My '02 Impreza 2.5RS has developed a problem. At highway speed it has a tendency to lose power, then recover. When starting from a stop, it "hiccups" several times before settling into a relatively smooth acceleration. At idle, it slows from 1100rpm to 600, then back to 1100, then to 500, then 1100, then 400, almost dies, back to 1200, and if I let it go long enough will eventually die. The hesitation and stalling during acceleration and at speed are worse if I let it do its idle dance long enough (as after being stopped at a light). I found that the hose connection to the air filter assembly was loose, although I've never touched that, and the air filter had a good amount of "oily" residue (just replaced it) and an unexplained hole, like a rock had gone through the filter. Originally I thought it was water in the fuel filter, as with my first '84 1600 sedan, but that is unverified at this point, as, without a manual, I can't locate the filter. Further, I've read on this site some post re: MAF contamination and the resulting mixture problems. Any thoughts or advice would be appreciated.

 

Stan

You should check the PCV valve for a problem. It sounds like the air filter is bad. Have you tried running the engine without it to see if that changes things?

IAC is also another thing to check. Sounds more like it's the IAC than anything else.

Front Oxygen Sensor. I had these same sympyoms on my 02 Forester. My buddy on his 01 Outback. When it is in the failing condition, it's telling the ECU you are running too rich and shuts the injectors down. Happens AFTER warmup.

Next time it acts up, open the hood and unplug the front O2 sensor. Yes this will set the Ck engine light, but the engine will now run in 'open loop' mode (preset pattern for fuel/air mixture). This will stop the bucking and you can safely get home. Only way with OBDII to even try to detect it is to map the front O2 sensor while driving in the failing condition.

P.S. his outback was sooo bad it died on the interstate, it was flatbedded to the dealer. 1st time car started and acted normally, dealer blamed bad gas. 2nd time the front sensor was replaced by a technition who saw some unusual responses.

Thanks all for your advice. I changed the air filter two days ago, and this seemed to help a bit. After checking the security of all of the intake ducts to the air filter unit, the noise under the hood is decidedly more of a growl when accelerating. Is that what all of those different ducts do; tune the noise? Anyway, I'm a bit of a neophite when it comes to looking under the hood. What is IAC? How can I locate the front oxygen sensor? Can it be replaced without going to the dealer (one tiny little dealership, one huge conglomerate, neither do I really trust)? Wouldn't the CEL be on if the sensor was bad? Where can I find a good, or at least decent, home maintenance manual? Thanks again.

Check engine light should go on if the O2 sensor goes. Just had the same symptoms in my '00 Outback. Diagnostic showed that the first sensor was bad. Changed it ($79 at dealer for part) and now its running fine.