*This is not a thread for posting your own misfire problem!*
Please start your own thread in the General Maintenance & Troubleshooting forum. Please do feel free to post your own resolution to misfire problems though in this thread.
Once or twice a week an individual posts about having a misfire issue. This is a very common problem and there are alot of threads addressing this very problem. So here are the possible issues (read this first,then post about an issue if you have no resolution after trying said solutions):
1. Wrong, worn or incorrectly gapped spark plugs. Generally NGK work best gapped between .028"-.030".
2. Clogged/dirty/bad injectors or fuel filter. Try changing the fuel filter. On the 04+ WRX's/STI's double check the fuel pump"sock" in the tank. The maintenance schedule for the filter is every 30K miles. Have the injectors professional cleaned or at least run several bottles of a good injector cleaner through the gas tank. Also make sure you are using the correct octane gas! Either 91 or 93 unless you are specifically tuned for another octane!! (IE: race gas or E85) You can swap injectors from another cylinder and see if the misfire follows. If so this would indicate an injector issue if it does move..
3. Check valve clearances & compression. Perform a leak down/compression test first.The FSM has the valve clearance specs. This is a complete PITA with the motor in car.
4. Dirty MAF sensor. This causes an incorrect airflow readings. Misfires can result. Clean with CRC MAF cleaner or NON chlorinated brake cleaner. Make sure you are actually cleaning the MAF sensor and not just the IAT sensor. Common mistake.
5. Vacuum leaks or preturbo leak. Check where the FPR hose connects (generally you could get a P0457 or P0171 with this as well). Check the turbo inlet pipe where it connects to the turbo. This pipe is famous for ripping and causing a huge preturbo leak (P0171 as well).
6.Poor grounds/grounding. Clean the battery ground cable and ones that run to the intake manifold. Make sure all are connected. These are sensor grounds and must be kept clean.
7. Cam timing. Check cam timing per the service manual. Clean the cam sensor and crank sensor face.
8. Excessive PCV blow by. PVC gases will effectively lower the octane content of gasoline/petrol and can cause misfires along with knock. Get a catch can or air/oil separator.
9. Carbon build up in heads/cylinders. You can use Seafoam or top end cleaner as well as other type of carbon remover.
10. Lightweight flywheels or crank pulleys. (phantom misfires) Can cause false misfires. Opensource tuning can disable.
11. Bad/loose coil pack or coil pack connector. Test coil pack and move to other cylinder to see if DTC changes. If so this would indicate a coil pack issue. Also check the tower boot for corrosion. Use dielectric grease on all connections.
12. Faulty fuel pressure regulator or fuel pump. Check fuel pressure. It should be between 35-38 psi idle @ sea level. Removing the vac line at idle should give you 43 psi. The FPR is a 1:1 rising rate. For every 1 psi manifold positive pressure the fuel pressure should rise 1 psi. You will need a mechanics fuel pressure gauge and have to install a schrader valve in the fuel system before the FPR. Best place is right after the fuel filter on the bug eyes or right before the hard lines under the manifold.
13. Faulty front O2 sensor. Replace with new. That easy. The OBDII system will probably let you know.
14. Damaged crank timing gear. The one behind the crank pulley.
15. Misfires all cylinders: Coil pack connectors mixed up (correct way:black to back,white to front),light weight flywheel,bad/unplugged crank or cam sensor,incorrect gaped plugs,cam timing off or wrong spark plugs.
FYI-The bugeyes were plagued with misfires. Updating the ROM in the ECU to the latest revision from Subaru helps alleviate most false misfire issues. This can be done through the stealership or opensource software.
Hope this helps! Will add as other causes are found.