i have a 03 wrx. recently it will occasionally idle a bit rough and seems like it's trying to stall. it usually idle at 750rpm at -18in.hg with everything turned off. but when it's going rough, vacuum becomes unstable, either rise to about -14 in.hg or drop to about -22in.hg (but never ocsillate between -14 and -22). it also feels like a cylinder not firing when it happens and it will go away if i give it some gas. this problem will appear maybe once in 3-4 days and i drive 50miles a day. it first started when i got some bad fuel south of monterey along highway 1 but that was 6-7 tanks ago. could it be still bad fuel in my system? by the way mods on the car are helix dp, magnaflow muffler, removed intake silencer, drop in air filter, and autometer boost gauge. i will be bringing my car to a dealer for a recall in a week or two and i'll ask them to take a look at this problem, but i'm curious if anyone know what might be wrong with my car off hand. thanks.

 

If it started with bad fuel, there could very well still be some crap in the tank and/or fuel filter. At the very least the filter should be changed, and if the contamination is very bad the tank may need to be drained as well. Could also have clogged an injector.