Rough running 3.8
I had my 87 Cougar in and spent $2500 trying to find why it runs rough, changed virtually everything. I just discouvered what it is. Go outside where it is pitch black. Wait for 15 to 20 minutes, then (no lights of any sort allowed here so prepare ahead of time to cover or disconnect any that may work) open hood, start engine and peer into engine compartment approximately where you would expect to see the spark plugs, spark plug wires, coil wire, and the coil. Now, take your time and watch for little spikes of electricity jumping to ground. It looks like miniture lightning strikes. If you see any you need to replace the defective part. Mine turned out to be the coil, it must be defective because it is arcing to ground along it's sides, not at where the coil wire is attached at the top. I can see there are one an maybe two of those brand new plug wires arcing too. The engine would run fine for sometimes a week, then go crazy. Not one mechanic could find the friggin problem.