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'15 MM ST2
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Discussion Starter · #1 ·
I've been trying to pin down this problem for a while now on my '15 ST2 103k, looking to get some insight.

Main Symptoms:
  • Car seems generally underpowered
  • Really loud exhaust when accelerating (mind you, this is the FP exhaust...)
  • Hardly any down low torque, takes several seconds for torque to really hit on WOT but still feels
    severely underpowered
  • Displays boost on gauge as completely normal
  • No CEL, no funky sounds or rattles, nothing leaking, the car otherwise runs fine.
I basically have to floor it to make it not feel like I'm driving a 6 speed prius, whereas before, the car was nothing but instant torquey, rowdy fun.


Possible Causes & What I've Tried
  1. Charge pipe popped off/has hole - (Unlikely; I've inspected virtually every vacuum line/hose and tightened every clamp.)
  2. BPV Stuck open/leaking - (Unlikely; I have mountune recirc valve and it's oiled and seated properly.)
  3. ECU limiting boost/power due to heat - (Unlikely; I have mountune FMIC and all temps are ok)
  4. Boost solenoid failed - (Unlikely? using true vacuum source. Unless the one on the turbo is bad?)
  5. MAP or IAT sensor(s) failing - (Unlikely; no CEL, all sensors are accurate, and I cleaned all of them to be sure)
  6. Loose WG or WG related - (Unsure; but absolutely no play in the WG actuator/arm when I wiggle it. Unless it's stuck open?)
  7. Other turbo problem? - (Unsure; but I looked in the turbo w/ a snake camera and the turbine appears fine and spins normally)
  8. Exhaust leak - (Unsure)
  9. Clogged/blown cat? - (Maybe?? This seems the most plausible out of the options right now.)
  10. or something else?
I've also tried changing plugs (gapped properly)


TL;DR: Car feels like it's NA but everything is in order as far as I can tell. HELP.

I'm driving myself crazy trying to fix this. Any input is truly greatly appreciated before I keep blindly ripping things apart and sticking money into it. TIA :)

Mods:
FP Exhaust, Mountune Recirc Valve, Mountune Symposer delete, Mountune MRX FMIC, True vacuum source, Mountune RMM, Mountune Sport Springs

No tune/AP
 

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2014 ST3. Garrett GTX2860R Gen 2. FBO
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How long is "for a while?". Since the weather got super hot? intercooler or not, theres a noticeable Decrease in performance with these engines in the heat.

If the above happens to not be the case, your description "exhaust got louder" points me to a turbo or wastgate flapper seating issue, but you would most certainly get a p0299. Unless the PCM is purposely limiting boost due to the high ambient temps? A clogged cat would make the exhaust Quieter and/or give you a vacuum cleaner sound with an overall performance drop and probably some codes.

Assuming the car does have an issue, it seems the next plasuible step would be to remove the downpipe, inspect the WG flapper, spin the turbo by hand/check shaft play, and check cat substrate.
 

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2017 Kona Blue ST1
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So, let start with a welcome and work our way to a, when did this issue start?

You mentioned "true vacuum" you mean for the BPV correct? Also remember that not all DTC's will trigger a CEL. However regardless if you have an aftermarket tune or not you will need a way to monitor certain PID's to determine the issues.

A visual inspection is great however an actual boost leak test above your target PSI is still required.

Factory downpipe?

J
 

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2017 Kona Blue ST1
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3,092 Posts
How long is "for a while?". Since the weather got super hot? intercooler or not, theres a noticeable Decrease in performance with these engines in the heat.

If the above happens to not be the case, your description "exhaust got louder" points me to a turbo or wastgate flapper seating issue, but you would most certainly get a p0299. Unless the PCM is purposely limiting boost due to the high ambient temps? A clogged cat would make the exhaust Quieter and/or give you a vacuum cleaner sound with an overall performance drop and probably some codes.

Assuming the car does have an issue, it seems the next plasuible step would be to remove the downpipe, inspect the WG flapper, spin the turbo by hand/check shaft play, and check cat substrate.
Beat me by seconds LOL!
 

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'15 MM ST2
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3 Posts
Discussion Starter · #5 ·
How long is "for a while?". Since the weather got super hot? intercooler or not, theres a noticeable Decrease in performance with these engines in the heat.

If the above happens to not be the case, your description "exhaust got louder" points me to a turbo or wastgate flapper seating issue, but you would most certainly get a p0299. Unless the PCM is purposely limiting boost due to the high ambient temps? A clogged cat would make the exhaust Quieter and/or give you a vacuum cleaner sound with an overall performance drop and probably some codes.

Assuming the car does have an issue, it seems the next plasuible step would be to remove the downpipe, inspect the WG flapper, spin the turbo by hand/check shaft play, and check cat substrate.
I'd say for about 6 months it's been an issue, and has gotten progressively worse. There was always a noticeable difference with ambient temps and whatnot during winter vs summer, but this is much much worse. The car just feels dead now as far as power goes. Even on cooler 50-60 degree F nights.

The point where started seemed to be when I first installed my BPV and the car pulled harder than ever before.. for about 4 days. After that, it seemed like my boost wasn't hitting quite as hard and it's just been a downward spiral from there. Not sure if that is helpful at all.

And I'll try that next. Hopefully find the culprit, or at least cross a few things off the list.
 
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'15 MM ST2
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Discussion Starter · #6 ·
So, let start with a welcome and work our way to a, when did this issue start?

You mentioned "true vacuum" you mean for the BPV correct? Also remember that not all DTC's will trigger a CEL. However regardless if you have an aftermarket tune or not you will need a way to monitor certain PID's to determine the issues.

A visual inspection is great however an actual boost leak test above your target PSI is still required.

Factory downpipe?

J
Hi, thanks for the welcome. :) I have some details of when it started in another reply in this thread.

For the true vacuum, yes. It's run off the symposer delete to the boost solenoid on the top of the engine. Also, I have checked out all the sensor values on forscan a few times and everything appears to be in working order (and no DTCs). However, I don't know off the top of my head what each value is supposed to be. I have a pretty good idea for most of them, but not all. Is there a resource somewhere with expected values/ranges for our car's sensors?

I have the factory downpipe, yes. About 103k mi on it now.

And I plan on taking off the downpipe and looking at everything (and turbo/wg). If I don't find anything, my next step will be a boost leak test.
 

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2017 Kona Blue ST1
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Do the leak test first.
 
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