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2003 Forester XS Manual
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194 Posts
Discussion Starter #1 (Edited)
2003 Forester with 185,000 miles on it. 5spd.

A month ago it started to bog down at lower rpms when I'd give it gas. I replaced the plugs, they were pretty old. The old girl started running like a champ again.

I found oil in the plug wells, I replaced the valve cover gaskets, bolt grommets and plug grommets two weeks ago.

Now...she's bogging down again. Almost stalling when at idle and the A/C kicks in. It's getting progressively worse though. Now it's stumbling, running really rough and bogging down at low rpms (especially at idle).

I have a ScanGauge II on it. I'm still getting good MPG's out of her (27-30). Seems to have plenty of power and runs fine onces out of the lower rpm range! A few days ago the check engine light came on...the ScanGauge is telling me it is a P0304.

What do I check/replace next?
 

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2013 WRX Manual
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135 Posts
do you have an electric throttle body? if so my friends car was doing the same thing and thats what it turned out to be.
 

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Administrator
2004 Forester XT Premium 4EAT
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29,458 Posts
P0304 is a Cylinder 4 misfire. Might be that your coilpack is going bad.
 

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2003 Forester XS Manual
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194 Posts
Discussion Starter #7
Almost stalled out at a light this morning...bogged down hard core and I could hear it knocking like MAD as it tried to recover, gave it a little more gas and she pulled away from the light just fine. This is maddening.

I tested the coil pack last night, resistance across the secondaries was a little high but within tolerances (just over 14k ohms - supposed to be at 12.8k ohms +- 15%). I could not get a reading on the primaries for some reason, I'm going to try a new battery on my multi-meter tonight.

Anyone know of a way to test the front O2 sensor?

These freaking parts are a little spendy to do the old "multiple guess / replace and see if it works" method.
 

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1998 Forester
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39 Posts
Trust your OBD (wannkanobi)

I had similar issues with my ECU barfing up a 0304 code. I know what a pain in the SSA it is to replace plugs 3 and 4 but.......remove #4, examine it and make sure the gap is correct. I think the spec of .032 is a bit wonky so I tried a .027 and Voila, she runs very nace. ( spoken with a french accent)

Your car is misfiring and runs like crap at idle because of that plug. Of course it will smooth out when the RPMs increase.

A faulty O2 sensor will cause the ECU to barf up a code, so unless you see one don't waste your money.
 

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2003 Forester XS Manual
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194 Posts
Discussion Starter #9
Well...it rain really nice and smooth for about a week after I replaced the plugs, I left them at the factory gap. Why would it all of a sudden change performance?

I put in Premium gas last night, it was almost empty and she ran a bit better.

What is the knock sensor, how does it work and could it have anything to do with this problem?

I also bought a propane torch last night, I saw an article on how to test the O2 sensor so I'll try that today!
 

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1998 Forester
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39 Posts
Knock sensor

Well...it rain really nice and smooth for about a week after I replaced the plugs, I left them at the factory gap. Why would it all of a sudden change performance?

I put in Premium gas last night, it was almost empty and she ran a bit better.

What is the knock sensor, how does it work and could it have anything to do with this problem?

I also bought a propane torch last night, I saw an article on how to test the O2 sensor so I'll try that today!
Are the plugs fouled? Also, if the plug boots are not properly seated in the head, the spark can arc over to the head instead of the plug.

Premium is harder to ignite. The consequence is that it doesn't pre-ignite as easily when compressed and cause a knock, a problem with high compression engines and turbo charging. To achieve maximum power, the spark is advanced so that ignition occurs prior to top dead center on a cylinder occur. The knock sensor is generally used by the ECU to retard the timing. Some engines have two. One is for spark timing and the other is for injector timing.

The knock sensor is a microphone for the engine computer. It is used to detect the pre-ignition of the fuel. You can test the knock sensor with an ohm meter.
 

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2003 Forester XS Manual
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194 Posts
Discussion Starter #11
Plugs were not fouled at all, just worn (and replaced).

I replaced the coil pack yesterday and got the exact same result. Stumbling, missing at idle and hesitation at times when driving at lower rpms.

I pulled the connector for the O2 sensor and started her up and tried to test the sensor. The voltage reading was bouncing around a bit...but it was throwing voltage so does that mean it was working?

Now...my troubleshooting brain is trying to figure this out:

The engine acts the same (missing/stumbling and running rough at idle) regardless of whether or not the front O2 sensor is connected.

So frustrating!

I don't really feel like shelling out $200 for a Subaru specific O2 sensor and have it not end up being the culprit. Anyone here have a huge reason NOT to use a generic O2 sensor such as this one:

Generic Subaru Forester O2 Sensor for $28
 

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Registered
1998 Forester
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39 Posts
What does your code reader say?

Are you still getting the 0304 code? What about other codes? I know the ECU is not infallible but it's the best place to start. Have you checked that plug and re-gapped it? I know it's a PITA to get at but stop chasing your tail.......
 

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Administrator
2007 Forester Sports XT 4EAT
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38,750 Posts
I don't really feel like shelling out $200 for a Subaru specific O2 sensor and have it not end up being the culprit. Anyone here have a huge reason NOT to use a generic O2 sensor such as this one:
The front O2 sensor, or Air/Fuel sensor is critical for correct engine operation. This one part that I replace with a factory/OEM part. I just ordered one from Jackie, SubaruPartsGirl, a supporting vendor here on the forum. Her direct number at Annapolis Subaru is (866) 509-3019. I just ordered on from her for our '03 X, which we're doing a project on, see our Member Journal below. It cost $152. expensive yes, but should be good for many miles. :wink:

Subaru part number: SEN AY-A/F RATIO) - 22641AA280

Other's have used aftermarket O2 sensors & have had good luck... some have not. It's not going to hurt to try one.

Disclaimer: I'm not saying your problem is caused by a defective or failing O2 sensor. I'm no mechanic, so anything I say is just a guess. :smile:

Bobby...

['07 FSXT MODding Journal] ['03 X MODding Journal]
 

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2003 Forester 2.5L
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122 Posts
I'm going to bump this. My engine is doing the same exact thing. When at stop lights I feel it bog down. I recently changed out my entire exhaust system (everything but headers) and replaced the rear O2 Sensor, does it have something to do with that?
 

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1999 L 4AT
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1,183 Posts
The Check-Engine-Light is off, right? Does the odometer read over 100,000 miles?
 
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