Part-Throttle Surging Under Load

[chapelheel]

Looking for diagnostic help!

I have a 2015 Fiesta ST that I bought as an insurance salvage. It appears so have full Cobb Stage 3 kit - intake, intercooler, piping, full exhaust, and tune, but the previous owner removed the Accessport so I don't have the ability to revert to a stock tune or read parameters in real-time. I can however use the Cobb functionality like switching to 'Eco' mode with the cruise control buttons so I know it has at least a Cobb OTS tune.

My issue is a surging behavior that happens most frequently in 6th gear when going uphill under medium throttle, generally between 3k-4k rpm (see video below). It only occurs under partial throttle in a very specific range; if I mash the pedal or let off it goes away. I've had the car for a year and this behavior was not present for the first 6 months; it first occurred when I was driving up a mountain pass and had not touched anything under the hood for quite a while. No codes, no CELs, and it will also sometimes do the surging in lower gears, maybe more often when the engine is cold. Otherwise the car runs fine.

https://www.youtube.com/shorts/5hJkBOPFfNU

Any ideas?

 

[FiestaSTdude]

My car does the exact same thing! I have a thread that I'll link about it, but I still don't have a definite solution. I've noticed that my dizzy tune makes the problem much less noticable. It's worse with the Cobb tunes. I feel like it's the boost getting released for some reason.

https://www.fiestastforum.com/threads/power-pulsing-while-acceleration.28219/

Also, welcome to the forum from another NC Fiesta ST owner!

 

[chapelheel]

My car does the exact same thing! I have a thread that I'll link about it, but I still don't have a definite solution. I've noticed that my dizzy tune makes the problem much less noticable. It's worse with the Cobb tunes. I feel like it's the boost getting released for some reason.

https://www.fiestastforum.com/threads/power-pulsing-while-acceleration.28219/

Also, welcome to the forum from another NC Fiesta ST owner!

Thanks for the welcome! Maybe the issue is a North Carolina thing?

I looked at that thread and am following it. The electronic throttle vs demanded torque might very well be the issue; I wish diagnosis was as easy as throwing on a $125 tune from @Jason@dizzytuning -- well, it is but I'd also have to spring $600 for the AP3 + tune. As long as it's not hurting anything and is just an annoyance I'll probably wait for an AP3 to pop up on the secondary market unless some extra cash pops up first!

 

[rcnut14]

Thanks for the welcome! Maybe the issue is a North Carolina thing?

I looked at that thread and am following it. The electronic throttle vs demanded torque might very well be the issue; I wish diagnosis was as easy as throwing on a $125 tune from @Jason@dizzytuning -- well, it is but I'd also have to spring $600 for the AP3 + tune. As long as it's not hurting anything and is just an annoyance I'll probably wait for an AP3 to pop up on the secondary market unless some extra cash pops up first!

they pop up on here often, that's how I got mine.

there are usually a few on facebook as well

 

[Jabbit]

Thanks for the welcome! Maybe the issue is a North Carolina thing?

I looked at that thread and am following it. The electronic throttle vs demanded torque might very well be the issue; I wish diagnosis was as easy as throwing on a $125 tune from @Jason@dizzytuning -- well, it is but I'd also have to spring $600 for the AP3 + tune. As long as it's not hurting anything and is just an annoyance I'll probably wait for an AP3 to pop up on the secondary market unless some extra cash pops up first!

It's $595 for AP plus Tune from Dizzy fyi. Might be worth kicking that off sooner rather than waiting.

 

[chapelheel]

UPDATE:

I got ahold of an AccessPort and got a Stage 2 Tune from @Jason@dizzytuning - no more surging! Looks like problem solved.