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Rough idle, P0302 and P0304

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My 2005 Legacy GT has a very rough idle and recently threw both P0302 and P0304.


The car had a compression test and came back clean and even across all cylinders (150 +/- a few). The car also has new spark plugs (OEM), which were gapped properly.


Its also not burning any oil. Its stage 2 and tuned properly. Anyone have an insight into what could be going on? I'm going to try to take it to a shop Wed.


Thanks in advance for the help.


Note: Car ran great for 80 miles with new coils on 2 and 4 and now is back to the same problem. Its back at the shop with the mechanic.

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I bring up the leak down test because I've heard of instances where cars pass a compression test but fail a leak down test.


It's running rough at idle and misfired at anything above 1/4th throttle. At half throttle it just falls on it's face acceleration wise

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All of my rough idle and 030x codes have been either a vacuum line that came loose or bad injectors.

I have 150k on mine and have replaced two of the injectors over the last 8 months.

First showed up as a low rpm rough idle. Mechanic tried everything and finally replaced injector and issue was solved.

Second one totally died at highway speed. Felt like piston three just quit working. Terrible idle.

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Have you replaced the black o-rings between the intake manifold and the TGV's with the new larger orange one's ?



Start there.


Where do you live ? In colder weather the black one's don't seal, will give rough idle and may be stall.


Also, when the intake manifold and TGV's are off the car, tighten all gas line clamps and clean the ground bar on the back corner of the intake manifold.


TGV gaskets and o-rings will run about $50.00

305,600miles 5/2012 ej257 short block, 8/2011 installed VF52 turbo, @20.8psi, 280whp, 300ftlbs. (SOLD).  CHECK your oil, these cars use it.


Engine Build - Click Here

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My mechanic swapped the left and right side oil control valves and now it's running correctly. Xt2005 and others thanks for the suggestions, not trying to blow you guys off, life is just crazy around the holidays and with the phd program.


Anyone have any insight on why the OCVs helped when it didn't throw those codes?

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If the OCV were to be at fault, I would seriously consider getting some new ones if I were you. The act of swapping them, and more than likely cleaning in the process, may have fixed the issue for now, but should be replaced. They are considered a wear item.


As to why they were causing your idle misfire, those with more knowledge than I will need to chime in.


Do you have an AP for your stage 2? Is it a v3? If so, you can set up a live monitor gauge for your OCV rates and verify they are running at the same levels of operation. Most people start seeing potential failures when they don't match.

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I think Amazon has them for about $90 each. There is another thread around here somewhere that has the part numbers.


Be sure to wait for other, more experienced advice before plunging on the buy. I am not knowledgeable enough to give you expert advice on these cars. MrTris and MaxCapacity are among many that can give you better advice.

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  • 1 year later...

can someone tell me what is OCV?


and someone here mentioned some orange O rings? What O rings?


I have P0302 code. I changed plugs (OEM) and swapped coil btw cyl #2 and #4. Still throwing a code. Should i swap injectors first or do compression test?

Car is 2005 lgt with 150K km (not miles). ALL stock. 5MT.

Sry if i'm hijacking this thread, i can open another one if necessary.


Thank you!


p.s. I just want to add that OBDII reader showed that CEL was triggered at 1227rpm. I don't have the other data, i can provide. I noticed that CEL (with P0302 code) would trigger when i start the car. Car still runs fine, at least i haven't noticed any issues. I will try compression test first before swapping injectors.

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OCV refers to the oil control valve, some tyoe of solenoid that control timing for the intake cams. There is one ocv on each side of the engine. The unit of the values you see when you log these is in degrees. They should both be in sync at all times.they can get stuck overtime where you'll notice that they're no longer in sync.


As for your misfire, when does it show up? Cold idle only? Warm idle only? Or both? Or at any rpm range?


The orange intake gaskets are the ones that go between the intake manifold and the tgvs. The old gasket type can cause misfires at cold idle when it is really cold outside. But it usually shows up on all cylinders.


You can swap injectors before you do the compression test. It is free for you.

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