Low Speed RPM Bounce

[Bladee]

Hey guys,

2015 67k non PP. Just got back from a roughly 45 minute drive, did a few pulls of course but nothing too crazy. When i was nearing home i noticed an odd noise, a repeating sound that seems like either air being sucked in or blown out, on and off maybe twice a second? kind of hard to explain, almost like blowing a raspberry but slower and deeper. the tachometer was also fluttering maybe 50 RPM either direction but no major hesitation or shudder in the car itself. basic research on the forum tells me the TC is a common issue with low RPM shuddering and hesitation. The car still drives fine and has plenty of power it only was noticeable right when it starts to build boost and then the noise goes away. Only modifications currently is an Airraid intake and the VTA mod with chair leg caps...classy.

I did do a quick check with a flashlight and all the airflow related components i could see with a flashlight from the engine bay looked OK. Tommorow i will try and go over everything more thoroughly. This car is still under the 90 day purchase warranty and honestly im going to end up taking it to the dealer and have them fix it but for my own edification I still want to know what went wrong, why it went wrong and how to fix it. I'm a mechanical guy by trade but ill be the first to admit i know almost nothing about cars, but i plan to keep this thing for a long time so its in my best interest to try and learn as much as i can. thanks

 

[kryptto]

my thoughts, check your blow off valve diaphragms, ever borescope the intake valves from the MAP sensor hole in the top of the intake?

The MAP sensor might need a cleaning, or the airbox sensor - either can have a quick spray from local store.

Airraid intake and the VTA mod with chair leg caps...classy.

Much like you I would prefer a "real" cap - however common and cheap. If you keep her - call Steve at UPR for Catch Can options, caps etc. Tell him his neighbor sent ya. I have a '14 so parts should be same.

Check the following, and make sure all these vacuum lines are not loose or dry and splitting.

 

[Bladee]

my thoughts, check your blow off valve diaphragms, ever borescope the intake valves from the MAP sensor hole in the top of the intake?

The MAP sensor might need a cleaning, or the airbox sensor - either can have a quick spray from local store.


Much like you I would prefer a "real" cap - however common and cheap. If you keep her - call Steve at UPR for Catch Can options, caps etc. Tell him his neighbor sent ya. I have a '14 so parts should be same.

Check the following, and make sure all these vacuum lines are not loose or dry and splitting.

tonight I’m going to check all of the above and re install my factory air filter/BOV hoses so Carmax doesn’t deny my warranty service lol. I plan on ordering a catch can from UPR after I make sure the intake valves are clean, I have the GH tune coming with the MAP, stat etc and I assume with more boost more oil will find it’s way into places it shouldn’t so it’s a good investment. If I find anything tonight I’ll post some pics

 

[kryptto]

tonight I’m going to check all of the above and re install my factory air filter/BOV hoses so Carmax doesn’t deny my warranty service lol. I plan on ordering a catch can from UPR after I make sure the intake valves are clean, I have the GH tune coming with the MAP, stat etc and I assume with more boost more oil will find it’s way into places it shouldn’t so it’s a good investment. If I find anything tonight I’ll post some pics

Well if you keep her start with reading the maintenance bible.

 

[Bladee]

Well if you keep her start with reading the maintenance bible.

I have it printed out lol

 

[Bladee]

Reviving my thread, the situation has evolved:

Never really figured out the root cause of my issues, didn’t find any major boost leaks or disconnected hoses. Put the stock intake back on and it was a little better to drive, could still tell the issue was there but not as pronounced as it was with the Airraid.

Well, yesterday I finally installed my PPE downpipes and found that the rear turbo wastegate was disconnected from the linkage and was hanging open. Likely the root of a lot of these issues? I reconnected the linkage and took it for a test drive and sadly it drove even worse, threw overboost code for the rear turbo, would not build any boost except for a little down low. I could hear the car hissing loudly when accelerating.

My assumption is that the solenoid or actuator that opens the wastegate isn’t working, and perhaps someone disconnected the linkage on purpose for a cheap fix. Can anyone with more experience tell me if I’m on the right track? I disconnected the linkage and let the wastegate stay open and the car drives like it pretty much always has. I guess I’ll try to pull the rear turbo and replace the actuator but I’m really not excited about doing that with no lift my neck and back are killing me from doing the downpipes on a concrete floor

 

[kryptto]

Likely the root of a lot of these issues? I reconnected the linkage and took it for a test drive and sadly it drove even worse, threw overboost code for the rear turbo

Yup, I will wait for others to weigh in - sounds like that turbo needs to come off and address the wastegate actuator that is not working. I admit I have a friend with a lift and it was the only way I was doing the DP's - and I am still sore from a year ago. I hope you checked the plastic charge pipes for splits or cracks. Infamous problem.

Off subject, did you possibly check the health of the turbine by checking its wiggle? Your car has twice the miles of mine, but that some decent amount of carbon, but I could be making something out of nothing. Did your other turbo have that level of carbon on it?

 

[Bladee]

Yup, I will wait for others to weigh in - sounds like that turbo needs to come off and address the wastegate actuator that is not working. I admit I have a friend with a lift and it was the only way I was doing the DP's - and I am still sore from a year ago. I hope you checked the plastic charge pipes for splits or cracks. Infamous problem.

Off subject, did you possibly check the health of the turbine by checking its wiggle? Your car has twice the miles of mine, but that some decent amount of carbon, but I could be making something out of nothing. Did your other turbo have that level of carbon on it?

perhaps the vacuum line on the back of the actuator has popped off i didnt have the time to put the car back up and pull everything off to check it...no leaks or cracks in the plastic piping (will be getting the metal pipe kit soon anyway). both turbines were rock solid and had no wiggle, similiar carbon buildup.

Also, can anyone explain how the wastegates work with a higher boost tune? i have the 3 bar tune kit from gearhead, and i thought wastegates were designed to limit the amount of boost you can make. since these wastegates are analog and pre set to a certain spring tension (i think?), how is the tune and MAP sensor able to make more than factory boost with factory wastegates? maybe you guys are upgrading them and not telling me

 

[Bladee]

OK

the line that goes to the charge piping from the boost solenoid on top of the engine popped off, which i check almost every time i pop the hood . reconnected that and the wastegate linkage and the problems went away. no more stuttering, no more overboost. car pulls as good as ever. i still think i have a few minor boost leaks and the car still seems to be making a lot of pressure on the boosted side, perhaps due to the intake and downpipes on a stock tune. BOVs blow off hard if you even tap the throttle and kind of whistle even when your cruising...i did just install the Torque Solutions BOV adapters maybe they are affecting it idk. but the worst of my issues seem to be gone for the time being

 

[MrBrownPants]

I have practically exactly what you are describing. It sounds like the the BOV is rapidly opening and closing and the car's rpm's jump and the car surges a bit. I am trying to diagnose what is going on. The line that goes to the charge piping from the vacuum solenoid that popped off on you, is that what kryptto was referencing? I have checked that multiple times and everything always looks connected. I guess I will double check everything again. I have EPP hot and cold pipes, with vta BOV's, so I don't know if that makes a difference.

 

[kryptto]

so the diaphragm on the BOV could be cracked as @MrBrownPants points out check that out.

 

[Bladee]

I have practically exactly what you are describing. It sounds like the the BOV is rapidly opening and closing and the car's rpm's jump and the car surges a bit. I am trying to diagnose what is going on. The line that goes to the charge piping from the vacuum solenoid that popped off on you, is that what kryptto was referencing? I have checked that multiple times and everything always looks connected. I guess I will double check everything again. I have EPP hot and cold pipes, with vta BOV's, so I don't know if that makes a difference.

check everything related to the wastegates i guess...few days since i fixed it and the problem has completely gone away. make sure the linkages are connected to the wastegate arms sticking out of the turbos, make sure the hoses on the back of the wastegate diaphragms are connected, all boost solenoid hoses as well. I have no idea the mechanism behind why the BOVs open and close like that when the wastegate is stuck open, and this issue seem to have a variety of cuases and solutions. good luck to you

 

[MrBrownPants]

Found the issue! Thank you so much for your advice revolving around the wastegates. On the rear turbo, the wastegate linkage was disconnected. Now I just need to find out what size snap ring goes there.

 

[Bladee]

Found the issue! Thank you so much for your advice revolving around the wastegates. On the rear turbo, the wastegate linkage was disconnected. Now I just need to find out what size snap ring goes there.

That’s awesome brotha. Pretty funny we had the exact same issue. Harbor freight sells C clip kits with a ton of sizes, good to have around

 

[Ryan Beach]

Yeah with my car I'm having the 50-100 rpm jump when I'm just cruising sometimes. I have an intake leak on the front turbo I believe so I'm hoping replacing that would fix the issue. I don't think I'm having the wastegate issue but if the pipe doesn't fix it. I know what to do now.