I have a 98 LX/auto Trans with 186,000 miles. A few days ago the check engine light started flashing and the idle seemed a little rough. I read the DTC codes and got a P0303 Code. I replaced the cap/rotor and wires and reset the DTC's. The problem seemed to be solved but after driving a few miles the light started flashing again and the rough idling returned. This time its a P0302 code. The plugs were replaced recently with NGK platinums and they still look fine. Other than the rough idle it seems to run fine. Any ideas?

 

When did you change the plug wires last?
With a meter check their resistance. See if they're still within spec.

Did you adjust the distributor at all when you did the rotor?
Check the timing.

 

the plug wires are new and the timing is good. I just realized something today: It only has the problem when it rains.

 

Do you have any moisture coming into the cabin?

How does your air filter look, and is it a stock air system or aftermarket?

 

Idle the engine and mist some water on the cap and wires to simulate rain. If it starts to misfire, replace those parts. Some aftermarket wires are real crappy. Bad wires also tend to spark on the outside which can be seen if you watch while running the engine in the dark.