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P0303 - Cylinder Misfire Help!?


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Hi All,

 

I've been having some issues diagnosing a Cylinder 3 misfire issue for a while now and I am hoping somebody can point me in the right direction.

 

The Car:

2005 Subaru Legacy GT Limited, stock.

140,000 miles

Original drive train

 

Here are the symptoms:

CEL CODE P0303

Rough Idle - It seems to happen fairly randomly, not specifically when cold or warm. The misfire usually triggers the CEL at idle but I've also

had it trigger in low (<3k) RPM.

Stutter during acceleration. This is hard to quantify but it feels like there is some stutter between taking off and 3500 RPM during medium acceleration.

There also seems to be a significant flat spot arount 2200 RPM

 

Here's what I've (read:local subaru specialty shop) done so far:

New spark plugs

Compression test: All cylinders between 125-130psi.

Swap Coil Pack between cylinder 3 and 1.

Injector Resistance all within spec. - After this test the car ran very smoothly for a few days before triggering the code again.

The shop thinks it may have been a ghost electrical issue or poor connection somewhere.

Today they mentioned that the (something) tumbler wasn't working and this could contribute to the rough idle.

 

Any help would be most appreciated. Thanks in advance!

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I would put money (no pun intended) on a bad injector.

I've had two fail in the last 6 months.

The first was just like yours. Ended up the injector wasn't functioning correctly.

The second was last week and failed completely. It was a much worse rough idle than the first and I actually thought I had had a piston failure.

Turns out the injector had just totally quit.

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  • 3 months later...

I'm just now getting back around to this issue. I took my car to the Subaru dealership this weekend and let them diagnose the problem. They told me that compression in cylinders 1 & 3 are the same (they didn't give me pressures). They swapped coil packs and plugs on cylinders 1&3 and the misfire stayed in cylinder 3. They are assuming that the problem is a burnt valve and wanted $5-6k to do all the work.

 

I am wondering how compression could be good in a cylinder if you had a burnt valve? Can someone explain this?

 

Yesterday, I decided to swap the injectors between 1&3. The process was a fairly simple process, so I'm wondering why none of the 3 Subaru shops that I've been to had decided to test this. (I did have a hard time getting the injector from cyl. 1 into cyl. 3, but it may be because I didn't replace the o-rings on the injectors.) I've driven over 50 miles and idled the car for up to 10 minutes at a time in between stops and I've yet to see the CEL come back on. Also, during idle, you can feel that the motor is running very smoothly compared to before I swapped the injectors. Again, if the problem were a burnt valve, why would it take so long for the CEL to come up and why would the motor be running so much more smoothly?

 

I'll post again when the CEL trips.

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Compression / leakdown test would tell them what's going.

 

Surprised they just ran a compression test....

 

My car ran smoothly and basically idled fine. The CEL only came on in 30 degree weather. But, my compression and leakdown test numbers were 75psi and 80% leakdown.

 

usually shops can diagnose with a boroscope and don't have to "assume the problem."

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  • 2 months later...
I'm back at it again this weekend. The CEL hasn't come up since I swapped the two injectors. And the car runs and idles more smoothly than before. It's still not perfect though. The Subaru Dealership wanted $450 to do a leak down test on my turbo engine ($300 for non-turbo). Any idea why the test on the GT is so much more expensive? So instead I bought a leak down tester on Amazon and plan to do the test myself tomorrow.
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I had I similar issue, I was getting a P0301

 

had compression test done, new spark plugs swapped coil packs.

 

Last resort took it to a shop to just swap injector 1 and 3. Best 40 bucks I paid.

 

CEL hasnt come back and idle has been solid for almost a year now.

 

 

I chalked this up to either bad / dirty fuel, or a weak injector.

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Another reason I'm planning the leak down test is because I am burning about a quart of oil every 1,000 miles. The only leak is around the passenger side valve cover, but it's not responsible for a quart every 1,000 miles!
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Yeah, a quart every 1k is borderline in my opinion. My car has 133k and burns barely 1/4 of a quart every 4k. But the dealer will tell you a quart is still ok...

 

Man, 450$ for a leakdown test? What the heck? Glad I bought my tester and a compressor..

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In for #ynansb.

 

Oil consumption, misfire -- it's not looking good.

 

OP, we swap injectors and coilpacks, so as to rule them out when diagnosing misfires. If the problem moves with the injector or coilpack, then, that particular piece is the culprit. If the misfire stays, well then, you might have a bigger problem, hence, #ynansb.

 

Although, misfires in 3 are usually indicative of a faulty valve, as opposed to misfires in 4, which are indicative of a faulty whole-shebang. :lol:

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My car ran smoothly and basically idled fine. The CEL only came on in 30 degree weather. But, my compression and leakdown test numbers were 75psi and 80% leakdown.

 

Very surprising that your car ran smoothly with a cylinder leaking 80%! That's a lot.

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In for #ynansb.

 

Oil consumption, misfire -- it's not looking good.

 

OP, we swap injectors and coilpacks, so as to rule them out when diagnosing misfires. If the problem moves with the injector or coilpack, then, that particular piece is the culprit. If the misfire stays, well then, you might have a bigger problem, hence, #ynansb.

 

Although, misfires in 3 are usually indicative of a faulty valve, as opposed to misfires in 4, which are indicative of a faulty whole-shebang. :lol:

 

Earlier on we swapped coilpacks and spark plugs to rule them out. And like I said the CEL hasn't come back on since I swapped injectors 1 & 3.

 

And yeah... $450 for a dealership leak down test wasn't sounding good, but $65 for a tester on Amazon shipped next day is right up my alley!

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From my limited experience,

tighten your hose in your spark plug hole,

put your finger on it as you slowly turn the crank with a ratchet. It has to be set at TDC on the compression stroke. So before it reaches TDC, you will feel air coming out of the hose. Keep going until no more air is getting pushed. Then you should be at TDC. If you pass TDC, it should suck the air out of the tube.

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Yesterday I watched this you tube video from "Eric the Car Guy" doing a compression test and leak down test on a truck. I assumed it was a very similar procedure. While this guy is a little quirky, he still has some good info in the video. He used a screwdriver inside the spark plug hole to determine when the cylinder was at TDC. He used a bit of paper in the hole to determine if the cylinder was on the compression stroke. The theory being that the paper will be moved or blown out of the hole during the compression stroke.

 

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