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I have a 2003 Forester XS with 217,000 miles and a misfire code for cylinder #3 (P0303). The plugs were replaced with NGK's 7500 miles ago and the wires about 17000 miles ago with NAPA lifetime made by Belden. After looking for obvious problems (loose wires, shorts), I replaced the coil pack with a used one from a '04 Forester. The junkyard just said that the engine ran well that it came out of. When I switched out the packs, it ran better, but its still skipping occasionally and throwing the same code. I also have a small evap leak (P0442), but I doubt that's related. 

I decided to check the wires with a timing light. FWIW, these are the results: 

Original Coil: #1 Fast light, occasional skip; #2 Slow and intermitted light; #3 slow and intermittent light; #4 fast light, no skip. 

Junkyard Coil: #1 Fast light, occasional skip; #2 Slow and steady light, occasional skip; #3 Fast light, occasional skip; #4 Fast light, no skip. 

I'm wondering if I got a second bad coil pack. Also, is there any protocol for bench testing the coil pack? 

 

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lots of people say only ngk or subaru ignition wires.   I would start with new good quality wires.  Might be a good idea to tighten teh gas cap as much as you can or replace it to go after the p0442 code.

 

Here is what some other sites recommend for troubleshooting:

P0300 SUBARU - Random Cylinder Misfire Detected.... Possible causes - Faulty spark plug (s) - Faulty ignition coil (s) - Clogged or faulty fuel injector (s) - Intake air leak - Fuel injectors harness is open or shorted - Fuel Injectors circuit poor electrical connection - Ignition coils harness is open or shorted - Ignition coils circuit poor electrical connection - Insufficient cylinders compression - Incorrect fuel pressure... The P0300 code means that a cylinder(s) is misfiring or is randomly misfiring. Start by checking for intake leaks, intake gaskets are common caused of multi-cylinder misfiring. If no leak are found the next step is to replace the spark plugs. If the problem persist more tests needs to be done to diagnose problem,............................P0442 SUBARU - Evaporative Emission Control System Malfunction... Possible causes - Missing or loose fuel cap - Incorrect fuel filler cap used - Fuel filler cap remains open or fails to close - Foreign matter caught in fuel filler cap - EVAP canister or fuel tank leaks - EVAP system hose leaking - Fuel tank leaking... The P0442 code means that the control module has detected a small leak in the Evaporative Emission (EVAP). Loose fuel tank filler cap is the most common cause that triggers the P0442 code.

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