02 outback chug or misfire at mid-throttle

OK, sounds like you have done some reading or have had experience with soobs in the past.

While you're under the hood, wouldn't hurt to at least clean, maybe replace the PCV valve. Inspect the battery cables, maybe refresh.tighten any ground connections you find, inspect vacuum/vapor hoses, hose out the bugs from the radiator fins, inspect the accessory belts, inspect all fluids, replace any that are darker than normal, smelly or suspicious. make sure the tabs are seated at the bottom of the air filter 'box'. If you mess with the coolant, read up on 'burping' the system when refilling. new thermostat should be OEM or 'maybe' Stant Xacstat. best to test them in a pot on the stove before install. they should open at 170*. New radiator cap might be a good idea. Inspect the axle boots - particularly the inner ones, for splits/leaking grease.

You might look for a cabin air filter. Behind the glove box. Could be dirty if equipped. (cars101.com will have instructions)

brake fluid is hygroscopic - might need flushing/bleeding if old.

make sure all 4 tires are the same model and same amount of wear. Many folks like to inflate them 10% or so above the recommended pressure.

I bet some other folks will have some tips for you. good luck with your new car!

Well, here's what I've done so far....

Picked up OBDII reader (Canadian Tire had'em on sale) and scanned. No codes, none pending.

Checked what I have for service records, and the plugs and wires were done about 60,000m ago. Found a service record from '09 with similar stated symptoms with CEL, and front O2 was replaced. Timing belt done about 50,000m ago. All significant maint. (beyond tires and oil) done by local Subi dealer, it seems. I don't have complete records, but maybe they do.... I'll look into it.

Pulled plug boots, no oil on any, no signs of tracking, white silicone grease visible. Pulled coil ends of wires, no corrosion, silicone grease visible.

Haven't spray tested yet, not dark yet and wifey just took the car for a girls thing. I'll update again when I try next...

Unless spray test turns something up, I guess I wait for it to throw a code? Misfire is very intermittent, often only one incident (hill comes, ease in gas, chugga, more gas, off we go!) in 15mi drive.

I'll add some seafoam once I've burnt the tank down some.... She's full right now.

Appreciate the suggestions so far!

J

Another non-Subaru forum suggested some possibility of xmission difficulties masquarading as engine misfire/stumbles.... Is this likely? How might I rule this out?

I'll resummarize here, and try to be as clear as possible:

2002 4 cyl auto.

Generally occurs during a gentle to moderate hill climb when a little throttle is eased in for climb, at somewhere near 30-35mph.

I drive along, reasonably level street, and a hill is approached. RPM's are at about 1500 on approach, doing 30-35mph. I feed in gas to maintain speed up the hill. TC unlocks, then tranny shifts, 2500rpm give or take. Hesitation is felt, usually at about 2500rpm, and occasionally the tach can be seen to twitch slightly a time or two (dropping 200rpm or so), or engine may just seem to bog/fail to deliver power. As more throttle is fed in, the engine seems to recover, the tranny shifts again, now in the 3K range, and off we go. Around 3200, the engine seems to come "on cam".

Haven't been able to reproduce it reliably otherwise. No miss noted in neutral. Seems to require some load.

I'm loosing sleep.

J

Well, the problem's finally been identified and corrected, so I figured I'd post the solution here so others may benefit.

I disconnected the battery for the weekend 2 weeks back to see if that had any effect on the symptoms, and lo and behold, it did. The car's performance improved for a time, slowly deteriorating again, thus pointing firmly at an EFI related problem. Disconnecting the battery basically set the computer to defaults, and then it slowly "learns" based on sensor inputs.

Armed with this info, I brought the car in to the local Subaru dealership, and told'em have at it. They insisted on dropping the manifolds to check the exhaust valve guides (their leading suspect, as they were resistant to the idea that there could be a sensor problem without a light or code). All was found mechanically fine.

Hooking the car up to their diagnostics computer, they eventually determined that the front O2 sensor was giving erroneous data, leading the computer to tweak the mixtures lean. Simply replacing the O2 sensor has got the car performing normally, much to my relief.

Slightly annoyed with Subaru's insistance on checking for a problem I ruled out (the exhaust valve guides), but that is tempered by knowing that the engine is mechanically in decent health.

Ultimately, the ordeal set me back about $500. So, all told, I've got about 3500 into this car (including purchase), so with luck I've got a decent thing going now.

It is nice when power output corresponds with throttle position.

Thanks for your input, fellows.

J