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I have a 2004 Subaru WRX and a few weeks ago my check engine light came on and when I checked there were five codes, P0301, P0302, P0303, P0304, and P-0171. When I'm driving everything seems to run okay, but when I stop and let it idle it idles very rough and almost sounds like the engine is going to die, but it never has. I've read a few of the threads on here about things that can resolve the problem but so far nothing I've done has helped.

Some things that I've done so far include running several bottles of fuel injector cleaner through the gas tank, replaced the mass air flow sensor, replaced the fuel filter, and replaced all four spark plugs. When I had the spark plugs out I performed a compression test on all four cylinders and they average out to around 50 PSI which seems really low to me. I've been reading on one of the threads about misfires and it seems like the next step I should take is to replace the Cam Sensor? I've already verified that the coil packs aren't mixed up, that I have the right spark plugs and that they are gaped correctly.

Does anyone have any suggestions of what step I should take next or what the issue may be?
 

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O2 sensor! I had a misfire on cylinder 3 its a quick 70$ piece that could lead to a misfire if it detects too much oxygen in the exhaust, although you should have a code for that too your call, it fixed my misfire

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Discussion Starter #4
So I tried replacing the O2 sensor and it eliminated the P0171 code, but I'm still stuck with misfires on all four cylinders. Any ideas of what could be causing misfires in all four cylinders?
 

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So I tried replacing the O2 sensor and it eliminated the P0171 code, but I'm still stuck with misfires on all four cylinders. Any ideas of what could be causing misfires in all four cylinders?
Just to double check spark plugs gapped at 0.30, it would be weird if all 4 coil packs went out at the same time but maybe replace one see if that helps? Im glad the o2 sensor helped with the one code


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If the cam sensors not too much maybe you should do that too of not maybe take it to a shop unless someone with more experience than me has another reccomendation

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Thanks for the suggestion. I recently replaced the timing belt and replaced the crank shaft sensor and cleaned the cam sensor. It was time to replace the belt, but it didn't make the problem go away either. :( I may have to take it to a shop if I can't find the problem soon.
 

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Discussion Starter #9
Thanks. I seem to be running out of simple things to check and replace. I may take a look at the vacuum hoses next to see if there's any obvious problems there.
 

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I would hate to assume that you checked this and it was actually a problem but all four cylinders could it possibly be the wiring harness that plugs into the coil packs?

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Wait, you have 50psi per cylinder? Isn't it supposed to be closer to 150?

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Yeah, it should be up higher than that. I'm wondering if they didn't open the throttle plate or something.

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Discussion Starter #15
When we did the compression test we removed all for spark plugs and then connected the gauge. I opened the throttle and then cranked the engine for a few seconds. All four cylinders measured anywhere from 48-52 PSI.
 

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I may be mistaken but the psi should be compression ratio x 14.7

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I have a 2004 Subaru WRX and a few weeks ago my check engine light came on and when I checked there were five codes, P0301, P0302, P0303, P0304, and P-0171. When I'm driving everything seems to run okay, but when I stop and let it idle it idles very rough and almost sounds like the engine is going to die, but it never has. I've read a few of the threads on here about things that can resolve the problem but so far nothing I've done has helped.

Some things that I've done so far include running several bottles of fuel injector cleaner through the gas tank, replaced the mass air flow sensor, replaced the fuel filter, and replaced all four spark plugs. When I had the spark plugs out I performed a compression test on all four cylinders and they average out to around 50 PSI which seems really low to me. I've been reading on one of the threads about misfires and it seems like the next step I should take is to replace the Cam Sensor? I've already verified that the coil packs aren't mixed up, that I have the right spark plugs and that they are gaped correctly.

Does anyone have any suggestions of what step I should take next or what the issue may be?
Did you ever fix the issue? Mines got the same thing and I’ve done the same stuff you have, please help!
 

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I may be mistaken but the psi should be compression ratio x 14.7

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Old thread, don't care -- this is true for the dynamic compression ratio (not the same thing as the static compression ratio specified). Dynamic compression ratio is dependent on cylinder filling -- intake runner design and velocity, valve timing, static compression ratio.

PSI = Ambient pressure * Dynamic compression ratio
 
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