2003 Buick Park Avenue change spark plug wires

Think of the oil cap extention as if it were the cap. That is it comes off the same way. Once you have the neck out and cover off, you could just put the cap onto the valve cover directly.

So turn the cap counterclockwise and remove it and then turn the neck counter clockwise and remove it. The whole cover just comes off.

HAving said all that I doubt you needs plugs yet. Pull one out and check the gap. Probably right on the money. IF it came out tough, put a little anti-seize on the threads about 1/4" up the threads AWAY from the eletrode.

IF you still have the desire to replace them, I think A/C delco is a good chice on that one. I would use Autolite double platinum myself only cause that is what I usually use and would have in stock.

The plugs wear significantly better than the old resistor style. If it was misfiring it would set a po30X code.

The 3.8 engines have had a intake manifold problem that "may" be part of the problem. IF it was, you would be losing a little coolant, so check the level.

IF not, I would suggest having the code numbers checked at that point and get back to me. Autoparts stores sell inexpenve code readers.

Yeah I would. Doesn't mean one will or won't be there, but it sure is easier to chase down a problem with more clues.

Coolant level been changing at all?

There are many possiblilities as to what is going on. Some will set a code, some won't. The some won't side of it can become a bit more of a ghost chaser.
Maybe an o2 sensor is weak or other powertrain management input sesor is out of spec, maybe a plug wire or coil is shorting, maybe carbon build up in the intake valves. Maybe the fuel pump is weak.

IT becomes procedural testing versus theoryand component testing.

No, your doing it about right.
I believe there is a metal shield around them on that one, we pull the metal and all out. Sometimes wires break.

The wires usually break when they don't come off easily. It would be a good idea to have a set on hand when you try to remove them.