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2009 Subaru Legacy Intermittent Problem; Uncertain Diagnosis

About 2 weeks ago, I got about half a mile from home in my Legacy when the “check engine” light started flashing, and the car started hesitating, jerking, etc. I heard weird noises coming from (what sounded like to me) underneath the car. I immediately returned home. We have two Subarus, both of which are prone, like many of them, to leaking and burning oil. So my first thought was that I’d been driving on severely low oil. But it was down only about a quart. Took it to my mechanic the next day, and he couldn’t find anything wrong, and there was no stored code in it. Drove it for a few days without any problem, then the engine light came on again after I’d been driving it on the highway. But it was steady on, not flashing, and the car was running fine. I got off the highway, turned it off, and waited a bit. Started it up and it was fine. However, a couple of days later, the car started running rough again and the engine light started flashing. This time, after not running it overnight, the car ran fine but the light was back on, steady (not flashing), and the traction light came on, and cruise control light started flashing. Took it back into the garage and this time, there’s a code for #4 cylinder. My mechanic (who has been servicing both our Subarus since we got them) thinks it’s probably a bad ignition coil. He says that’s his best guess as to what would cause a disappearing/appearing problem. He’s driven the car all over and the problem won’t duplicate. He said there’s a possibility that driving low on oil might have damaged the engine, but his bet is the coil. I admit that with my other Subaru, I at one time forgot to keep an eye on the oil and drove it almost dry–which caused the car to buck and stall and the dash lights to flash like a Christmas tree. But once we filled the oil, it’s run fine since.

I have the car home with me now, and ran it last night. It ran rough, but no dash lights are on, and the problem wasn’t anywhere near as severe.

I’m just not sure what to do at this point. Hate to spend the $$ on the ignition coil and not solve the problem. Then again, it’s not that expensive (a bit over $200), and if it works, yay. But if it doesn’t, I have no idea what to do from there. Any help is appreciated; TIA.

is it 2.5 or 3.6 engine?
what is mileage?
how long since you replaced spark plugs?
wires?

assuming 2.5, it would have “one coil pack per 2 cylinders” system, so if it was coil, likely you would get misfire in two cylinders

from what I recall about my last Subaru, 2007 one I used to have, plugs are to be replaced every 60K miles with exact OEM type (NGK) and I had to replace wires on multiple Subarus I owned.
some of them had running issues, which they not always were throwing up as “Check Engine” light, but were back to “runs like new” once wires were replaced.
never had to change any coils

Hi, and thanks! It’s the 2.5 engine and has 106,000 miles. We talked about it possibly being the spark plugs or/and wires, but mechanic feels that this would be a constant problem, not intermittent. The car acts similar to how it would if it were the plugs or wires (running rough, hesitation, etc.), but it generally runs smooth as glass–until it doesn’t. I had to replace those on my Forester, but only once in the 10 years I’ve had it. I’ve had my Legacy about a year and a half, and it had 89,000 miles on it when I got it. If I had to guess, I’d say that the former owner did not replace those items!

if you still have owners manuals in glove box, Subaru has one of the best forms to present maintenance schedules in their manual: one page to show all maintenance items against age/miles, including fluids, plugs, etc…

look there, it will also list correct plugs type, which happen to be always NGK on 1996…2007 range of samples I used to have.

replacing plugs is surprisingly simple even with basic tools, you only need to have a set of extensions to get to proper plug well depth, but not much longer so you would be able to get around in tight opposite design they have

few sets of wires I had to replace all looked decent, but still were faulty in the end, as after replacement engine was running silk-smooth for me

be aware of cheapo-wires: AdvanceAuto used to have some set for something like $35 and another for around $60: I bought the first one, wires were equal length, so were very tight to get to far side bank, I had to get for more expensive replacement set , which worked great in the end

If the maintenance is up to date, and given the engine was run with low oil at some point, then next step is probably a wet/dry compression test.

That is false logic, in my opinion

I have seen several double-sided coils fail on “one side” only

I feel it’s worth gambling on a coil, assuming compression, fuel and the rest of the secondary ignition system are proven to be acceptable

I don’t feel running the engine only 1 quart low would somehow be the cause of the misfire

for OP’ sake, @db4690 has more in-field experience, so I would not argue his point if double-sided coil might still be under suspicion or not (although I think we are both in guessing game here)

from my perspective, I did not yet see if @amc654 ever had spark plugs replaced, as on 106K miles they are due for replacement, no questions asked

once this “must have” item is addressed, I see a guessing game “coil vs. wires” here

if I was to do it the cheapest way possible, I would swap the current coils to change where suspected failed coil is working and watch if misfire returns but in different cylinder now

I would expect next possible outcomes:

  1. car runs smooth => plugs were the root cause
  2. misfire returns, different cylinder => we know what coil to replace
  3. misfire returns, same cylinder => replace wires
  4. misfire returns, MUCH WORSE or bad idle develops => replace wires

note on #4: I might expect that after wires would be disturbed, they will develop “worse than before” condition, this is what I’ve seen on one of my Subarus after plugs replacement, cured by wires replacement

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Thanks so much to all of you! In doing further research, it sounds as if this is not uncommon in these vehicles, and that the coils are frequently the culprit. I’m going to ask my mechanic if he’ll switch the coil to another cylinder and see if the problem follows the coil. We’ve been using this guy for years and years, and his instincts are usually spot on–but I’d still rather have him test this out before replacing the coil.

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