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Rough Idle

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Hi all,


Thought I'd post here to get a better gauge of the issue, got a 2014 Legacy Spec B (just under 80k kms) idling a bit low and causing a mild/moderate rough idle in gear only. When car is shifted to neutral or park it subsides entirely. Sometimes engine will idle fine causing rough idle to be almost unnoticeable, but revs will drop too low for just a moment causing a shudder and it picks itself back up. Other times it is just a bit low causing the rough idle at traffic lights and what not. Info about the car as follows:


* Car has had compression test returning values of ~150psi for all 4 cylinders which is within normal range.

* No vacuum leaks

* MAF sensor cleaned

* Throttle body cleaned

* Spark plugs average condition, will be replacing soon as part of service regardless 

* Not throwing any codes

* No warning lights

* Cam belt replaced recently as due on the 10 year mark but issue preceded this replacement


Saw a forum about AVCS solenoids (with a banjo bolt, couple of gaskets and a filter) causing a similar issue but not sure whether this was specific to the car being in gear and it appeared to be causing much more severe symptoms. Took mine to a shop recently and they said they didn't really notice the issue (which was difficult to believe) so they didn't end up investigating further. 


Any suggestions? 


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Out of my scope really, but could be your idle air control valve, if that's playing up it will give you issues with idle, especially if the engine gets loaded up a little as the valve may not be letting enough air through, hence it tries to stall. 


I know you said vacuum leaks are out, but check if any air is getting pulled in after the MAF, if you imagine the air filter creates a bit of a restriction and this can cause a slight vacuum in the intake even before the throttle, and if unmetered air gets in you could be running lean, and this could only give issues when idling, 


Just ideas for food, or thought or what ever the saying is, either way I'm hungry.

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  • Admin

EThrottle cars don't have a IACV. They just open the throttle a bit when they need more air.


So, uh, the spark plugs. How many did you pull out? And if you pulled them out to inspect them, why didn't you just put new ones in?

"Rough idle. Haven't changed my plugs. Any suggestions?"


Yeah I've got one I can think of...




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Had a shop do the compression test for me so they pulled them out and looked at them for me then as part of that. Both those guys and another shop told me the plugs should still be fine, but yea will be replacing them soon anyway. Plus did think it would be a bit strange for the issue to only be happening in gear if it was the plugs but happy to be wrong there. 

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Hmm okay maybe it is a more curly one. Is it caused by _any_ load on the engine (having the car in gear will slightly load it, viscous coupling blah blah blah) or only in gear? Try loading it up a bit (Air con, headlights + heated seats if you have them) in Park and see if it dips.


How does it run with some throttle? If it was a spark/miss issue I feel like it would be more pronounced with more RPM and load, things like coils or plugs usually behave like that.

It does feel a little bit like a slight air/fuel problem. Inclined to say a very small vacuum leak (more likely after the throttle body) or perhaps an issue with one of your exhaust oxygen sensors, although those usually throw a CEL.


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I have noticed it's slightly worse with the AC and headlights on and whatnot, but again, only once it's warm. With a bit of throttle and under more load its perfectly fine, it really does just appear to be an idling too low issue, but without it being a vacuum leak I'm a bit stuck as to what the root cause would be. I'll get vacuum leaks checked again, wondering if theres any plausibility behind leaky injectors? 

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