Rough idle in gear, dies

djott70 · Jul 12, 2014

Hello. 2005 Subaru Legacy Outback, 2.5 4-cylinder, no turbo. 120,000 miles.

Started running rough within the last month. Felt like a miss and rough idle when stopped but still in gear. Unsuccessful OBD scan.

Check engine light finally came on the other day with cruise light blinking. OBD scan showed cylinder 3 misfire.

Original spark plugs. Replaced with new NGK plugs today. Oil came out with three of the four plug wires. Recognized that valve head gaskets will need to be replaced soon. The 4th plug wire was semi-hardened with a rust/burned appearance. Still flexible but it appears that new wires should be considered also.

Check engine light now gone as well as blinking cruise light. Idle is fine in gear but now car runs more rough in gear when stopped and starting dying.

Basic repair knowledge but mainly American cars (Pontiacs). I wouldn't think my changing of the spark plugs would exasperate the idle problem but maybe I messed something up? Leaking valve gaskets won't do this, would they?

Something else related to the cylinder 3 misfire I should be considering? I have a reliable and affordable Subaru mechanic in the area but anything I can fix is always nice for the pocketbook.

Thanks in advance.

rasterman · Jul 12, 2014

If you removed the battery to access the plugs (common) then the car will have forgotten learned idle settings. It will get them back again in a day or two.

You can force it to learn more quickly by starting and running it several times without touching the accelerator pedal at all.

djott70 · Jul 13, 2014

Thanks. I'll give this a try and that was unknown to me. I didn't have to remove the battery but I did unhook the terminals since I was working on the engine.

The rough idling seemed to be present before the plug change but I'll go out in a little while and try your suggestion.

eagleeye · Jul 12, 2014

Review this maint schedule, anything else you think has not been done to cause the problems?
(so this should have been the 4th set of spark plugs?)

The good news? you are right at the big service interval anyway so you can get all that done and fix the problem in the same go round.

and you need OEM wires, ...do not substitute,...if and when it gets that far OEM coil.

don't forget to throw in a new PCV valve. cheap and easy held in place by 2 screws,...and the cause of many a engine problem.

Subaru maintenance schedules and new car break-in period- 2000 through 2009, links for 2010, 2011...

djott70 · Jul 13, 2014

eagleeye said:
Review this maint schedule, anything else you think has not been done to cause the problems?
(so this should have been the 4th set of spark plugs?)

The good news? you are right at the big service interval anyway so you can get all that done and fix the problem in the same go round.

and you need OEM wires, ...do not substitute,...if and when it gets that far OEM coil.

don't forget to throw in a new PCV valve. cheap and easy held in place by 2 screws,...and the cause of many a engine problem.

Subaru maintenance schedules and new car break-in period- 2000 through 2009, links for 2010, 2011...

Thanks for the maintenance schedule link. Not sure what set of plugs this is -- not the original owner -- but I can attest to not doing the 3rd set according to the maintenance schedule.

Rough idle in gear, dies

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