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Old Yesterday, 11:44 AM   #1
xX_STI_Xx
Scooby Newbie
 
Member#: 493470
Join Date: Nov 2018
Location: Chicago
Vehicle:
2019 STI
Crystal White Pearl

Default Rough Idle 2018 & 2019 STI

Sup Guys,

I know this topic has been discussed many many times...however, I think I may have discovered why STI owners complain about a rough idle, especially with the tachometer not showing a bouncing needle.

Okay, so I have a 2019 STI with 700 miles on the clock (purchased back in November of 2018). The car is brand new, but recently I started driving it more and more because of the nicer weather. So, the other day I noticed in traffic the car was idling a bit rough. Not terribly rough, but noticeably rougher than normal. RPM tach was dead still...no bouncing or moving. I assumed it was something else and not engine related. So, I called my cousin who is a Subaru Master Tech and he said, "check under the car for anything that may look loose or disconnected." I checked, and everything was good...nothing out of place. I drove the car to his dealership the following day, and he inspected it...and everything looked fine. Then, he asked if I hit anything under the car or bottomed out. I told him no, it was barely driven. So, he told me to get in the car, turn it on and stay in the car while he lifted the car high enough to get under it. He noticed the rear mufflers vibrating and slightly bouncing from the exhaust harmonics and pulses. He said, "this is normal, but why is it apparent now?" So, he looked at my fuel gauge and saw that I was fairly low on fuel, and as an experiment....advised I go fill up to add some weight to the rear of the car and see if it balances out the vibrations or "rough idle" feel. I went to a nearby gas station and filled up....and sure enough, no more "rough idle." I guess the STI's are lighter in the rear and because of the dual mufflers, has stronger exhaust pulses that vibrate up onto the chassis??? After filling the car up with gas, the "rough idle" went away. Not sure if this is the same case with everyone else's complaints, but it's worth checking for all you 2018 & 2019 STI owners.
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Old Yesterday, 01:30 PM   #2
dingobite
Scooby Specialist
 
Member#: 82391
Join Date: Mar 2005
Chapter/Region: MWSOC
Location: Lou. ky
Vehicle:
08 OBP Sti

Default

Spark plugs can cause rough idle or stall your motor even while driving anything thats loose at the coil packs i think i learned my lesson at mandatory cold plugs changes every 12k miles.

I dont know much about weight of the car causing idle problems.


Map sensor cleaning is also a issue, i just had to clean a brand new one 90 days later to get my idle correct.
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Old Yesterday, 02:08 PM   #3
xX_STI_Xx
Scooby Newbie
 
Member#: 493470
Join Date: Nov 2018
Location: Chicago
Vehicle:
2019 STI
Crystal White Pearl

Default

Quote:
Originally Posted by dingobite View Post
Spark plugs can cause rough idle or stall your motor even while driving anything thats loose at the coil packs i think i learned my lesson at mandatory cold plugs changes every 12k miles.

I dont know much about weight of the car causing idle problems.


Map sensor cleaning is also a issue, i just had to clean a brand new one 90 days later to get my idle correct.
I agree, and your right weight won't cause engine idle problems....I guess i am talking more about "rough idle like" symptoms not necessarily a rough idle.
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