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Persistent P303 Misfire

2004 Legacy EJ251 P303 Misfire

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8 replies to this topic

#1 GoatFei1337

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Posted 15 September 2017 - 10:29 AM

Hi, I am driving a 2004 Legacy with the EJ251. I've been having trouble diagnosing the P303 3rd cyl misfire.

 

I tried changing the wires, plugs, and ignition coil. It went away for four days and then it came back.

 

I've noticed that it would sometimes turn off during cooler days and it would stay off for like two days before coming back on again. On hot days it would take longer to start and it would come back on.

 

I've also noticed that when the fuel tank is near empty it turns off and then comes back on when I fill up.

 

My only conclusion now is possibly the fuel injector. I am planning on replacing it and see what happens.

 

Any advice and information is greatly appreciated.

 

Thank You

Goat

 

 



#2 1 Lucky Texan

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Posted 15 September 2017 - 11:14 AM

was there any oil on the plug wire boot?



#3 Downbound

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Posted 16 September 2017 - 10:22 AM

Had the same code with my 99 forester. Finally found out it was a burnt exhaust valve. Simple test to check is to hold a dollar bill over the exhaust when it is running and if the bill gets sucked in then blown out it is the exhaust valve.  



#4 GoatFei1337

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Posted 16 September 2017 - 12:47 PM

1 Lucky Texan. There was some dirt/residue on the wires, when they were removed to get the head gaskets and valve cover gaskets replaced not long ago almost immediately after I got the car. 

 

Downbound how big of a job is it to replace the valve? If it is the same problem?

 

Thanks for the replies.



#5 Bushwick

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Posted 16 September 2017 - 08:43 PM

If it's a valve, you'd have to pull the head. Since the heads on these are like baby inline 4 heads, it's pretty straightforward to remove a head. However, before doing that, connect a vacuum gauge to the engine and see what's it's doing or if points at a valve. Can try reading this: https://www.motor.co...a-vacuum-gauge/  or google "engine vacuum testing". Lots of sites have tips on how to read. Gauges cost $15 roughly. You can T into a proper vacuum source, and rest the gauge on the windshield (slip under hood gap with hood up) and watch from in the car. 

 

FWIW, I bought a 99' Continental with running issues (it barely ran) @84k miles. Guy I got it from was in over his head with it and bought it at auction. It had one CEL code (forget what atm) and the guy kept saying something was wrong with cylinder 3. After a quick inspection, I found the wires going to the coils (V8 engine, so entire side i.e. 4 cyl. had a lump of wires going to coils) had been chewed on; the rodent was thorough, as every single wire in the bunch had a 3/4" section where the rubber was completely stripped from EACH wire! I cut each wire and soldered back and had heat shrink on each. Car ran better, but still had a misfire (it wasn't throwing a code though). This time, I removed each coil out of circuit and took my ohm meter and probed the center (output going to spark plug) and then carefully probed a connector pin on the coil. I checked both pins separately while leaving other probe on center output. 7 of the coils had virtually identical readings. The last one was 2 ohms too low. Got a new coil and have a great running car I got for nearly nothing.

 

Anyhow, the point of that anecdote was there was code, but the underlying issue was the wires had been tampered with (stupid rodent, found it's nest under the intake upper) and many were touching each other, and even after fixing that, it took checking each coil to verify a secondary fault was there as well. I'd check each coil. If they all check out with identical readings (mind you, my good coils read 4.5 ohms, and the bad one read 2.4 ohms, so that's enough of a difference) , try swapping coils around, reset the code, and see if it appears on a different cylinder i.e. P0302 (cylinder 2), etc.

 

Trace the wiring to that coil back as far as you can (if it's wrapped or under plastic sleeves, probably OK, anywhere the actual wires might be exposed i.e. you see the color of the wire, is where you wanna see if there's copper showing. Also look closely for burns or melted sections.



#6 06SubLegTx

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Posted 17 September 2017 - 04:53 AM

Hi, I am driving a 2004 Legacy with the EJ251. I've been having trouble diagnosing the P303 3rd cyl misfire.

 

I tried changing the wires, plugs, and ignition coil. It went away for four days and then it came back.

 

I've noticed that it would sometimes turn off during cooler days and it would stay off for like two days before coming back on again. On hot days it would take longer to start and it would come back on.

 

I've also noticed that when the fuel tank is near empty it turns off and then comes back on when I fill up.

 

My only conclusion now is possibly the fuel injector. I am planning on replacing it and see what happens.

 

Any advice and information is greatly appreciated.

 

Thank You

Goat

Are these subaru parts or aftermarket ones?



#7 GoatFei1337

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Posted 19 September 2017 - 11:07 AM

06SubLegTx, they are aftermarket ones...



#8 montana tom

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Posted 19 September 2017 - 11:26 AM

Might check your valve adjustment,  in 04 I believe it is .16 intake and .21 exhaust 



#9 GoatFei1337

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Posted 28 September 2017 - 07:27 PM

Hey guys thanks for all the replies. Sorry I've been busy. But I've noticed that whenever I turn the air conditioning on the tach will fluctuate every few minutes or so and the CEL will blink once. Then sometimes it will shudder (misfire?) once or twice every few minutes at idle. This happens only when I turn on the a/c. Also if it the day is cold like under 60 degrees then the CEL will shut off and the car will turn on really fast, but if it is above 70 degrees and warm/hot then the CEL comes back on and it will take several seconds for it to start.

 

Thanks

 

Goat


Edited by GoatFei1337, 28 September 2017 - 07:28 PM.






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