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Problem started last September on normally aspirated 2.5 motor, when all of a sudden car idled and ran poorly. CEL came on along with blinking cruise control light, and brake antilock dash light indicator. I thought it may be bad gas, so added 8 gallons of E-85. This fixed the problem entirely within about 50 miles, so thought all is well. The same problem came back just before Christmas. I pulled codes, but only said spark plug miss-fire. I cleared codes, and no codes appeared. So, replaced the knock sensor, and fuel pump and fuel filter unit. Upon start up ran fine for a few seconds, then miss fires started again. Expected a new fuel pump to fix the problem, but not so; but it seems to be a fuel problem, as sometimes it takes additional throttle to start engine. It won't necessarily start by just turning the key with no extra accelerator assistance.

Got me stumped on how to fix with no CEL to give direction. Doesn't seem like spark plug trouble, as motor can run so well,  then instantaneously run so poorly. Maybe a bad computer??

Any advise really appreciated.

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Post exact code(s) or which cylinder(s) misfire 

Misfires = replace spark plugs and wires with OEM or NGK or equivalent only.  You can test them if it’s worth your time.  They’re a consumable and cheap and need replaced regularly.

Check spark plug tubes for oil.  If wet, replace valve cover gaskets and spark plug tube gaskets  

If you think they’re good and it’s worth your time, you can swap plugs and wires and see if the misfire moves. 

List age, brand, and mileage if you think they’re fine.

Edited by idosubaru

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I think it's noteworthy that these cars aren't meant to run on E-85.* Just because it can, and might've coincidentally fixed the misfire, doesn't mean you should do that. 

*on a stock vehicle 

Edited by golucky66
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Agree.  Thats correlation not causation.  

Similarly, the throttle to start the car is common with a poorly running engine and isn’t really indicative of any one system or part. So it doesn’t really tell us much except the engine isn’t running well

Misfire = Oil in plug tubes, wires, plugs.

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I added the E-85 because of high alcohol content. If I had gotten gas with water,  the alcohol would have helped get rid of the water. Have had this work in the past.

Will pull a spark plug to check condition. Bought as a used car,  so don't know condition of the plugs.

Thanks for the good advise.

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19 minutes ago, Rooster2 said:

Any chance a bad coil pack would be the cause of my problem??

Of course. Lots of things “can” cause misfires. I only outlined the starting point, not an exhaustive list.

check for oil in the spark plug tubes first.  

If it’s notably better when dry or worse when wet those are tell tale signs of plug/wire related issues. 

 

 

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The trouble may be within the alternator. To see if this is the case disable the alternator by removing the small wire connector going to the alternator. Start the engine and see if the trouble has cleared. It id does then the alternator may have some bad diodes inside it and too much ripple voltage is occurring.

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My son and I chased a poor running issue on two Subaru's, both ended up being a coil.  I went to the pull and pay yard and got a Subaru OEM one and when I installed them all running issues went away.

Stay away from the aftermarket part store ones, I have never had much luck with them but don't want to spend $$$ for an OEM part from a dealer.  The pull and pay yard worked for me.

All that being said is this a Coil Over Plug engine?

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Subarus require spark plug replacement every 40k miles or so. You might check those.

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Well after replacing the fuel pump, plugs & wires, and ignition coil, my car still ran like crap. I finally took it to a dealer. It turned out to be a bad EGR valve. Apparently, this is somewhat common on 05-09 Legacy/ Outbacks. I hope this info helps someone in the future.

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