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Discussion Starter #1 (Edited)
*This is not a thread for posting your own misfire problem!*
Please start your own thread in the General Maintenance & Troubleshooting forum. Please do feel free to post your own resolution to misfire problems though in this thread
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Once or twice a week an individual posts about having a misfire issue. This is a very common problem and there are alot of threads addressing this very problem. So here are the possible issues (read this first,then post about an issue if you have no resolution after trying said solutions):


MISFIRES


1. Wrong, worn or incorrectly gapped spark plugs. Generally NGK work best gapped between .028"-.030".

2. Clogged/dirty/bad injectors or fuel filter. Try changing the fuel filter. On the 04+ WRX's/STI's double check the fuel pump"sock" in the tank. The maintenance schedule for the filter is every 30K miles. Have the injectors professional cleaned or at least run several bottles of a good injector cleaner through the gas tank. Also make sure you are using the correct octane gas! Either 91 or 93 unless you are specifically tuned for another octane!! (IE: race gas or E85) You can swap injectors from another cylinder and see if the misfire follows. If so this would indicate an injector issue if it does move..

3. Check valve clearances & compression. Perform a leak down/compression test first.The FSM has the valve clearance specs. This is a complete PITA with the motor in car.

4. Dirty MAF sensor. This causes an incorrect airflow readings. Misfires can result. Clean with CRC MAF cleaner or NON chlorinated brake cleaner. Make sure you are actually cleaning the MAF sensor and not just the IAT sensor. Common mistake.

5. Vacuum leaks or preturbo leak. Check where the FPR hose connects (generally you could get a P0457 or P0171 with this as well). Check the turbo inlet pipe where it connects to the turbo. This pipe is famous for ripping and causing a huge preturbo leak (P0171 as well).

6.Poor grounds/grounding. Clean the battery ground cable and ones that run to the intake manifold. Make sure all are connected. These are sensor grounds and must be kept clean.

7. Cam timing. Check cam timing per the service manual. Clean the cam sensor and crank sensor face.

8. Excessive PCV blow by. PVC gases will effectively lower the octane content of gasoline/petrol and can cause misfires along with knock. Get a catch can or air/oil separator.

9. Carbon build up in heads/cylinders. You can use Seafoam or top end cleaner as well as other type of carbon remover.

10. Lightweight flywheels or crank pulleys. (phantom misfires) Can cause false misfires. Opensource tuning can disable.

11. Bad/loose coil pack or coil pack connector
. Test coil pack and move to other cylinder to see if DTC changes. If so this would indicate a coil pack issue. Also check the tower boot for corrosion. Use dielectric grease on all connections.

12. Faulty fuel pressure regulator or fuel pump. Check fuel pressure. It should be between 35-38 psi idle @ sea level. Removing the vac line at idle should give you 43 psi. The FPR is a 1:1 rising rate. For every 1 psi manifold positive pressure the fuel pressure should rise 1 psi. You will need a mechanics fuel pressure gauge and have to install a schrader valve in the fuel system before the FPR. Best place is right after the fuel filter on the bug eyes or right before the hard lines under the manifold.

13. Faulty front O2 sensor.
Replace with new. That easy. The OBDII system will probably let you know.


14. Damaged crank timing gear. The one behind the crank pulley.

15. Misfires all cylinders: Coil pack connectors mixed up (correct way:black to back,white to front),light weight flywheel,bad/unplugged crank or cam sensor,incorrect gaped plugs,cam timing off or wrong spark plugs.
Flashing CEL = Gross misfire. A DTC is not always stored. The resolution is generally one of the ones listed above.
FYI-The bugeyes were plagued with misfires. Updating the ROM in the ECU to the latest revision from Subaru helps alleviate most false misfire issues. This can be done through the stealership or opensource software.

Hope this helps! Will add as other causes are found.
 

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Thanks for writing this up Donkey. Misfires are one of the hardest things to track down and they're almost impossible to diagnose over the internet. This gives people a great place to start.
 

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Yesterday I've replaced 4 plugs and 1 coil pack. P0304 came back today. I've did some of the things that Donkey posted, cleaned the MAF sensor, clean the coil pack connectors, replaced the PCV valve and pure in 2nd bottle of injector cleaner. I also replace the air filter and fuel filter as well.

It seem to be improving as I drive home this afternoon. I hope this will fix it, next step will be cleaning the grounds and add more fuel injector cleaner.

Thanks for the posting.
 

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Great post!
10. Lightweight flywheels or crank pulleys. (phantom misfires)
I have heard this, but never really knew the reasons for this occurrence. Care to clarify?
 

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Discussion Starter #6
Great post!

I have heard this, but never really knew the reasons for this occurrence. Care to clarify?
Apparently it has to do with the crank's rotation speed and how the ECU interprets it via the crank sensor.
 

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I think the check valve clearances should be moved to like number #2 or #3 position fore Ive dealt with alot of these misfire codes for years and with my experience if its not a plug/coil pack/ backed out or broken wire somewhere, than it seems to be the valve clearances, we've had more than a couple higher mileage WRX's with tight valves... Of course unless your rating them in terms of intrusiveness... ;) Just my opinion of course... BTW excellent idea for a sticky....
 

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Discussion Starter #8 (Edited)
I think the check valve clearances should be moved to like number #2 or #3 position fore Ive dealt with alot of these misfire codes for years and with my experience if its not a plug/coil pack/ backed out or broken wire somewhere, than it seems to be the valve clearances, we've had more than a couple higher mileage WRX's with tight valves... Of course unless your rating them in terms of intrusiveness... ;) Just my opinion of course... BTW excellent idea for a sticky....
Done! You guys change out the shims/buckets with the motor in car?
 

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Done! You guys change out the shims/buckets with the motor in car?
Yup, Its a tight fit but It can be done.. we have a pretty decent assortment of them, fore personally whenever I have one that needs adjustment I order a couple bucket sizes smaller and a couple larger just in case...extremely tight clearances japan haha
 

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Ok, so I really need help on this I got the P0304 code. So far I've cleaned the MAF, Changed the FPR hose (fixed P0171), cleaned injectors, changed the spark plugs 1000 mi ago, and this afternoon changed two coil packs (rear driver and passengers side)... Basically now I'm stumped, I don't know what to do next, FYI the idle tends to move up and down ( don't know if it is because its cold engine since i sometimes also occurs when the engine is warm.)



Thanks in advance to all that help me :).. .. . .. .
 

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Ok, so I really need help on this I got the P0304 code. So far I've cleaned the MAF, Changed the FPR hose (fixed P0171), cleaned injectors, changed the spark plugs 1000 mi ago, and this afternoon changed two coil packs (rear driver and passengers side)... Basically now I'm stumped, I don't know what to do next, FYI the idle tends to move up and down ( don't know if it is because its cold engine since i sometimes also occurs when the engine is warm.)



Thanks in advance to all that help me :).. .. . .. .
What year WRX? With the idle going up and down, I'd suggest cleaning and re-gasket-ing your IACV if applicable.
 

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My 02 has 197K miles on it. I keep getting the P0304 I found out that it is sticky valve in # 4 cylinder. Only when I'm hard on the gas when the car is not fully warmed up. If the car is warmed up before I drive I don't get the code.

I'm going to keep driving like that. Will be throwing in STI block will rebuild the heads then.
 

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Ooo forgot, I have an 04' WRX. I really have no clue what to do next and I'm not planning on spending 2k on this issue, cause I'm saving that for the possibility of a spun bearings my uncle (mechanic) said he heard, so possible rebuild ( which I have no idea what to get)




THanks guys, either way your all awesome.
 

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Ooo forgot, I have an 04' WRX. I really have no clue what to do next and I'm not planning on spending 2k on this issue, cause I'm saving that for the possibility of a spun bearings my uncle (mechanic) said he heard, so possible rebuild ( which I have no idea what to get)




THanks guys, either way your all awesome.
If you have a spun bearing, you have a spun bearing, you'd be hearing knock and possibly other noises along with deminished oil pressure, they wouldn't be just random come and go things. I wouldn't worry too much about that until it's actually a problem.

What I would recommend is cleaning that IACV, which you do have since you have an '04, and make sure that you use a new gasket when reinstalling. There are how-to's on this board, so do a quick search and it will yield the information you need in order to do it.

Beyond that, I would recommend a compression and leak down test be performed to make sure your engine is still within spec. A blown piston will cause a misfire, so hopefully this isn't the case, but it's good to rule out.
 

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Discussion Starter #15 (Edited)
Ok, so I really need help on this I got the P0304 code. So far I've cleaned the MAF, Changed the FPR hose (fixed P0171), cleaned injectors, changed the spark plugs 1000 mi ago, and this afternoon changed two coil packs (rear driver and passengers side)... Basically now I'm stumped, I don't know what to do next, FYI the idle tends to move up and down ( don't know if it is because its cold engine since i sometimes also occurs when the engine is warm.)



Thanks in advance to all that help me :).. .. . .. .
Unless you have tried everything on the list above then thats what to do next:).

*Check valve clearances & compression. Leakdown test.
*Clean the ground cables that run to the intake manifold. These are sensor grounds and must be kept clean. Clean coil pack grounds (bolt holes).
*Check cam timing
*Carbon build up on motor. Real common on EJ205's. Get some BG Chemtool, GM top end cleaner or other type of carbon remover. GM top end cleaner seems to work really well.
 

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OK, so after a long hard drive to campus and after class I turned the car on it looks like the CEL turned off on its own. I'm guessing that is a good thing, but as I put in the diagnosis computer the code (P0304)was still there except for the CEL.

What does this mean? and does it come off after a few more miles?





Thanks in advance.
 

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grounding turned out to be the issue

I was having problems with my 02 WRX and cyl 3 misfiring. After replacing the spark plugs and running injector cleaner through the system a few times with no positive results, I was about to drop hundreds on new coil packs when I noticed a post about the engine grounding. $5.00 and 1 hour later I had installed a #6 ground wire, which grounded the engine in about 5 additional places. And SNAP!!! the car immediately was running noticeably smoother and I haven't had any other problems with the cylinder misfire. I urge anyone having the Cyl Misfire problem to try this inexpensive, simple modification.
 

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I was having problems with my 02 WRX and cyl 3 misfiring. After replacing the spark plugs and running injector cleaner through the system a few times with no positive results, I was about to drop hundreds on new coil packs when I noticed a post about the engine grounding. $5.00 and 1 hour later I had installed a #6 ground wire, which grounded the engine in about 5 additional places. And SNAP!!! the car immediately was running noticeably smoother and I haven't had any other problems with the cylinder misfire. I urge anyone having the Cyl Misfire problem to try this inexpensive, simple modification.
yeah same here. ... any pics
 

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Just to reiterate. I've worked on lots of cars old and new and weird things happen. Example:

I have a 2004 wrx all stock except for the resonator box was removed. Sounds kinda nice:)

I was driving and the CEL came on. Ran the code P0301 cylinder 1 misfire. I started with the plugs and all of them were fine. I replaced them anyway because it was cheap. Then I switched the coil packs from 1 to 3. Nothing changed. Still pulled the same code. Then I checked the wires. All were fine. Then I ran a fuel injector through 2 tanks of premium gas. Finally my buddy said to pull the MAF plug while it idles and see if it improves. Guess what...

So I cleaned off the MAF and it runs fine. Just because it says P0301 doesn't necessarily mean that's the issue. That's just the first place the computer detected a problem so I had to go "further upstream."

Sometimes I wonder why I even bother lifting up the hood but there's always something new to learn.

Anyway, don't get tunnel vision like I did and if you submit something to the forum, give as much info as possible because it wasn't until I stopped thinking about the plugs (which were totally clean anyway) and considered that only the idle was rough. I never experienced a loss of power. Be thorough and provide details. Cheers
 
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