Misfire code P0302

[Harold56]

Have a '99 Century with the 3.1 L Vin M engine. Rebuilt the engine about 5k miles ago. Starts and runs like new except for occasional rough running and throwing a #2 engine misfire code. Below are the conditions under which the problem occurs:

-engine warm
-@idle
-very light engine load (low speed cruise)
Engine runs fine when warming up, or medium to heavy load. I ruled out #2 plug/wire/coil is bad as it pulls hard under acceleration. If the fuel supply was at fault then I would get random cylinder misfires and not just #2. Which leads me to think the #2 injector does not work well under light fuel requirements. (short injection pulses) Am I on track or should I consider other problems?

[richtazz]

Just because it runs better under hard acceleration is no indicator that the plugs/wires/coils are ok. I would try swapping all three (one at a time, plug first, then wire, then coil) to see if the miss moves with the component. If it stays at #2, you can eliminate the above parts as the cause of the miss. This leaves you with either a bad ignition module (they will fail due to heat and can fail on only one cylinder), or a fuel/compression issue. These are harder to test because they require special tools (DMM, noid light set and compression guage) but not technically difficult. The ignition module can be tested for free at Advance and Auto-Zone, but if it's cooled off when they test it, it may not detect the problem. Try the simple things first, and let us know if swapping the ignition components helped.

[Harold56]

Okay, will try swapping out the plug, wires, coil. I have a set of everything from a previous known good engine. Lucky for me the #2 cylinder is right in front and easy to get too.