My girlfriend has a very reliable 2005 Mustang V6. The previous owner replaced the old engine with a rebuilt one with only 20k miles, everyone’s kept up on maintenance, we’ve replaced nearly everything on that car; it’s just been treating her great.

We have had one issue, though, that we can’t figure out. Cylinder 1 misfiring. We reached out to the guy who previously owned the car. He said it was probably the coil pack, since it was original. I replaced that in ten minutes on the side of the road, car ran great. Four or so hours later, engine lights flashing at us, and the cylinder’s misfiring again. We move to replacing coils, then plugs, both only helping for a few minutes before the engine light comes back on.

What should we do next? Original owner suggested we replace the MAF with a spare he gave us. No luck. He’s suggesting crank position sensor next. What do you guys think is the best course of action?

 

First, double check the wiring on the coil pack and plugs. Make sure you have them connected properly. One side (driver I think) is in numerical order and the other side is not.

Then, you don't mention the wires being replaced so check them to make sure that they are still good, or replace them with a new set (again, watch the firing order). Eliminate the cheap potential problem things first.