What causes a misfire on a 99 pontiac grand am if spark plugs or good and the car does not have wires

How do you know the spark plugs are good??

Main causes of misfire would be loss of spark; the air/fuel mixture is too far out of balance to ignite; or loss of compression.

Loss of spark includes anything that prevents coil voltage from jumping the electrode gap at the end of the spark plug. Causes include worn, fouled or damaged spark plugs, bad wires or even a cracked distributor cap. A weak coil or excessive rotor gas inside a distributor would affect all cylinders, not just a single cylinder.

"Lean misfire" can occur when the air/fuel mixture is too lean (not enough gasoline in the mixture) to burn. This can be caused by a dirty, clogged or inoperative fuel injector; air leaks; or low fuel pressure because of a weak pump, restricted filter or leaky pressure regulator.

Loss of compression means the cylinder loses most of its air/fuel mixture before it can be ignited. The most likely causes here are a leaky (burned) exhaust valve or a blown head gasket. If two adjacent cylinders are misfiring, it's likely the head gasket between them has failed. Also, if an engine is overheating or losing coolant, it's likely the head gasket is the culprit.

You can isolate the weak cylinder by temporarily disconnecting each of the spark plug wires, one at a time, while the engine is idling. When there's no change in the idle speed, then you have pinpointed the weak cylinder.

Here are the common causes of a missfire, defective ignition coil or ignition coil wiring for that cylinder, defective fuel injector, defective powertrain control module, intake manifold vacuum leak, low cylinder compression.