2.0l WRX misfires. Help!
 

Sue's '02 Wagon was having random misfires on different cylinders, 1 every week or 2 for a couple months. 130K miles.

We were going to replace plugs when it appears she popped a head gasket.

GST pulled the motor, checked the heads (not sure if they did any work on them) and assumedly put the new plugs in when reassembled. (Also swapped the clutch in the process.)

As soon as she picked it up it threw a P0457 Evap/fuel cap and she took it back and they cleared it.

2 days later it has now misfired on 3 out of 4 cylinders, P0301-3, and thrown P0457 again.

Fuel pressure? But doesn't that throw a code? Without a coil, I don't know what causes misfires on more than 1 cylinder other than maybe fuel pressure. MAF or O2 sensor? Again, don't they throw their own codes?

Fuel Filter is relatively new.

Thoughts, ideas before it goes back to GST?

My 02 had a misfiring issue once, even threw the P0457 code and it turned out it was caused by bad fuel (either water or something in the fuel) from a crappy gas station...This sounds like something more significant though...

P0457 is an EVAP leak code, and there's many EVAP system parts, not just the fuel cap.

If there's a leak in the EVAP system at the intake manifold, you could be getting a post-maf vacuum leak, and thus, multiple cylinder misfires.

my $0.02

I had a persistent P0457 (every 3-4 days) prior to pulling my motor to cure a spun rod bearing. New fuel cap made no difference. After reinstalling the motor (about a year ago now) I never got another P0457. Must have been just something in the engine compartment associated with the fuel tank evaporative system. Reconnecting everything during the install cured it...