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Old 11-16-2020, 05:07 PM   #1
Jfheisenberg
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2015 BRZ - Rough idle + acceleration struggles

Hi All - Cant figure out whats going on with my car, so here i am.

The car: 2015 brz 54k miles, full exhaust/tuned with OTF stage 2+ and perrin cold air intake. Spark plugs were changed 4 months ago. Never had this kind of problems with the car and all maintenance is done on time. Always use 93 octane and try to go to the same Shell gas station

The problem: Few days ago, i started to notice that the car idles rough. Normally, the car idles at 800 rpms and are steady. Now, the rpms drops to about 600~ rpms and car kind of wants to halt but doesn't, then the rpms come back to 800 again. Also, when accelerating on 1st and 2nd gear i can feel that the car is struggling to run throughout the rpm band.

These are the only to symptoms as there is no CEL code. Today, i cleaned the throttle body (not that it was that dirty) to see if it make a difference, but nothing.


I definitely feel that something is off with the car. Any ideas?

Last edited by Jfheisenberg; 01-12-2021 at 06:32 PM.
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Old 11-16-2020, 09:03 PM   #2
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Which plugs did you use? Put some air in your tires
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Old 11-16-2020, 10:15 PM   #3
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That Air Fuel Ratio and timing going up and down is not right. whatever is causing that is the problem. An air leak or fuel pressure maybe, dirty air flow sensor even reload the tune.
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Old 11-16-2020, 10:39 PM   #4
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The afr swinging at idle is your issue. I would clean the maf and see if you can get your hands on one and swap it.

Also check your air filter.
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Old 11-16-2020, 11:38 PM   #5
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You might consider getting a new MAF -
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Old 11-17-2020, 12:58 PM   #6
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Quote:
Originally Posted by gen3v8 View Post
That Air Fuel Ratio and timing going up and down is not right. whatever is causing that is the problem. An air leak or fuel pressure maybe, dirty air flow sensor even reload the tune.
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The afr swinging at idle is your issue. I would clean the maf and see if you can get your hands on one and swap it.

Also check your air filter.
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You might consider getting a new MAF -

Thanks for the advise. I just clean the MAF sensor, it was a bit dirty but not much IMO. Put it back in and its still the same (same numbers on the OTF live stats). The air filter was changed on June. At this point im just going to the shop i usually go for diagnostic.
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Old 11-20-2020, 07:11 PM   #7
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o2 sensor? Or a small exhuast leak? I had the acceleration issues without a cel and found a leak at one of the header gaskets, but that was my issue.

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Old 11-21-2020, 04:04 PM   #8
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Yesterday, i went to the shop where i get all my stuff done to the car. The tech mentioned that there is some air leak, so suggested to swap my perrin throttle body inlet to the OEM one. So i did that, and still having the same issues. We also replace the clamps and to my eye, everything looks correct. Also, i tried to one of my friend's inlet hose that he runs in his FRS and issue is still there.

Next, i flashed back to stock with the OTF and then reflash to stage2+. Unfortunately, problem is still there.

I was thinking maybe the gas was bad? So i needed to refill and went to shell and got vpower 93 octane. Sadly, this was not it neither. Car still idles rough and sluggish acceleration is more pronounce on lower gears from 1k rpm - 3.5rpm. After passing 4k rpm the issue is not felt.

No idea what the problem can be as there is no check engine light.

My next step will be to remove the perring CAI and put back the stock air box. However, if the issue was with the air filter, wouldn't the maf sensor throw a code?
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Old 01-04-2021, 02:40 PM   #9
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Car is still having the same issue. Cant seem to figure out what the problem is.

About 1 month ago, i remove my bumper and noticed that the CAI filter has come off (I thought this was the problem), so then i remove the CAI entirely and put the stock box and new filter back on. This didnt fix the problem.

I have taken the car to the shop and, they dont see any leaks. Shop refer me to go to Subaru dealer to have my ECU check. I went to Subaru dealer, they see nothing wrong with the ECU, and since im catless, they refer me to a Tuner shop (which i dont have one close to me)

Also, attempted refreshing ECU with OTF again, but didnt help. No CEL codes, but problem still there.

Any one has any advise on where i can look next?
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Old 01-04-2021, 03:16 PM   #10
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Is your air conditioning/defogger on?
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Old 01-04-2021, 03:51 PM   #11
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Is your air conditioning/defogger on?
No
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Old 01-04-2021, 07:39 PM   #12
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Garage
Any check engine lights? Can you use a scanner to see if there any past/pending codes?
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Old 01-04-2021, 11:14 PM   #13
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I think you're going to have to start posting some data logs for some of the guys on here to see in order to help you out.
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Old 01-05-2021, 02:34 PM   #14
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Did you have a look at your throttle body if its stuck open?

Normally you should have had CEL.
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