Hello Joe,
I've not only checked the 3800 and grand prix forums, but also the owner's manual, and they both say 91 or higher is required. I've heard people drive these with regular gas and they are fine as long as you don't go over a quarter throttle. As far as I know the previous owner ran regular gas in it throughout her ownership of the vehicle, and that is what the dealer was running in it too. We don't have 91 out here in Chicago so I've been running 93.
I've replaced the plugs and wires, and the issue still continues. Under full throttle in 4th gear, it still knocks and there is a vibration at idle, but the vibration could be blown motor mounts. I've been running injector cleaner and octane booster in my fuel for a while now and it has gotten a lot better. I think the issue was there was still some contamination from the regular gas left over in the system somewhere. It only takes one drop of regular to detonate all of the fuel prematurely.
I have not touched the coils or the DIC but if it comes back those will be one of the first places I look. I know for a fact the original set of coils is still on the car, and I recon they will be out the door soon. I plan on switching to a MSD or ZZP set of coils. I've also read it could be a problem with the engine's knock sensor or the Throttle Position Sensor. I think I'm also going to try a fuel system flushat some point, as my lower intake manifold gasket is leaking, and the fuel lines running over the supercharger need to be disconnected to remove the intake and S/C.
Tuesday, January 17th, 2023 AT 1:07 PM