Sputtering could be fuel, air, or spark. But yes, spark sounds like a likely culprit.
- Generally fuel related items will generate a check engine light and codes. (You've not mentioned check engine light or codes, so I'm assuming there are none. If there are Onboard Diagnostics (OBD2) codes you should get those items fixed first.)
- Plugged air filter is not all that likely, you'd have other problems besides sputtering at idle.
- Spark related items are a whole other story. My guess here is bad spark plug wires. You will have to check each wire from the distributor / ignition system ALL the way to the sparkplugs. Please pull the wires OFF at both ends and check that nothing is burnt or worn.
- One fun test is to look for spark plug wires shorting out... You open the hood at night in a dark place, keep all the lights off. Place the car in park, and start it up. Then carefully inspect under the hood looking for stray sparks. Please be careful around moving parts (fan belts, cooling fans, moving alternator. I got my hand caught in an alternator pulley with cooling blades, at night Twack! and it wasn't a pleasant experience and visit to the emergency room.)
- Another thing to look at is the distributor cap and rotor (Not sure if your car has this or distributorless ignition coils) I've seen cars with loose caps lately. The bolts were tight, but the plastic inside had cracked and the cap was loose.
- I really doubt you've got spark plug issues. Again, my guess is a bad / burnt adapter on one spark plug wire where it is attached to the top of the spark plug.
Update:
- One way cool test is to remove the wiring from each spark plug, one at a time and read the tachometer. If the engine is idling rough, and you remove one spark plug's wiring, and nothing changes, that circuit is suspect.
- Somebody suggested that you've got a Coil on Plug distribution system. Way cool. They still need to be checked between the coil and the top of the spark plug. If memory serves correctly, Coil On Plug (COP) systems DO indeed set an engine code if the coil itself fails. The code will even tell you which cylinder is not firing. An great test of the individual coil is to clear the codes swap the coils between different cylinders and retest again.
Let us know what you find.
