1999 Oldsmobile Aurora 9 plug wires on a V8?

I just verified all 8 wires are connected to the coil pack. All 4 are connected to the plugs on the front of the manifold. As far as I can tell by feeling around all 4 are connected at the rear of the manifold too. I've just discovered the mystery wire is indeed a plug wire cause I've found both ends but can't get it out of the wire holder now. Also my gas gauge is going crazy now. It quickly shoots up and down by between 1/4 and 3/4 of a tank span. So now I'm super confused. I'll pull it apart in the morning to make sure all wires are connected on the rear of manifold but wouldn't it be missing really bad and constantly if that were the case? At a slow to moderate acceleration or if going up hill it'll stutter really bad but if I put more gas to it then it settles out and drives fine. I also don't have this problem once I get up to hwy speed only at slow to moderate acceleration. Also no check engine light. Can I still get the trouble codes if the check engine light isnt on? Anyway I'll dive further in tomorrow but thank you very much for your reply.

So figured out the extra wire I found was just that. Extra. The guy I had Change the plugs left one of the old ones loose but on the clip. The plugs on back of manifold and wire are a real pain to get to. I've verified all plugs and wires are good. Fuel filter is new. Compression test was good. Fuel sending unit ohms were good. Still no check engine light but was able to get an intermittent cylinder 1 miss fire code. Everything has checked out tho(plugs, Gaps, and wires). So now I'm thinking maybe the #1 coil is dropping out or maybe injector? I'll replace the coil this weekend cause their cheap enough, but how do I test/check an injector? Thanks a lot for all your help so far.