I've been told there is a G118 blk/white striped ground wire that grounds to the passengers side head between it, and the firewall. It is a ground for the ECM. Sources say if you have an ECM acting up, this can be a culprit. The only thing is I can't locate it on mine. What harness does this ground wire come out of ? The one that snakes over from next to the coil to the heater/ac box ?

 

Only thing I've ever seen was a regular groundstrap (woven bare copper) from pass side head to firewall.

BUT - I have seen 2-3 grounds connected to the top pass side tranny/engine bolt. Maybe that's what you are hunting for? I never paid attention to colors - those wires are at best a PITA to reconnect let alone actually being able to see the colors - but they have to be grounds for something.

I'm surprised that a ground for the ECM wouldn't just attach to the body somewhere inside the car, since the body should be grounded at the battery as well as at the pass head/firewall groundstrap. But no one ever said GM engineers were rocket scientists.

 

True even though they claim to be rocket surgeons

That does seem odd- even for GM- to run a ground from the passenger compartment to the engine bay. I seem to recall the grounds on that bolt to be (at least one of them) for the HVAC items on that side (?). Bear in mind, my car was a wiring nightmare when I bought it, so who knows if mine was correct.