Check the plugs. I'm here to tell you loose plugs will make the engine idle rough. I also cracked a plug one time. This didn't have any apparent effect on idle quality but made the engine lag and run rough during acceleration; the symptoms were what you might expect with a low fuel pressure problem.
This engine has aluminum heads so I only remove or install plugs when the block is dead cold and always use a torque wrench. A tip I found in a book is to use anti-seize or graphite. Be careful what you use so you don't contaminate your O2 sensor! Also don't over tighten since anti-seize reduces friction. I found some O2 sensor anti-seize (part number 5613695) at the AC/Delco parts house. It was expensive (something like $25 for a 4 ounce bottle) but you only use a little and if it's ok for the O2 sensor, it should be should be safe for spark plugs.
There is a second school of thought on using anti-seize on spark plugs; the AC/Delco spark plug installation procedure page says not to do it:
These days I do not use anti-seize.Do not use any type of anti-seize compound on spark plug threads. Doing this will decrease the amount of friction between the threads. The result of the lowered friction is that when the spark plug is torqued to the proper specification, the spark plug is turned too far into the cylinder head. This increases the likelihood of pulling or stripping the threads in the cylinder head. Over-tightening of a spark plug can cause stretching of the spark plug shell and could allow blowby to pass through the gasket seal between the shell and insulator. Over-tightening also results in extremely difficult removal.
A few years ago I tried the new AC/Delco Rapid Fire plugs. I don't have a dyno so I can't tell you if they increase power but I like them. One problem though is that there is a threaded piece where the plug wire goes on. I found nearly all of mine were loose. You can use channel locks to put them all back on tight but don't twist too hard or you'll damage the ceramic! (At $4 a pop, you don't want to destroy one of the plugs!)
If you do pull any plugs, examine them for color. From what I've seen and read (including a nice pictorial in Stock Car Racing magazine), off-white with a greenish tint indicates a good fuel mixture.
The picture on the left is an AC/Delco Rapid Fire plug and the one on
the right is a regular AC/Delco plug. Both were removed after about 20K
miles. It's doubtful the digital camera used to take these pictures or
the monitor you're using to view them reproduce the colors accurately
but in fact both show pretty reasonable colors.
Check the plug wires and replace them if they are too old. However, this can be a lot of work. But as plug wires age, the insulation breaks down and this has a larger effect on modern high voltage ignition systems than with older vehicles. One book I have says a good plug wire should measure between 1k and 10k per foot with a DMM. Snap-on says each wire should measure at least 20k.
Since it's nearly impossible to replace the plug wires one at a time (as is usually recommended), make a diagram of what goes where. The service manual is helpful here. The original plug wires probably have plug numbers on them (mine did). The factory replacements didn't so I just used a permanent marker. You can also use a DMM to buzz out the wires to the back side of the engine after routing them.
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