Just swapped a 4.8 in the nova . Vats is off . After i rerouted the wires It wouldn’t run anymore .the only thing i changed was the ground locations . I’ve tried them every which way since and it still only fires briefly then stops . The engine does run with a spark plug pulled out of the head grounded to the exhaust manifold (on 7 cylinders). I think i have a ground issue but I’m looking for input . Maybe a low reference issue .

Grounds as follows: ecm to each cylinder head , battery ground to the block, ecm to body, battery to body

 

What about block to body?

 

Yes. Tried it.

 

Did you cut and splice any wires? Did you check continuity of those wires if so?

Ground location shouldn't change a thing. They all meet a mutual source.

Are you saying it will run as long as you pull one spark plug out of the motor?

 

That's what I was thinking but I read the grounds had to run to each head. The engine will run if the plug is out of the head, plugged in the spark plug wire, body of the plug grounded to the exhaust manifold . The manifold doesn't touch/ground to the body anywhere though, which is weird . To me it seems like it grounds the circuit when it's touching the manifold somehow (running on 7 cylinders bc one plug out )

 

Sounds really odd.

The heads are grounded to the engine block, the engine block is grounded to the battery, the body is grounded to the battery, the exhaust manifolds are grounded to the heads that are grounded to the block. It's all theoretically and literally a mutual ground. In this case ground is ground. The wire has no idea what its attached to, it just needs to be grounded. The wires are grounded to the heads from the factory because it's a short path to ground for the sensors on the engine, it uses less wire which saves money and weight in long runs of wiring.

As for the plug being pulled and the engine running that is very strange. There may be some sort of back feed going on in the ignition circuit that I'll have to scratch my head on for a while to see if I can make sense of it but before going there.

What all exactly changed. It was fine before, changes were made, now it's not. Was any wiring cut? If so did you ohm across each side of each splice to check for continuity?

Did you pull the coils or do anything with the coil wiring? If so try unplugging the main coil harness connector on the side of the engine that you are pulling the plug out of. Put the plug back in and disconnect all coils by pulling the main connector. See if it will run on the other side of the engine only. I've had to do this before to diagnose incorrect coil wiring (coil wiring was hooked to the wrong coils on one side of the engine). See if that gets you anywhere to start with. If it runs fine on one side of the engine and not the other you can isolate things to that side.

Does it pop, sneeze, or backfire at all out the exhaust or intake?

You could also pull a wiring diagram and look at the wires you cut, if you cut any, and go to which ever sensor or item that wire feeds and check for ground there (or power whichever you cut) to check to see if your connection was good.

First evaluate whatever changes you made with high scrutiny, before moving on to what if's. It's real easy to start thinking of other items and start swirling all the what if's in your head. Keep it simple at first and only concentrate on the changes you made and make 100% sure there are no issues there first. I went through this last night with my son. He's checked over his car 20 times in the last 2 days and swore there were no issues with the changes he made. He changed the door speakers in his car and the radio stopped working. I ohmed a circuit for him and told him there was a short and where to look and sure enough he pinched a wire and shorted it. Take your time and pick every change apart with a fine tooth comb so to speak.

 

Also, have you scanned it for codes?

Have you verified the security system is not activating? Just a double check on it?

 

That's what problem I'm having with my 2003 Tahoe

 

That's nice, and really tells us a lot.... NOT.....