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Discussion Starter #1
Hello there.
I have an 04 Outback (4 cyl). It's been good overall except for the all too common headgasket that had to be replaced at 102 miles about 7 months ago.

I'm now at 111K miles and the engine light has been coming on and staying on. once in a while it goes off but mostly on.

As soon as it came on i had the code read and it was P0302, the spark plugs are pretty new, what should I look into ?

I'm wondering if the diagnostic is right, it runs mostly well, except that it "putters" a little but only it a specific condition .... usually when "Coasting" around 40mph (no gas & delerating) and the RPM going down/low (~ 1500) and then pressing the gas lightly. But when pressing the gas a little more(goes down a gear) it goes and sounds fine.

It seem if i put the car on the 3rd gear (automatic shift on "3" position) it doesn't happen at all.

I don't know what a cylinder misfire effect would be but it seems weird to me that it seem to have no effect at all EXCEPT in that particular scenario.

Anyway, I've used the car only for short trip as I'm a bit uncomfortable with this even though it runs mostly fine and I know hoe to avoid that "spot" where it putters a little.

Anyway, if anybody as any advice on this and had similar symptoms I'd love to hear opinions. Thanks.

Hoping for something easy to resolve / DYI as after spending a small fortune on the head gasket and associated work 6 month ago I'm really not looking forward to spend more.
 

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2004 Outback Wagon EJ259 - 4EAT
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121 Posts
You mentioned the spark plugs were "pretty new"...how new? What about the wires? Condition of spark plug boots and tubes?
 

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Discussion Starter #3
The spark plugs where changed (OEM by the dealer) at ~95K, a couple years ago. So yeah I guess it's not that new but reasonable.

Boots and tubes I'm not sure, I'll see if I can dig that up, ... I suppose I could change those, cylinder 2 plug is the one that is a pain to access though right ?
 

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2004 Outback Wagon EJ259 - 4EAT
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Removing both the battery and windshield washer fluid reservoir made it much easier for me to access those plugs (as well as change the valve cover gaskets.)
 

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2001 VDC/SC One of a Kind
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Check the fuel filter. Check fuel pressure. Check the coil.

Cylinder 1 and 3 are the right side of the engine. 2 and 4 are the left. Cylinder 2 is at the end of the fuel rail circuit and has the pressure regulator above it. If there is a fuel delivery issue, cyl 2 will misfire due to lack of fuel as it is last to receive the flow.

The coil generally last a long time, but can fail causing a misfire only on one cylinder. If the voltage output is low, it will idle rough and smooth out with engine speed.

It will also act this way with a fuel issue since the ECM ups the voltage to the pump at higher rpm.

If the above checks out, check the battery, grounds and alternator.
 

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Discussion Starter #6
Thanks for the suggestions, i'll check into that.

One thing that's weird about the idle is that the true idle (car running but parked) it runs perfectly well. It seem the only time I can tell something is wrong(mifire) is when accelerating from a low RPM but while the car is in motion(ie, reaccelerating after decelerating, 20mph+) .... That's the part i find very odd, while would misfire only happen in this scenario ??
 

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2001 VDC/SC One of a Kind
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Fuel delivery. Fuel demand is low in idle and cruising because the engine demand is down. It needs more fuel to accelerate and when the fuel is not there, misfire.
 

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Discussion Starter #9
First thing I was going to check was the fuel filter ... unfortunately I have a late 04 where the filter is in the tank.

I read somewhere that you can access it via a plate behind the back seats, can anybody confirm that ?
 

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OBW H6 VDC, Tribeca, XT6
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you should pull the offending spark plug and make sure there's not oil in the tube or on the plug/wires. go ahead and check the plug while you're at it. the valve covers have spark plug tube gaskets that can leak into the spark plug tubes.

First thing I was going to check was the fuel filter ... unfortunately I have a late 04 where the filter is in the tank.

I read somewhere that you can access it via a plate behind the back seats, can anybody confirm that ?
Confirmed.

2003's have fuel filter in engine bay, starting in 2004 they migrated to being integrated into the fuel pump. they are accessed via a metal plate under the rear bottom seat cushion. im' sure you can google it, it's really easy. like two 12mm bolts for the seat bottom, it pulls out, then remove the bolts/screws holding the cap in place, remove fuel lines.
 

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2017 Outback 3.6 Touring, which replaced '05 Outback XT
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Wouldn't a plugged fuel filter affect more than one cylinder? Bad fuel injector.
 

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2001 VDC/SC One of a Kind
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Wouldn't a plugged fuel filter affect more than one cylinder? Bad fuel injector.
Not necessarily. It depends on the restriction level. I note a restricted fuel filter in the following video. The fuel trim was higher on the end of the fuel circuit after the car ran for a while. It was due to the fuel filter condition, slowing the flow and therefore delivery by the injectors.

2001 Subaru Outback LLBean P0420 Diag - YouTube
 

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Discussion Starter #13
OK, fuel filter didn't seem to help.
Anyway I got a proper code reader since and realize now autozone had only gave me one of the codes:

The error codes I get are (P302, P0032 and P0030) ... I'm thinking maybe my issue is O2 sensor as that seem to be a fairly common issue, does that make sense ? Can this theory or the sensor be tested without replacing it ?

-- Full diagnostics ----
**i/m readings**
Those all show "OK"

** Eror codes:
1: P0302 : Cyl2 misfire detected
2: P0032 : Heated O2 sensor, heater ctrl circuit HIGH bank 1 sensor 1
3: P0030 : Heated O2 sensor, heater ctrl circuit bank1 sensor 1

** Error freeze data for P0302:
abslt TPS % : 50.6
RPM : 2938
calc load: 44.7
MAR: 6.91
MAP: 28.1
coolant temp : 187F
IAT : 88
IGN AD : 15.0
ST FTRM1 : 16.4
LT FTRM1 : 0
Speed : 14mph
Fuel system 1: closed
Fuel system2 : N/A

-------------
 

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Discussion Starter #14
I juts did a small run with the front O2 sensor unplugged and it seem to run much better.
it was a very short test though, but usually i was able to "cause" the misfire purposedly fairly easily.
I'll try going up a hill later as that use to make it happen easily before.

If it runs better without the sensor plugged in I assume it makes it pretty likely it's the culprit right ?
 

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Discussion Starter #15
The author of that video say the O2 sensor should have a resistance between 6 and 20 ohms ... can somebody confirm that ?

I get full resistance (open circuit) on mine, so at that point pretty safe bet it's shot right ... anybody can confirm this ?

 

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2001 VDC/SC One of a Kind
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Replace the sensor. Use an OE sensor, no universals. Also, the ECM is adding fuel on STFT causing a rich condition, which at this level could cause the misfire. 16% is a lot of fuel. Run a Sea Foam treatment through to clean out the excess carbon.
 

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Discussion Starter #17
Will do ... actually had signs that it was probably running rich before (sometimes smell at start-up. millage not as great and so on) so that would make sense.

Wish it had been the back sensor, easy access. But seems like the front one should be ok too from under with ramps.
 

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Checking on this thread.....was the 02 sensor the culprit? I just took my car to the dealer today for a diagnostic...same issues....really low idle at the traffic light. I would need to put in neutral...or, if I had the A/C on it would rev up just enough.

So, the dealer basically says the same thing and recommends new spark plug cables and plugs. I buy the cables there and even though I probably put new plugs in about 6 months ago I purchased new bosch plugs from Autozone and he said they were gapped correctly.

NOW, it run worse and it really seems sluggy and a fuel issue. I did replace the original fuel filter about a year ago.

Autozone got the codes P0302 and P0303.

OEM O2 sensor means the dealer or online?
 

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2001 VDC/SC One of a Kind
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I wouldn't use Bosch plugs; they tend to be a problem. Denso or NGK are best. Same with the AF sensor, Denso.

You can pick up a Denso AF sensor at the parts stores.

Fuel filter should be replaced every 15-20k miles.

Check your battery output and grounds. Clean any corrosion and make sure the bungs on the intake, firewall and fenders are tight.
 

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I wouldn't use Bosch plugs; they tend to be a problem. Denso or NGK are best. Same with the AF sensor, Denso.

You can pick up a Denso AF sensor at the parts stores.

Fuel filter should be replaced every 15-20k miles.

Check your battery output and grounds. Clean any corrosion and make sure the bungs on the intake, firewall and fenders are tight.
As always, thanks cardoc for checking in...so, this is what I ended up doing...went to a different autoparts store and got some NGK plugs and the guy gapped them for me. Went home and started from scratch again. Put the new plugs in and made sure everything was tightened and looking good. Fire up the car and still the same problems. Trying to deduct what was different it was the Subaru plug wires I purchased from the dealer. So, I put 3 of the 4 old wires back in place. In hindsight I realize problem what the issue was....looking at the engine, the front left plug wire cap at the plug side the connector has separated from the wire inside. I bet that had happened before and that was my issue all along. So, not sure why the replacement wire worked but just using the old other three now my car runs tip top. Digging around to learn about potentially replacing the 02 sensor I saw alot of people recommending the NGK spark plug wires also. Funny how the OEM wires through me off.
 
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