2005 Subaru Outback 2.5i Cylinder 4 Misfire help

Need some help regarding my 2005 Outback. I was driving on the highway uphill and started feeling a loss of power and some shaking of the car. The cruise control light started to blink and the check engine light came on. I pulled the car over and stopped for 15 minutes, tightened the gas cap and continued driving. The light went off and the car drove fine up several more hills for 15 miles, then started to hesitate and lose power again. This time the check engine light and cruise both blink, and after about 5-10 blinks the check engine light stays on. I drive the car for another 150 miles with no further performance issues. I finally took it to an auto parts store to get the code read and it comes out as “P0304 Cylinder 4 misfire.” I drive the car home and it sits for another half hour and the check engine light goes off and doesn’t come back on the next day when it’s driven another 40 miles.

I got gas about an hour before this whole thing started and I thought I might have gotten bad gas, so I filled up again at another station to mix it out and haven’t had problems since. Or it’s just a coincidence. I’ve had nothing but trouble with this car, done the head gaskets, suspension, brakes, emissions and I’m at my wit’s end.

So my question is: Did I do any damage to the car by continuing to drive it when the check engine light came on, and what can I expect to pay to fix it? I’ve been reading a lot about spark plugs and wires as well as ignition coils and so I’ll probably start there. I think I did spark plugs and wires 2-3 years ago but can’t be sure. Thanks for any help!

The car should not be driven with a blinking CEL.

My personal opinion of what you should do before replacing parts on a guess is to run a compression test to verify that cylinder pressures are where they should be.

It’s possible that a valve could have tightened up on the cylinder head. Valve lash should be checked and adjusted as necessary every 30k miles but this is seldom ever done.
You state the head gaskets were done and that would have been an ideal time to perform this procedure.

Another possibility is a compression issue related to the head gasket failure cause or replacement.
Bottom line is make sure the compression is good as a first step.
If you run a comp. test post back with the numbers and we’ll try to sort it out.

If you are DIY swap your coil pack with #2 and see if you get “P0302 Cylinder 2 misfire.” to see if your coil pack is good or not. If no check your spark plug too otherwise go to a compression test and pray your valves are good.

I went thru a valve job on a 2005 Legacy albeit turbo engine.

Quick update: Took it to the shop and they got a code for a cylinder 3 misfire this time. From what I understand, a misfire moving around can mean bad air to fuel mixture. They didn’t do a compression test because I had the head gaskets done recently.

The spark plugs I had installed when I did the head gasket 3 years ago were burnt up when the technicians took a look at them. Supposedly they weren’t rated to the correct temperature for Subaru engines which could have led to their early failure. Did the wires and the coil pack at the same time since its February in the northeast and I don’t have the means to check on this myself

No issues since then, but I hadn’t had issues with it for days before either. Hopefully this is the end of it and they didn’t miss anything by not doing a compression test

Did they install new plugs exactly the brand/model and gap that the underhood sticker or owners manual specifies? Anything else is at worst trouble, at best an experiment.

I guess the shop I did my gaskets at, not a Subaru dealer but deals exclusively with Subarus, used plugs a step down from recommended (gold vs platinum). Did the work at a dealer this time so they used what was recommended

Lowered compression can cause early failure of spark plugs. When the plugs are out why not run the compression test at that time and know what you have? It only takes 10 minutes or less.

The temperature range has to do with the amount of thermal insulation used it the tip of the plug, it has nothing to do with gold or platinum series plug. For cars that see a lot of highway use, it is not uncommon to go down one heat range. Going down one heat range on a car driven around town on mostly short trips will cause them to foul earlier.

Your car calls for a heat range 5 in NGK. A higher number will be colder so a heat range 6 is an alternate plug for cars used mostly on the highway. If the plug retains too much heat, it will cause pre-detonation (spark knock). Too cold and deposits won’t burn off properly.

When a complaint surfaces that the car misses or pings when going uphill, but not otherwise, the first thing I wonder about is the spark plug gap. Too wide of gap can cause this. The gap widens over time, in any event even if the plugs are fairly new might make sense to verify the gap is within the manufacturer’s recommended range for this car.

Edit: And of course only one of the exact spark plugs recommended in the owner’s manual should be used.

Does your shop have an ignition analyzer machine? If so, might be worth it to pay them to put your car on it, see if they note anything unusual going on with the spark. These machines display the spark waveform for each plug, so can identify otherwise difficult to diagnose ignition system problems.

Whatever plugs they did end up installing didn’t seem to do the trick. After less than a week and about 100 miles, check engine light comes on again. This time it’s cylinder 4 misfire again and catalyst system below threshold. So that’ll be the catalytic converter because the car is sending fuel into it. Not sure if it’s a valve stuck open, part of the fuel injection system, or something else entirely at this point.
The reason the dealer didn’t do a compression test or ignition analyzer is that it would’ve cost them money and time to do is my guess at this point. Either way they’re doing it now for free so we can decide to kill the car or not

Have the swap the cylinder 4 coil and injector to another cylinder and see if problem moves

It just boggles my mind that considering the chronic problem with a misfire, prior head gasket work, and the fact they changed spark plugs that they couldn’t have devoted an extra 10 minutes tops to checking compression while the plugs were out.

Any fuel system or ignition system fault can be worked out. Any problems due to compression or tight valves is not so easily or cheaply resolved.
Verify mechanical fitness first has always been my mantra but to each their own I guess.

They ended up doing a compression test and said that the numbers were good. I didn’t get the numbers so I can’t verify independently. They did a visual inspection and said that they couldn’t see a valve stuck open, so the next guess was to use an ethanol buster and carbon buildup fuel additives and see if the problem goes away. The car had been driven short distances for the last 3 years and may have led to buildup within the engine causing misfires, or so was their best guess.
The next best guess was a fuel injector, but I told them I wasn’t going to spend big money on another fix, so they didn’t spend a lot of time on diagnosis. They didn’t even bother doing any tests on the catalyst system to see what the problem was there.
I ended up driving it straight to another dealer and bought another car while the check engine lights were off. Sorry I don’t get to post a happy ending to this story, but I got a happy ending by not having to deal with this car anymore

This continues on with what I consider to be bad practices. Any time a compression test is performed the numbers should be put down in black and white. Failure to do so leads to memory lapses or flat out amnesia and leaves things too open-ended for my tastes.

Visual inspection for a stuck valve comes across as suspect unless they removed the valve cover and checked the lash. In borderline tight lash cases the valve may be closed when cold and may be held open when the engine is up to operating temperatures.

This still comes across as iffy to me and I also point out the following. It’s amazing how many times poor compression numbers are called good by mechanics when the fact is they are not good in any way, shape, or form. I’ve got service manuals that are as dead wrong as can be and not just in relation to compression figures but other areas also.

Sorry I can’t be of much help.

Well, it’s a done deal

I hope OP is happy with the “new” car . . . whether it be new, or new to him

This shouldnt be hard to suss out actually. Its either the plug, the coil or the wire IF it has wires and not coil over plug by 05’ I know on the 02 Subaru it has one coil PACK for all 4 plugs. I like it when they have the pack divided so that you dont trash a whole coil pack for one port failure… Start small and work your way up…replace the plugs… See what happens… then move to wires and lastly the coil pack. I like to use salvage yards in these instances…but I know thats not an option for everyone. Not a hard issue to figure out on this one.

Re-reading your post…you tightened the gas cap right BEFORE the misfire…Did you also try to loosen the cap to see if it affected the misfire? The misfire could be gas cap related…try that real quick…its unlikely but possible

The blinking CEL light was caused or is usually associated with the transmission codes that can pop up…the transmission is saying to not drive because something is wrong with the transmission…what has happened is that the misfire caused the transmission to think there is something wrong with it…but almost always this is caused by the engine misfire. The transmission doesn’t know about the misfire and thinks the tranny is in danger…but it was caused by how the engine was turning the transmission the trans is most likely just fine.