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Old 02-25-2021, 07:31 PM   #1
cwoodford
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Member#: 450950
Join Date: Jul 2016
Default 2018 WRX 6MT rough idle

Hi everyone,

I'm dealing with what seems the somewhat infamous WRX weird idle. It seems while the car is warming up it will idle weird when coming to a stoplight, dipping down to sub-700 rpms, feeling like it's going to die then popping up to 1200 rpms or so and then resetting to normal idle. It also seems to occur after starting the car while it's already warm, the car's tone flutters so much it's like an old carbureted engine that needs some throttle to actually get going.

I have been doing some digging and saw the OCV was suspect in this occurence so I went ahead and ordered one. I then looked at my AP log as apparently you can see a bad OCV in the AVCS in left/right and out left/right (Specifically they don't track together). It seems based on my log the intake tracks together, but the exhaust never leaves 0. Am i on the wrong track with a OCV replacement?

Car is a 2018 WRX with 47k miles. I put a stage 1 tune on it about 1000 miles ago. I started seeing this intermittent issue before the tune, the tune seeemed to fix it for a while, then it came back.

Here's a log of the warm car right after it's started. I don't know if it is useful but it was definitely doing the weird idle right away.
https://docs.google.com/spreadsheets...it?usp=sharing

thanks in advance for any help
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Old 02-26-2021, 01:55 PM   #2
basyager
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Member#: 518233
Join Date: Aug 2020
Vehicle:
2019 WRX

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Why don't you get it sorted with warranty?
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Old 02-27-2021, 02:08 PM   #3
WrightWRX
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Join Date: Nov 2018
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There could be oil in your OCV sensor connector housing. Try unplugging the intake/exhaust OCV and seeing if there’s any oil that might’ve slipped passed in there. If so, clean it out
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Old 02-27-2021, 02:44 PM   #4
X49
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Member#: 386600
Join Date: Apr 2014
Location: Western Canada
Vehicle:
2015 WRX

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Look at your AFR after 10 seconds, when the engine transitions to closed loop fueling. It's at 19.6. It goes as low as 12.7 at times. It should be very close to a constant 14.7.

It could be a fuel system issue. Your fuel pressure seems good, but it might be an injector issue. Log Cylinder Roughness on all cylinders to see if one there is any misfire.

If it were only happening in open loop just after a start, I would suspect the MAF. If it was only in closed loop, I would suspect the O2 sensor. Since it happens in both regimes, I doubt either is to blame, but something weird could be happening with AF Correction, so I'm not sure if you should rule either out at this point.

Edit: The AVCS is disabled for a time after engine start. I don't see how an AVCS issue could be causing your issues, especially since your AVCS readings are pretty much pegged at 0.
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Old 03-15-2021, 03:34 PM   #5
Asm123
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Join Date: May 2017
Default Seafoam

Got a 2017 with 128k on it and ive noticed a periodic weird idle like youre mentioning starting around 40k. Thought it might be carbon on the intake manifolds so tried out seafoam since I didnt want to spend money on more expensive options and the weird idle cleared up. Now i do it as part of every oil change. Also noticed a decline in mpg, 27 ish when new and dropped to 22 ish after some decent mileage. Seafoam seemed to help with that as well, now sitting at 25.8 mpg.
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Old 03-31-2021, 05:05 PM   #6
Mycroft_Groks
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Member#: 522534
Join Date: Feb 2021
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2016 WRX
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I had an issue that was similar to this. My problem was corrosion on the MAF sensor extension that I had installed with my Cobb Big SF intake. I replaced that wire and it instantly went back to running normal.
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Old 04-02-2021, 08:36 PM   #7
newsubylvr60
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Join Date: Feb 2017
Chapter/Region: MAIC
Location: Allentown Pa.
Vehicle:
2016 WRX
Lapis Blue

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Do you have an AOS? I had a problem recently with idle fluctuation and it turned out to be a check valve in AOS line which was causing a huge vacuum leak. I also had to replace the stock turbo inlet housing which had a huge crack in the port for the Bypass valve. My car is a 2016 WRX with a stage 2 Bren Etune.
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