is operating improperly... what do you think that does for the ECU - harness -- lights --- I could go on. ALSO... if there is electrical noise... knock sensors, MAFs, TPS, and many other sensors can go ape shit.

Yep, most sensors operate between X and Y voltage...if it goes above or below that then they can't be read properly and everything goes nuts.