3.0 studdering at 2000-3000rpm

[Caboobaroo]

My '04 3.0 has started studdering within the 2-3k rpm range in every gear. It has done it before but I found a stored code for the IAT and since there was no light on, I tested and replaced with a generic. Now the issue has come back after a a couple thousand miles and being far from home with no tools or equipment, just thought I might see if anyone else has this issue. Car has 107k miles with no other known problems.

[cardoc]

When's the last time you changed the fuel filter? Only need a Phillips screwdriver for that.

Also, check the battery and alternator. Ground cable connections/condition also.

Specific rpm issues are generally fuel delivery or power/ground problems. You are also due for a thermostat and flush and a fresh set of plugs. Don't skimp on the plugs. Get the NGK designed for the car. Heat range 6. PLFR6A-11

[03outback]

Had the same issue with my 2.5... ended up being a misfire that the computer wasn't catching... once I got new spark plugs and wires it stopped.

[Caboobaroo]

I have yet to do the plugs since I bought the car (95k, now has 107k) so it is currently on the list but I do know they are OEM correct NGK as I've had one out. No external fuel filter as its a 2004 model, only one is in the tank.

All battery connections and grounds check out. When I replaced the IAT, I cleared the computer memory with the SSM2 and the problem was gone for a couple thousand miles. I'm thinking it might be a TPS issue because it feels throttle related. Cruising at 65, roughly 2250rpm, it will do it occasionally when it's flat but once the it gets more load on it, it will do it more frequently but only at part throttle. I can also power out of it and it goes away by 3000rpm.

I think I'm going to put the SSM3 on it this weekend and go for a drive. Hopefully I can duplicate the issue and have it record what's going on. Glad I get to work on the driver when I planned to work on the toy this weekend

[Sriggle]

Sounds like yours is doing exactly what mine did a few months ago. It started out as an occasional stutter, let off the gas and it would go away. Over the course of about a month or so it slowly got worse and worse. It got to the point where as soon as I hit about 2k it would come back, did not throw any codes the entire time. Took a shot at replacing the front o2 sensor and that was the problem, hasn't done it since.

[Caboobaroo]

Finally had some time to snag the SSM2 from work and go for a drive.

After driving for a bit, seems like the rear O2 sensor heater circuit is having issues. It's not switching as fast as it should be and every time its slow to switch, it hiccups especially under light load.

Ordered a Denso which will be in my hands Monday morning.

[Caboobaroo]

Wait, so after some time to think about this, is the heater circuit supposed to be oscillating between ~3.8v and .02v on the rear O2 irregularly besides throttle input?

[grossgary]

i don't know O2 sensors well. what you describe sounds odd but i wouldn't know.

Most common issue on that generation H6, besides the serpentine bearings, is the TPS.

[Caboobaroo]

Originally Posted by grossgary

i don't know O2 sensors well. what you describe sounds odd but i wouldn't know.

Most common issue on that generation H6, besides the serpentine bearings, is the TPS.

This is true so I checked it out as well. Ran the TPS through a sweep while at idle, while driving, and Key On Engine Off and found no issues. It has me stumped except something about the rear O2 doesn't seem right.

[cardoc]

I have a video of my car posted here: P0420 Diag

Another H6 here: 2001 Subaru Outback LLBean P0420 Diag - YouTube

If you are looking at the rear sensor and its spiking over 1 volt, you either have a bad sensor or too much fuel. You can rule out the fuel if the front WB are sending the appropriate data.

Another possibility is a failed or clogged CAT.