Subaru WRX Forum banner

IF YOU HAVE A MISFIRE CODE ,PLEASE READ THIS!!!

881K views 99 replies 48 participants last post by  Brenda 
#1 · (Edited by Moderator)
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.

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.
 
See less See more
  • Like
Reactions: TheJ
#2 ·
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.
 
#3 ·
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.
 
#7 ·
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....
 
#10 ·
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 :).. .. . .. .
 
#12 ·
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.
 
#13 ·
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.
 
#14 ·
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.
 
#16 ·
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.
 
#17 ·
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.
 
#19 ·
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
 
#20 ·
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
 
#31 ·
Have 03 WRX - been dealing with P0303 & P0301 CEL




I'm sure this is a noob question to ask but what is the MAF plug? I also have an after market flywheel with a K&N air filter, Cat Back HKS exhaust. Any help is appreciated since there are so many different approaches on this forum, Thanks!
 
#21 · (Edited)
My 02 Wagon is wigging out. It's got the cobb access port, turbo back exhaust, sti up-pipe. Bought it from a good home with all proper maintenance records. It ran great for a month after I bought it. I recently put new headlight bulbs in it. Had to disconnect the battery and the stock intake tube that opens above the radiator. Afew days after it started to misfire. Running terribly rich with misfire codes/CEL. Replaced the o2 sensor. CEL went away. Ran great on the test drive but started misfiring again on the way back to the shop. Checked records, the fuel injection hose was replaced in 07 (because of the fuel smell in the cold), as well as spark plugs and fuel filter. In 08 the MAF was cleaned. Is it due for a cleaning again already?? I'm going to check all of my ground wires and make sure the intake piece is sealed properly. Though I thought I put everything back in place after the bulb switch. I'm going to run down the checklist on this thread, but does anybody have any thoughts? I'm scared.

edit- it's not reading any codes but there's white smoke coming from engine compartment??
 
#22 ·
I had a 2006 WRX TR that I was getting misfires on all four cylinders. The dealer said it was because I installed a K&N air filter and it snapped the tabs holding the air box together causing a vacuum leak. Evidently the K&N's are too tight a fit for the airbox. Just thought it's another thing you could check.
 
#25 ·
My 02 ran great but then my CEL came on and registered 2 misfires. It still ran just fine but I thought I would change the plugs anyway. After changing all the plugs, I took it for a test drive. As soon as I gave it full throttle, the CEL came back on and the car would start jerking as if I was repeatedly pressing and depressing the throttle. I didn't use the plugs you stated before, but could that affect the power output that much?
 
#30 ·
OK my stock 04' WRX 145k miles has a P0304, and after buying the spark plugs, coil packs,cleaning MAF, and one bottle of injector cleaner still code. So a month later(today) I did a compression test......

Engine was Cold, results were: 150,150, 151, 149

Is this a good thing?? and if so what should I do next by the injectors or 02 sensor.
 
#100 ·
OK my stock 04' WRX 145k miles has a P0304, and after buying the spark plugs, coil packs,cleaning MAF, and one bottle of injector cleaner still code. So a month later(today) I did a compression test......

Engine was Cold, results were: 150,150, 151, 149

Is this a good thing?? and if so what should I do next by the injectors or 02 sensor.
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.
Can you post pictures of your new grounding scheme?
 
#32 ·
On the intake there is a small electric device with a harness clipped to it. Is is about the size of a match box and located on the top of the intake after the filter section, and is black. This is the mass airflow sensor. Periodically it can accumulate dirt (especially if the filtration is sub-par -- read: aftermarket) and generate a PO171 "lean mixture O2 sensor" code or more rarely in my experience a misfire.
 
#34 ·
Hi,

New here. Just brought my first Subaru as im mainly a Rotary or nissan guy. Anyway I have just brought a 1999 jap import wrx wagon which has a misfire. I have changed fuel filter which has helped a wee bit but my main question is how do i access the fault codes? Do i need a diagnosis tool or something? Also how do i read the fault code?
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top