2010 Subaru Forester P0304 code, 139k miles, unknown veh. history

I have a 2010 Subaru Forester. 139k miles. 2.5 engine non turbo. CA emissions vehicle info in engine sticker data under hood

Friend bought a Subaru back in Feb. He uses it for work locally. He took a trip to Minneapolis 500 miles round trip and a 1500 mile round trip to Indianapolis and back without issue. Apps 30mpg to Indianapolis and 22 coming back to South Dakota in headwind If you weeks ago he had a P0304 number four cylinder misfire code. I scanned it, save the data, clear the code, Took it for a test drive and the misfire did not return.


I did check out the wires, inspected them to make sure I didn’t see any shorts, went to pull them loose from the spark plug and found oil in the number four cylinder at the top of the spark plug tube.

I generally haven’t been a person for Subaru vehicles as I know they have their following. This guy previously had a Dodge so that’s why I’ve had to change the brand and start to learn something new which I’m fine with. My experience tells me that a wire or cylinder generally isn’t gonna have a misfire based on oil alone but then I’ve read in some places that it can happen and I should look at cleaning the oil out of the tube and going from there.

I did check the other spark plug wires at the spark plug tube’s and noticed there was oil there so that leads me to think that I don’t believe the oil in the number four cylinder is causing the misfire because otherwise the rest of them should be.


My friend drop his vehicle off at my house last night and I was going to put it on the driveway so I could drive it when it’s cold because he said it’s cold it seem like it misfired a little bit because I questioned him about that as last time I briefly looked at it and pulled the codes I had noticed it when I took it for a test drive to see if the code returned.

The friend wanted to wait and see if the code returned which I didn’t have a problem with. After a week he said the code returned which was about a 2 weeks since I looked at it. When I went out and switched the vehicles around so it would be cold on my driveway for any test I need to do such as leak down (read about head gaskets being symptomatic). I wanted to take it for a drive since it was actually cool before I went back to bed at 3 am (I wake up at the most random times of night even if after only sleeping 2 hrs, must be age catching up with me) So I waited about a minute after starting it and gave it light throttle and nom misses or anything. Then snapped the throttle a little and all seemed well.

Once it was warmed up I went on a test drive and I started to notice what it sounded like a wheel bearing growl so some of my focus shifted to that and I thinkIt does have a right front bad wheel bearing but we will check later. Running the vehicle checking on the wheel bearing got it warmed up pretty good and I didn’t notice anything wrong performance wise. This was 3 AM or so in the morning so hardly any traffic was out so I could get down on the throttle as all the cops are keeping 6 feet apart from each other and probably from the road so I didn’t see any other, ha ha.

Driveway and it was sitting there idling so I turned it off and started it back up and gave it some throttle right away and it was fine. I let it idle about a minute and snap the throttle and as I snapped it down it seem like it stumbled while the rpms we’re increasing during the WOT throttle snap. So I made it 30 seconds and did it again and it did the same thing stumbling. I tried it again after 15 seconds and it stumbled again. I then tried it again after 10 seconds and then five seconds to respond
So I made it 30 seconds and did it again and it did the same thing stumbling. I tried it again after 15 seconds and it stumbled again. And then tried it again after 10 seconds and then five seconds respectively and it still stumbled briefly while throttling up.

So I decided to stop the throttle as soon as the idol came back down and once again at stumbled and if I snapped it again I stumbled again. I or the owner don’t know the history on the vehicle as he purchased it back in February as he had lost control on ice and somebody T-bones him luckily in the passenger side of his Dodge Dakota pick up truck.

I thought his pull the plug wires and check the spark plugs as that’s probably gonna be the cheapest run but also doing a little bit of reading I have I seen and I’m not sure if this is totally true or some people have said that the leaking spark plug tube seals will cause a misfire but as I said earlier wouldn’t it be on multiple cylinders being that all of them have oil leaking?

My thought right now is take the misfire approach and go that direction but part of me says OK we need to check this, this, this and this being that we don’t know the history on the vehicle. I can also check the resistance on the spark plug wires and check for any burn marks. The last time I looked at it was just a quick pull the codes and let him know what it may be and if I’d need to look at it again.

I don’t see misfires often enough that it’s something that’s a use it or lose it skill which I need to start using more often as I have had a lot going on around the house lately and with the covid 19 stuff, us being indoors at ho,e more the wife is wanting projects here done sooner than later taking away from working on cars. I’m sure we can all relate to that at one point or another.

Should I focus on the spark plug tune seals or focus on something else at the primary and do the tube seals secondary. It’s one of those things that I don’t want to do the tube seals and a week later the guy calls and I have a bigger issue. It’s better if there’s something else (which i believe there is) to do that and do the tube seals / valve cover gaskets on the backside of the project.

Please let the thoughts roll in. I’ve searched for similar symptoms regarding this and have had no luck thus far.

I have access to various testing tools, a couple different scopes, pressure test methods, a leak down tester, compression tester, a wireless pressure tester, relative compression tester.

And I spoke side of things my experience is limited but today would be the perfect day to learn because it supposed be a nice day, nice day it’s been a week and why not learn something new today?

I think my direction as well it’s lagging and I need to get that proper direction. I’ve read Paul’s book, watched his videos but I think I read too much into it at times. I need to go dig of the chapters or chapter regarding misfires and misfire diagnostics & diagnosis.

When I come across things such as this it makes me squeeze my head and go “why did I look at this vehicle” as the fear wants to creep in but I keep it at bay knowing that all things can be conquered. Here’s the link. Also I seen they had good head gasket article.

mdhmotors.com/intermittent-cylinder-misf...valve-guide-problem/

I will pull fuel trim numbers, compression, freeze frame data if possible which should be. Just gotta refresh my brain.

May I add that the cruise control light was on the first time when I looked at it a couple weeks ago and also again when I went for a test drive early this morning.

So many things come to mind. Brain freeze seems to be coming on. Argh!!

Might be a good time to try some cool tools out. Yes I’d have to install the picoscope software but wouldn’t have to install it next time making me one step closer then....

Since you can get the miss to happen in the driveway, I'd agree that now is the perfect time to get some scope practice in. Maybe consider trying some secondary ignition testing?

www.scannerdanner.com/forum/lab-scope-di...need-to-buy-one.html

These engines definitely have several mechanical issues, but don't let that deter you. Cross that bridge when you come to it. For now, a miss on snap throttle definitely suggests an ignition or fuel issue, so K.I.S.S. and start digging there. Spark strength at the coil, at the wire, and so on. Move parts to another cylinder as needed. Use the Mode $06 miss counts to make sure the code isn't lying, and track if the miss truly moved with a part.

If you replace ignition parts, do the valve cover gaskets/tube seals as well. Stop the oil from pooling in the plug tube. They're not as bad to do as they look.

The cruise light happens when the PCM sets a code. Misfire, EVAP, whatever. Don't sweat it. When you fix the P0304, you'll fix the cruise light.

And look on the bright side! This generation of Suby uses bolt-in wheel bearings, instead of the nightmare press-in style. If you've done the old style rear bearings before, you'll know what a blessing that is.

Tyler wrote: Since you can get the miss to happen in the driveway, I'd agree that now is the perfect time to get some scope practice in. Maybe consider trying some secondary ignition testing?

www.scannerdanner.com/forum/lab-scope-di...need-to-buy-one.html

These engines definitely have several mechanical issues, but don't let that deter you. Cross that bridge when you come to it. For now, a miss on snap throttle definitely suggests an ignition or fuel issue, so K.I.S.S. and start digging there. Spark strength at the coil, at the wire, and so on. Move parts to another cylinder as needed. Use the Mode $06 miss counts to make sure the code isn't lying, and track if the miss truly moved with a part.

If you replace ignition parts, do the valve cover gaskets/tube seals as well. Stop the oil from pooling in the plug tube. They're not as bad to do as they look.

The cruise light happens when the PCM sets a code. Misfire, EVAP, whatever. Don't sweat it. When you fix the P0304, you'll fix the cruise light.

And look on the bright side! This generation of Suby uses bolt-in wheel bearings, instead of the nightmare press-in style. If you've done the old style rear bearings before, you'll know what a blessing that is.

Taylor you’re referring to mode 06 data as in the generic function on the scan tool correct? Also I can’t remember the name at this time because my memory has been screwed up the last few days from lack of sleep and being sick but there’s a file that should show the state of the system at the time the code occurred correct? The name will come to me here in a minute or when I get inTo the scan tool it’ll come back. It’s like it sitting on the tip of my tongue but when I take breaks a month here or there I tend to let things slip my mind. I’m not grinding every day unless I’m getting after it but just lately I’ve been down range.

I will do as you said and just stick with the basics can use my spark tester to begin with. We can always whip the power probe out if needed later or just hook up the scan tool/scope and use some of the tests built into the scan tool that are ignition related and such. It’ll be just like hands-on training, or wait it will actually be hands-on retraining. Lmao ....

Thanks for the input and if I get stuck I will be back. I will try Swapping the spark plug wires if I’m not making any headway with the spark plug tester then I’ll measure the resistance of the spark plug wires as I found some of that information today. I just procrastinate too much and here it is 10 o’clock at night and I’m just getting going on it but I do like working at night time except when it’s cold but it’s only 50° so it’s not too bad yet.swapping the spark plug wires if I’m not making any headway with the spark plug tester then I’ll measure the resistance of the spark plug wires as I found some of that information today. I just procrastinate too much and here it is 10 o’clock at night and I’m just getting going on it but I do like working at night time except when it’s cold but it’s only 50° so it’s not too bad yet

So I did some checking and found the number 4 cylinder wire had some Carbon on it.

All four of the photos are the number for cylinder wire where it goes and the spark plug tube.

All the spark plug tubes were leaking around the O-rings and not any further down in the bottom. So I just need to stick a rag in there and wipe around it and clean it up then pull the, plugs,
valve covers and replace the O-rings, valve cover gaskets and I will check the spark plugs as well.

I was able to get some video which I will have to upload later of the spark plug tubes showing how little amount of oil was in each one but clearly spark plug wires have been used over and over again as they have like a dried caked on Oily residual dirt on the spark plug end that goes down in the tube.

The video I took is a little bit of rambling and such so I will probably cut the audio and just do a voiceover. Maybe if I do it right I can fill in for Mike Rowe someday, I'm dreaming.

A little off topic but when the key is in the Forward accessory position just prior to start in the engine compartment there is a noise coming from around the throttle body area.

As I've mentioned before and probably will mention at some other Point again he got this vehicle back in February but the throttle body gasket has been replaced I noticed as well so wonder if it’s related to the noise. .

When it's making the noise you also hear like a little noise like a actuator is adjusting constantly in very small increments.



I will pull out the stops and run the component test tomorrow and hopefully being that I replace the wires as one was burnt lightly so I felt it was warranted plus it's soaked as they were with oil that it was hardened on the plug wire going down the tube.

I will check and make sure that it is getting good spark tomorrow as I wasn't able to get a coil this weekend if I needed one and that would take a couple days unless of course I went to the dealer I’m sure and I did notice the current coil is a Subaru so it's probably the original one and really expensive being it ha the Subaru name on it .

Roughly at 140,000 miles my guess is it could be on the down slope but we will figure that out when we run the tests with the component tester tomorrow or maybe I should say today anyways I should get inside as it's pretty cold out here and I am in shorts as I've been all evening. My gues is the coil will be good But you never know

While removing the spark plug wires and checking for any fouling on the ends and also on the coil and the coil end of the wires I noticed that down on each side of the engine just above the motor mount there's oil that runs down over the motor mount.

I haven't got on the ground and got down underneath and looked at it I will when I jack it up but I hope it's not head gaskets leaking but I do have some dye just in case it's needed.

I don't think I would do a head job in my driveway anyways.

I don't want to do my first head job on someone else's car as I would rather get my feet wet doing it on mine though I would do it by the book and it would probably take me twice the time because of double checking and double-checking and triple checking.

I just want to get this part fixed and we'll go from there. Here’s to high hopes later today.

Here’s. Link to the video of the noise somewhere around the throttle body area. Has me stumped but I’m sure anybody who works on Subarus regularly will probably identify the noise.



www.dropbox.com/s/5nhy9gzyn8csu5u/20200419_005212.mp4?dl=027z

I notice that noise on many cars with electric throttle bodies. I wouldn't let that lead you down a rabbit hole.
I spent a good portion of last week diagnosing an intermittent no shift on a six cylinder Subaru Outback and it sounded just like that.

If it was me and I was doing it at home with the wife "encouraging" me to get it done, I would sell plugs and wires, and the valve cover gaskets/plug well seals (probably come together in the same box) as Tyler suggested.

A little tricky given the orientation of the engine, but really not all that difficult.

Hey, that's a carbon track. Easy breezy. The Mode $06 and Freeze Frame data would be fun to check? But I'm pretty confident you found the problem.

Noah is right - that noise from the TB is totally normal. Don't sweat it.

I also agree with Noah about the repair. Do the plugs + wires, the valve cover gaskets, and leave the potential head gasket issue for another day. Give the vehicle owner the heads up and let them budget for it. These head gaskets CAN be done in the car, but it's not fun. So much easier to pull the engine and get it on a stand.

We've got full on problems now! Went to loosen the valve cover bolts and about 2/3 came loose and the others stripped the head. I was using a FDX deep well socket.

The heads of the bolts where they go thru the Aluminum valve covers corrode causing, ..... you know it seizure of the bolts. Not in a good way.

Snap-on guy won't be home till later tonight so that's out of the question of going and getting something to grab onto the stripped head.

Even after the first one didn't bite that slipped I went and got my map gas torch and used the porch and PB Blaster and used that back and forth a few times heating then spraying and even putting the socket on the valve cover bolt and tapping on it with a hammer.

Stripped 2 on drivers side and 3 on passenger side. I tried working the bolts forward and back. They'd go forward some and then I'd reverse them and the socket would get back to the point that I started forward at and then would stop. It's like something is holding it from going backwards.

I'll include a few photos for eye candy with this post. Not sitting in a good way right now. Don't want to dremel the bolt size down where it'll fit a standard size socket. Don't want to go to far forward either and snap the bolt off in the head.


What seems to have worked good for you guys with this? I'm not looking to yank the motor and do a head job or try in the vehicle either.

Any other suggestions to get these loose would be appreciated. I know I will use anti-seize on the shoulder of the bolts out near the end where they go through the valve cover to keep it from seizing up.

My day went from everything going good and a quick way too bad real quickly!



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Man those things are crusty. :ohmy: I've honestly never seen them that bad.

I say keep hitting them with the torch and the Blaster. Wet them all tonight and go to war again in the AM. That'll give you time to rest and go at it fresh tomorrow.

I really doubt you'll break the threads off in the head, since the threads aren't the real issue. Worst case, you damage the valve covers. Not great news, but it could definitely be worse.

Pour some ATF and let it soak. Weld (MIG or WIG) a big nut on top of the M6/10mm and you'll get them out. Heat will be very helpful.

I smacked em good with a hammer and socket on it as well figured the impact should knock those crusties loose. I'm guessing these are the original and there's probably been zero to nill work done on this engine in the past.

I was able to use a vin decoder for figuring out which specific model this was and it showed me recall information, etc.

Went into service in PA and at 30k miles had work done in Juneau AK. so this vehicles seen the country and then some. My thoughts are just a lack of upkeep overall thru it's life.

I'm still driving my 04 Chevy Tahoe I bought new in Jan 05 and it has just under 130k miles on it. Something breaks, goes bad then it gets fixed as soon as I have the time.

Like mentioned, prolonged oil soak, heat, small vice-grips, strip socket.

Well after multiple days of trying that spoke with owner last night and he wants to bite the bullet and since it's cheaper to cut and replace the valve cover we're just going to order one today.

Even drilled a bolt hole out yesterday and kept step drilling it up to about 3/4 of the bolt shoulder size and it wouldn't come out using several types of extractor bits I had.

The drivers side had 2 bolts bad so it is harder by a long shot to work them loose when it's the back 2 bolts.

I was lucky enough to get the passenger side off after grinding the front bolt. I'll throw some pics in the post.

Come to think this was all over a misfire. Goes to show how something small can turn into something big.


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Well that went South fast. I feel bad for telling you that it wouldn't be too difficult.

So what's the plan for getting the cut bolts out of the head? Should be plenty left to grab with vice grips?

That is awful. Jobs like that are the worst. Try not to let it beat you up and be persistent, which it sounds like you are. Then it's over, and it doesn't bother you anymore, and just becomes a story to tell.

I'm due for a good shit storm myself.

Nissan Technician

What a miserable POS.

I dunno if I'm gonna recommend these gaskets anymore? For years, I've just wedged my electric ratchet in there, got the anvil spinning at wide open and mashed the socket onto the bolt. :silly: They spun out every time! Now I'm wondering if I was just getting lucky.

There's the thicker part of the bolt which is the shoulder that actually corrodes to the valve cover so grabbing that with vise grips makes it easy removing it. Did the passenger side with ease though it took about 45-60 min of tapping with a pry bar and hammer from behind.

Sadly this wasn't feasible on the drivers side with 2 bolts stripped.

Rule of thumb. Heat then soak these bolts for a day or two with pb blaster or kroil to help loosen the corrosion. After that we're wasting our time and with a valve cover being not to expensive better off to advise friends and/or customers to just cut their losses vs either pulling the engine or nursing it along.

Getting my experience with these quick. Wasn't the misfire diagnostic process I was hoping for but as I've learned in life and advised younger people as well we can't have it easy and you have to take the good with the bad and opposite likewise.

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