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Old 03-19-2023, 09:45 PM   #1
transmutation
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Default STI bogs and almost dies after coming off throttle when warm

Hey All,


I've recently over the last few months noticed this issue and wanted to see if anyone had any suggestions I could try to correct the problem.

The car is a 2018 STI (i am original owner) at 19K miles with the following mods:

Cobb turbo-back exhaust (with GISI cat)
Cobb intake
Cobb fuel lines, rails, fuel pressure regulator and 3 port boost controller
ID1300 injectors
AEM 340 fuel pump
IAG AOS
Cobb Flex Fuel kit
Pro tune by Drive Auto Sports

Install of the fuel parts (except pump), intake, AOS and E85 kit were installed by the shop that also pro-tuned on a dyno in April 2021. They also replaced the spark plugs at that point before the tune. Car has been running fine up until roughly a few months ago.

What I've started to notice is after the car is warmed up if I come off throttle and come to a stop the idle will drop really low and the car will almost die but it hasn't actually died up to this point. Note it doesn't always do this when coming to a stoplight but has displayed the same but milder symptoms. I can recreate the issue 100% of the time after my drive home from work after i come to a stop at my driveway and prepare to back into my garage. I've also noted that the short term fuel trims will drop to -25 to -35 when the bogging happens. LT fuel trims are generally between -5 and -6 depending on RPM. After the idle drops and the car almost dies the idle will pick back up, then drop, then pick back up sometimes for a couple of cycles and then will idle fine. Car starts and runs fine otherwise, accelerates great no other issues or codes. I can hit target boost of 19 and idle is around -9.8. I've cycled through a few tanks of gas thinking that might be the issue but that hasn't helped.

I've tried replacing the Cobb air filter and also tried cleaning the MAF using CRC cleaner (did not touch anything just used the pressure from the cleaner). The MAF didn't look all that dirty but easy to do so figured i would try it. Does this sound like a vacuum leak? Ideas you all would suggest i try before throwing in the towel and taking to a shop?

Updated list of things I've tried: cleaning throttle body, disconnected battery for 30 minutes and doing idle relearn procedure. I've noticed that the car seems to be running rich so hoping a new O2 sensor fixes the issue.
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Last edited by transmutation; 03-30-2023 at 10:18 AM. Reason: Updates
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Old 03-19-2023, 11:02 PM   #2
19std
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Quote:
Originally Posted by transmutation View Post
Hey All,

...

The car is a 2018 STI (i am original owner) at 19K miles with the following ...

Cobb intake
...

...

The MAF didn't look all that dirty ...

Dude the first red flag i saw in your post was that your maf wasnt "all that dirty." At 19k on an 18, it should be spotless. If it not spotless I would be concerned about air filter bypassing somehow.

Heck i dont think i have ever had any of my cars with any miles ever have anything on the downstream side of the air filter.

After this, first troubleshooting suggestion would be to cut the battery, discharge any power in the system by hitting brakes or whatever your preferred method is, then letting the car relearn to idle. Turn it on and don't touch anything, let it get to temperature and then sit at idle for 15 minutes.

After that, my next step would be to clean butterfly valve.

Last edited by 19std; 03-19-2023 at 11:49 PM.
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Old 03-20-2023, 11:34 AM   #3
transmutation
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Thanks for your reply. To clarify, the MAF wasn't dirty but the main wire that is visible was slightly less shiny on the upstream side as compared to the downstream before it cleaned it - not sure if that is normal or not but figured I would try to rule out easy stuff first.



From what i have been reading it seems that negative fuel trims would indicate a boost leak or maybe a sensor is going out but also not sure if idle dropping really low for a split second would create the same fuel trim.
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Old 03-20-2023, 12:23 PM   #4
19std
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Quote:
Originally Posted by transmutation View Post
Thanks for your reply. To clarify, the MAF wasn't dirty but the main wire that is visible was slightly less shiny on the upstream side as compared to the downstream before it cleaned it - not sure if that is normal or not but figured I would try to rule out easy stuff first.



From what i have been reading it seems that negative fuel trims would indicate a boost leak or maybe a sensor is going out but also not sure if idle dropping really low for a split second would create the same fuel trim.
If theres no code, no knock, and no symptoms anywhere else, I wouldnt worry about a sensor yet, try the easy stuff like reset idle, clean butterfly valve and go from there.

But definitely make sure your filter is not passing, that is scary stuff! It sounds good from what you decribe.

Edit: check to see if your making full boost, and vacuum is above 8 or 9 inches of water.
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Old 03-20-2023, 03:40 PM   #5
Jedi03
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vacuum leak somewhere would be my thought
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Old 03-22-2023, 10:58 AM   #6
transmutation
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I'm sure this is a dumb question but to clean out the throttle body its as simple as pulling off the intercooler and scrubbing it out from the back? I can't find a specific walk-through for the STI or EJ WRX but some guides indicate with an electronic TB that its not a good idea to move the butterfly valve by hand as that can damage the motor. Just want to make sure i don't cause other problems.
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Old 03-22-2023, 12:29 PM   #7
19std
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Quote:
Originally Posted by transmutation View Post
I'm sure this is a dumb question but to clean out the throttle body its as simple as pulling off the intercooler and scrubbing it out from the back? I can't find a specific walk-through for the STI or EJ WRX but some guides indicate with an electronic TB that its not a good idea to move the butterfly valve by hand as that can damage the motor. Just want to make sure i don't cause other problems.
Its not a dumb question, yes pull off inter c. Use brake clean or whatever your choice of super fast evaporating stuff is and a tooth brush. Wipe clean with lint free rag.

Dont mess with the plate, its connected to electric gears. It may have a little play allowing you to get a little behind it, but don't force anything here.

You can get it to flap, but this is outside the scope of this thread lol.
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Old 03-26-2023, 08:53 PM   #8
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Update:

Cleaned throttle body (wasn't visibly dirty) and did a quick look-over for leaks (didn't find anything obvious) and then did the idle relearn procedure after leaving the battery disconnected for over 30 minutes. Still has the same issue where after reving to around 2000 or so and letting off the throttle car will decrease revs and then almost stall but catches itself. Short term fuel trims still hit -25 to -35 during the bog.

Leaning towards an O2 sensor issue now .
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Old 03-26-2023, 08:53 PM   #9
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duplicated post
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Old 03-26-2023, 09:10 PM   #10
19std
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Quote:
Originally Posted by transmutation View Post
Update:

Cleaned throttle body (wasn't visibly dirty) and did a quick look-over for leaks (didn't find anything obvious) and then did the idle relearn procedure after leaving the battery disconnected for over 30 minutes. Still has the same issue where after reving to around 2000 or so and letting off the throttle car will decrease revs and then almost stall but catches itself. Short term fuel trims still hit -25 to -35 during the bog.

Leaning towards an O2 sensor issue now .
Agreed, I have had an O2 sensor do this to me in my FB (idle issues, hesitation), but it still threw a code, an AF code, not an O2 code, but it still threw a code...

Would be my next step if your sure theres no leaks.

Swap in a new one....
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Old 03-26-2023, 09:20 PM   #11
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What is your vacuum pressure at idle? Before you go off dropping a 100 bucks on a sensor...
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Old 03-26-2023, 10:28 PM   #12
transmutation
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Recommendations on which one to try first? I did find a video of an 2019 STI who had very similar issues and he replaced the front one and that fixed it (
).
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Old 03-26-2023, 10:49 PM   #13
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Quote:
Originally Posted by 19std View Post
What is your vacuum pressure at idle? Before you go off dropping a 100 bucks on a sensor...

I remember seeing about -9.8ish on this last go around during the idle re-learn but before this issue -10 (+/- a few decimal points if i recall) was pretty common. This is based on the stock boost gauge (also an Ultra gauge I've been using recently) so not sure how precise that is.

Edit: I decided to go ahead and order a smoke machine as well just to rule out a leak.

Last edited by transmutation; 03-26-2023 at 11:12 PM.
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Old 03-27-2023, 01:09 AM   #14
19std
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Quote:
Originally Posted by transmutation View Post
I remember seeing about -9.8ish on this last go around during the idle re-learn but before this issue -10 (+/- a few decimal points if i recall) was pretty common. This is based on the stock boost gauge (also an Ultra gauge I've been using recently) so not sure how precise that is.

Edit: I decided to go ahead and order a smoke machine as well just to rule out a leak.
Lol, wel you have a smoker now. Guess I'm too late to say 9s sounds ok lol.
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Old 03-27-2023, 01:17 AM   #15
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Originally Posted by transmutation View Post
Recommendations on which one to try first? I did find a video of an 2019 STI who had very similar issues and he replaced the front one and that fixed it (Symptom of a Failing Front Oxygen | O2 Air Fuel Sensor | WRX STI - YouTube).
I can't remember, was years ago on mine, aren't they the same? If so you obviously only need one, I think you need a special tool to fit the sensor, get copper high heat manifold grease, i think it's called jet lube, don't get any on the sensor but a good but not excessive coat on the threads, whatever you do, don't strip/ cross thread the threads. If you do you'll need a helicoil repair, *ask me how i know lol*.

At 9.8 vac, it's probably not a vac leak, and if a hose blew off somwhere you'd have boost issues.
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Old 03-27-2023, 10:18 PM   #16
Eegoor
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Default Any luck yet?

Any luck with your STI? I am following because I just installed my Cobb Stumble kit and 1050X injectors, new MAF, new front 02 sensor, and not throwing any codes but...

It wont build boost above 1-3psi and it falls on its face at 3k RPM. My AF learning 1 doesn't even read anything, it stays at 0.00. My smoke machine should be here tomorrow. I hope it is something simple.

Installed the following before all this and it was running fine
1. IAG AOS
2. DW300 pump
3 Cobb SF intake
4. GS 3port boost controller
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Old 03-27-2023, 11:38 PM   #17
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Originally Posted by Eegoor View Post
Any luck with your STI? I am following because I just installed my Cobb Stumble kit and 1050X injectors, new MAF, new front 02 sensor, and not throwing any codes but...

It wont build boost above 1-3psi and it falls on its face at 3k RPM. My AF learning 1 doesn't even read anything, it stays at 0.00. My smoke machine should be here tomorrow. I hope it is something simple.

Installed the following before all this and it was running fine
1. IAG AOS
2. DW300 pump
3 Cobb SF intake
4. GS 3port boost controller
Why did you change the O2 sensor? Why a new MAF? Was there an issue?
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Old 03-28-2023, 07:14 AM   #18
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Quote:
Originally Posted by 19std View Post
Why did you change the O2 sensor? Why a new MAF? Was there an issue?
I changed my 02 sensor because I put a new header on and I couldn’t get the old sensor out of the exhaust manifold. I put a new (OEM) MAF sensor on because it was the original one from 2008 and I am getting a tune. I didn’t want any issues during the inspection. Please keep in mind it was running fine after I replaced the 02 and MAF. It just started doing this after I installed the injectors and Cobb stumble kit.
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Old 03-28-2023, 09:04 AM   #19
19std
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I think you have an unrelated issue, probably a fueling issue for you.

If the problem started after cobb regulator and injectors, then that is probably where to start.

Can you verify good fuel pressure after that cobb regulator you installed. If so, then go to injectors, pull out and try old ones.
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Old 03-28-2023, 09:06 AM   #20
19std
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What tune are you running?
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Old 03-28-2023, 09:23 AM   #21
Eegoor
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Unhappy no boost

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Originally Posted by 19std View Post
What tune are you running?
I am running a cobb stage 2+ 91tune at the moment.
I checked and fuel pressure is 43.5psi according to my AP

I ordered a automotive smoke machine last night. It should be here today. I can swap the Cobb stumble kit out when I get home and rule that out. I noticed my AFL1 reads 0.00 when I start it up and it never changes.
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Old 03-28-2023, 10:02 AM   #22
19std
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Quote:
Originally Posted by Eegoor View Post
I am running a cobb stage 2+ 91tune at the moment.
I checked and fuel pressure is 43.5psi according to my AP

I ordered a automotive smoke machine last night. It should be here today. I can swap the Cobb stumble kit out when I get home and rule that out. I noticed my AFL1 reads 0.00 when I start it up and it never changes.
Im not sure at what point that pressure measurment is, you know before or after the regulator...

But yes, just work backwards to the last time you had a working car lol, and slowly troubleshoot from that point to isolate most likely cause.
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Old 04-01-2023, 09:26 PM   #23
transmutation
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Update:

Replaced both front AFR (first) and rear O2 sensors and unfortunately no improvement. Maybe a slight improvement after holding 2k revs and letting off but still car almost dies and ST fuel trims drop to -25 to -35 as the revs come down and the engine almost dies. I did pay attention to the vacuum which was around -9.4 or so at idle so seems like not a huge vacuum leak but that is next.

My smoke machine doesn't want to work (yah china quality) so will work on troubleshooting and checking for leaks next. Wish me luck.

Last edited by transmutation; 04-01-2023 at 09:49 PM.
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Old 04-01-2023, 09:26 PM   #24
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dup post again.
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Old 04-02-2023, 11:09 AM   #25
19std
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Man im going with you got dirty injectors at this point, dump some fuel system cleaner in there and then next tank dump sume techron in there. Maybe a dirty fuel filter.

Are you using 93? Like only shell, marathon, bp top tier gas? Not convenient mart rusted underground special 89+ right?

Really don't think it's a vacuum leak, and if you find a tiiiny one with your smoker almost positive issue will persist.
Yeah I was skeptical of O2, it throws codes.

Are there any codes? Edit* any codes that happened any time, maybe you cleared one somewhere??

Throw up some logs if you have any.

Last edited by 19std; 04-02-2023 at 11:21 AM.
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