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1989 Subaru XT6, 1989 Subaru RX, 2004 Outback 3.0 SUS
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73 Posts
Discussion Starter · #1 ·
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.
 

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01 VDC, 05 R Sedan, 06 BAJA EJ257
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16,259 Posts
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
 

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2003 limited sedan 2.5
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6 Posts
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.
 

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1989 Subaru XT6, 1989 Subaru RX, 2004 Outback 3.0 SUS
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73 Posts
Discussion Starter · #4 ·
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:(
 

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02 2.5 outback, 96 grand cherokee(5.2zj), 96 legacy gt
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1 Posts
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.
 

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1989 Subaru XT6, 1989 Subaru RX, 2004 Outback 3.0 SUS
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73 Posts
Discussion Starter · #6 ·
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.
 

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1989 Subaru XT6, 1989 Subaru RX, 2004 Outback 3.0 SUS
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73 Posts
Discussion Starter · #7 ·
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?
 

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OBW H6 VDC, Tribeca, XT6
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12,385 Posts
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.
 

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1989 Subaru XT6, 1989 Subaru RX, 2004 Outback 3.0 SUS
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73 Posts
Discussion Starter · #9 ·
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.
 

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01 VDC, 05 R Sedan, 06 BAJA EJ257
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01 Outback LL Bean
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2,454 Posts
The rear O2 does not affect driveability. I would suspect that what you are seeing from the rear 02 sensor is a reaction to the issue you are trying to solve.

I would be looking at fuel pressure and the condition of your spark plugs. H6 fuel pumps have a known issue where the fuel pump will start to come apart and cause fuel pressure issues.

How does it drive if you floor it all the way to redline?
 

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1989 Subaru XT6, 1989 Subaru RX, 2004 Outback 3.0 SUS
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73 Posts
Discussion Starter · #12 ·
It'll run up to redline with no issues, makes me want to yard the engine out and stick it in my '89 XT6:D

I pulled all of the plugs and they looked good. No indications of anything out of the ordinary. Fuel pressure checks out but I have yet to pull the assembly from the tank for inspection.

Had the exhaust off almost a year ago due to a nasty accident the car was involved in so I was able to do a visual inspection on the rear cat with nothing out of the ordinary.
 

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01 Outback LL Bean
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2,454 Posts
If it will pull hard to redline I would not suspect exhaust issues.

Is there anyway for you to monitor fuel pressure while driving. The leaking fuel pump issue may allow you to have sufficient pressure at idle when little fuel is being used but may create an issue while driving. I would pull the pump and check. It is very obvious if you have the problem.

The TPS issue generally causes shifting issues before other significant problems.

Can you create a snap shot from the scanner while the issue is occuring? How do your long and short term fuel trims look?
 

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1989 Subaru XT6, 1989 Subaru RX, 2004 Outback 3.0 SUS
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73 Posts
Discussion Starter · #14 ·
Checked the fuel pressure while sitting idle and under driving with no issue.

If I get a chance, I will get a screen shot of the scanner when it happens. I believe the short term was always at +4.95% but my memory is being a bit fuzzy right now (go Seahawks!):29:

Hopefully have a bit more time this week to get the SSM2 back on it with more info.
 

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2002 Outback Sedan H6 3.0 VDC
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151 Posts
I have my old TPS I believe to still be good. I'm here in Mountlake Terrace, if you want to borrow it for a couple of weeks let me know.

-Tim
 

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01 VDC, 05 R Sedan, 06 BAJA EJ257
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16,259 Posts
Don't worry about the STFT. Look at the long term. If bank 2 is higher than bank 1 to the positive side its more likely a fuel delivery issue. If both banks are about the same positive it could still be a fuel issue or sensor. If they are negative, it's not burning proper or receiving too much fuel.

LTFT is calculated from the front WB Oxygen sensors. When the value is positive over 0 the ECM is adding to the injector pulse to deliver more fuel. Values in the negative, its reducing. Any LTFT over 5% is an issue starting and the higher the value, the worse the problem. STFT is the instant value and a value of 5% is normal if you are on the throttle accelerating or demanding power from the engine. STFT can alter pos/neg 15% and not be an issue. It just depends on what you are requiring of the car. So pos FT means the car is running lean and neg FT rich. 0 is stoichiometric which is the target value.

107k, I'm betting plugs or high alcohol fuel or both.
 

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1989 Subaru XT6, 1989 Subaru RX, 2004 Outback 3.0 SUS
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73 Posts
Discussion Starter · #17 ·
Time for an update and still not as much success as I hoped.

I got new NGK plugs, both front Denso A/F sensors and a few other bits for the car Friday. Saturday I took the car down, plugged in and noticed the LTFT was negative 1.6 on bank 1 and positive 2.8 on bank 2. Figured with 109k miles and counting, might as well do the plugs to ensure proper burning and both the A/F sensors. Plugs look normal, no signs of anything out of the ordinary. No oil loss of any kind, something I'm adamant about keeping my eye on. Still feels like it might have an issue but I'm not totally certain on as it takes it a couple weeks after an ECU reset. Hopefully the wife can tell me if there's an issue with it again soon.

On another note, I installed a TriBeCa front swaybar on it and man what a difference. I did have I drill out the metal insert in the end link on the swaybar side to fit the larger TriBeCa bolt but it seems to work good. The bar came with Energy Suspension poly bushings too but it was still a major difference in the feel of the car. She even told me she felt a major difference in front end grip while cornering.:29:
 
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