Rough idle, engine sputter/misfire.

Noob here. Longtime lurker/first time poster. I've been helped quite a bit by reading these forums and have bookmarked quite a few threads that I refer to from time to time (Thanks!).

So I have a 2005 Legacy 2.5i NA/non-turbo with about 180k miles on it.

A few week ago I noticed that, on acceleration, the car sputtered quite a bit and misfired a few times. Figuring I needed new spark plugs (I did), I replaced them with the help of GEE-OTTOs tutorial (and it only took an hour or so!).

That pretty much solved the problem. It ran perfectly.

For about a week.

Now, when the car is idling in drive, the idle is very rough and the engine will randomly sputter/shake. It's like a hiccup. From what I can tell, this only happens while idling in drive. Occasionally it will sputter just at take-off/initial acceleration but that's only when I start while the engine is 'hiccuping.'

This has been going on for about a week now and the hiccuping has becoming increasingly frequent. I really hope I'm not doing too much damage by driving it like this but I have no other way to get to work (it's only a 3 mile drive so it's not putting a ton of mileage on it).

Any ideas on where to start? I've searched these forums extensively and have a few ideas but just not quite sure.

Thanks in advance!

May be dump a can of injector cleaner in the gas tank.

Your you sure you don't have a vaccum leak, check all the lines.

Have heard that sometimes a dirty mass airflow sensor can do that, happens as car gets older etc. Other than that maybe an ECU reset, if you can do that by removing the battery, could help but have not tried it. Hope this little advice helps but i'd read into that sensor, or if someone you know has a diagnostic device plug it in and see if any errors are showing up.

Thanks for the help! I'll take an all-of-the-above strategy on this one.

Another symptom I noticed today that may help zero-in on the issue:

The sputter increases when I turn the steering wheel and I notice the headlights dim at the same time.

How old is the battery ?

Are the terminals clean ?

Originally Posted by Max Capacity

How old is the battery ?

Are the terminals clean ?

The battery is brand new(ish).

It was replaced when I replaced the plugs.

The terminals are clean, however, they were a little loose and the negative terminal/post had to be shimmed. You think it could be an issue of a bad connection? Because that would be beautiful. Haha. If I can solve this by replacing the terminals I would be one happy camper.

You need to have a good tight connection there. If it takes a new end on the cable...so be it.

After that put some grease on the top,bottom and sides of the treminal to seal out moisture.

So, since I last replied I have replaced the battery cable terminals and cleaned the MAF sensor. [I'm replacing the air filter today, as well. With the MAF that grimy, it's apparent that it needs a new filter.]

Unfortunately I'm still getting the rough idle, but what is really concerning me is now, whenever the RPMs drop (particularly on deceleration and idling), the engine stalls out completely.

Just a brief rundown of the sequence of events from start-up to stall-out:

1. Turn the ignition and engine starts with no problem; engine idles (in park) at around 1000RPM

2. Shift into drive or reverse and the RPMs drop to ~600-750RPM range.

3. Drive, accelerate: Generally, no problem.

4. Decelerate, come to a stop: RPMs drop to ~500 before completely stalling.

I've noticed that if, as I'm coming to the stop and the RPMs hit ~500, I give it just a little gas to bump up the RPMs, I can prevent it from stalling. After that, it typically idles with no problem in the ~600-750RPM range.

Even without stalling, the engine shakes and shudders pretty roughly during idle. It gets to the point that it feels like the entire car is shivering.

The odd thing is that the CEL is off and has remained off. I can understand the CEL being off after the ECU reset that comes w/ prolonged battery disconnect, but it has remained off for days now. I'd expect whatever is causing the erratic idle to have triggered the CEL by now.

Either wait for the CEL

Or take it to a local mechanic.

I wonder if replacing the O2 sensor's may be the next step.

Hate to throw money at it without knowing what's going to fix it.

Keep in mind that most of this happened after I replaced the spark plugs, so I'm pretty sure it's related (to something I did).

I think I'm just going to go home today and take apart the entire air intake "assembly" and put it back together again, just to make sure everything is fitted together properly.

Did you use the recommended plugs.

Yes people have not installed the air box correctly. I even did that back on my 00' GT wagon

Originally Posted by Max Capacity

Did you use the recommended plugs.

Yes people have not installed the air box correctly. I even did that back on my 00' GT wagon

Thanks for the quick replies.

I used NGK plugs. I believe they were NGK Laser Platinum Spark Plug (FR5AP-11) but I'd have to look at the box to make sure I have that part number correct.

Mines doing this also, but I have low compression, hope you dont

Replacing your OCV's wouldnt hurt.

So after taking apart and re-assembling the airbox and re-cleaning the MAF (just to be safe!) it appears the idle is back to normal, for the most part (by normal, I mean no stalls and back within a nice RPM range).

Unfortunately, a few hours later I got a CEL. I say "unfortunately", but it's actually kinda what I was hoping would happened. Pulled a code P0303.

I think I'm gonna change out the valve cover gaskets and spark plug gaskets for starters. Then replace the spark plug wires. The reason I'm tackling those first is because I noticed a little oil in the spark plug boot. Not a ton, but enough that I figure I should tackle it now before it becomes a problem. Do you think I should change the plugs as well? They're only a few weeks old. They're not too expensive so it wouldn't be a problem to change them, so I'd rather change them now if need be vs having to go back in after the fact to swap them out.

If none of the above work, I'll look into the coil pack, which I'm hoping is not the issue. With 180k miles on the Legacy, the gaskets and wires are probably due for a change anyway, so it won't be the end of the world if they don't work.