dsp4848
Well-Known Member
- Joined
- Oct 26, 2014
- Threads
- 32
- Messages
- 389
- Reaction score
- 115
- Location
- Hutto, Texas
- First Name
- David
- Vehicle(s)
- 2015 GT PP Magnetic
- Thread starter
- #1
So I'll preface this by saying that I've had this issue over the past several months and have just been putting up with it. It's still driveable, just annoying. This will probably be a wordy post, but I want to be detailed enough to share what I've noticed and tried throughout this process and hopefully learn a thing or two along the way. I contacted Lund (my tuner) last year and tried to get the issue solved to no avail after a long string of back and forth emails with him. My mods are in my sig.
The issue:
Over the past few months, I've had a lot of part throttle surging and occasionally bucking. The surging is noticed when I'm giving the car about 1/2 to 3/4 throttle immediately after an upshift and also when starting out in first gear. It's like when I push the gas pedal to 1/2 to 3/4 throttle, the car isn't making the demanded power until about a second or two later. The power then comes in all at once, almost like turbo lag. The bucking is a little more dramatic, resulting in the car bucking like a bronco for a few seconds until it resolves itself. This has only been noticed at part throttle (usually about 1/2 throttle or more) immediately after an upshift - I can recreate this the most when going from second to third gear. Driving casually around town at low throttle won't cause the issue - it's only when I'm giving the car at least about 1/2 throttle. Giving the car more throttle just makes the bucking worse, so I have to let off the gas for a couple seconds and then try again. WOT is fine, I don't really have any complaints here. Also highway speed cruising is great - my fuel trims at highway cruising are usually +-5%.
A few things I've noticed and some notes:
-When I was troubleshooting with Lund last year, the first suspicion was that I have a vacuum leak. I've replaced vacuum hoses and fittings, un/reinstalled the MAF, un/reinstalled the throttle body, un/reinstalled piping, and tried running the car with and without the UPR catch can. If I have a vacuum leak, it's the sneakiest son of a bitch ever because I cannot find it. Further, I've visually inspected all vacuum hoses and everything looks great. No cracked hoses and all fittings are super tight. I have clamps on all connections. I have a 2018 manifold here, so I guess I could just rip out the stock manifold and reseat literally everything and see if that helps, but I'll save that as a last resort.
-During the surging and even the bucking process, I can hear that the Tial bypass valve is closed. No boost leak here.
-Anywhere from 3/4 throttle to WOT, my LTFTs are pegged at about -16%. Why is this?
-Transitions between on and off of the throttle seem a little jerky. For example, LTFTs will go from -20% to +5% when transitioning from 0 throttle to about 10% throttle.
-Driving at a negative incline with light throttle, the LTFTs and STFTs are very negative. On average, I think about -15% each. To me that seems a little off.
-Don't know if it's related, but I've had a weird cold startup idle issue as well where the car hunts around between about 600-800 RPM for several seconds before bringing the idle speed up. I've noticed that during this time, my LTFTs are very negative (around -20%). Not sure why the car would be pulling fuel at startup? From what I've researched, I believe this is during open loop, because after several more seconds the STFTs come in (entering closed loop), adding fuel. After the STFTs start adding fuel, the idle becomes normal.
-With the UPR catch can installed, my fuel trims do seem to change a bit. They tend to be more negative (pulling fuel due to rich condition) without it installed. Initially, Lund and I thought this was the culprit (possible vacuum leak), but the car exhibits the same issues regardless of the catch can being installed or not.
So, with all of that said, has anyone else experienced this? Does this sound like a tuning issue? Something else perhaps? I will post a datalog on here as soon as I can to help provide info. I'd appreciate any insight into this issue, so I can finally solve it. Thanks!
The issue:
Over the past few months, I've had a lot of part throttle surging and occasionally bucking. The surging is noticed when I'm giving the car about 1/2 to 3/4 throttle immediately after an upshift and also when starting out in first gear. It's like when I push the gas pedal to 1/2 to 3/4 throttle, the car isn't making the demanded power until about a second or two later. The power then comes in all at once, almost like turbo lag. The bucking is a little more dramatic, resulting in the car bucking like a bronco for a few seconds until it resolves itself. This has only been noticed at part throttle (usually about 1/2 throttle or more) immediately after an upshift - I can recreate this the most when going from second to third gear. Driving casually around town at low throttle won't cause the issue - it's only when I'm giving the car at least about 1/2 throttle. Giving the car more throttle just makes the bucking worse, so I have to let off the gas for a couple seconds and then try again. WOT is fine, I don't really have any complaints here. Also highway speed cruising is great - my fuel trims at highway cruising are usually +-5%.
A few things I've noticed and some notes:
-When I was troubleshooting with Lund last year, the first suspicion was that I have a vacuum leak. I've replaced vacuum hoses and fittings, un/reinstalled the MAF, un/reinstalled the throttle body, un/reinstalled piping, and tried running the car with and without the UPR catch can. If I have a vacuum leak, it's the sneakiest son of a bitch ever because I cannot find it. Further, I've visually inspected all vacuum hoses and everything looks great. No cracked hoses and all fittings are super tight. I have clamps on all connections. I have a 2018 manifold here, so I guess I could just rip out the stock manifold and reseat literally everything and see if that helps, but I'll save that as a last resort.
-During the surging and even the bucking process, I can hear that the Tial bypass valve is closed. No boost leak here.
-Anywhere from 3/4 throttle to WOT, my LTFTs are pegged at about -16%. Why is this?
-Transitions between on and off of the throttle seem a little jerky. For example, LTFTs will go from -20% to +5% when transitioning from 0 throttle to about 10% throttle.
-Driving at a negative incline with light throttle, the LTFTs and STFTs are very negative. On average, I think about -15% each. To me that seems a little off.
-Don't know if it's related, but I've had a weird cold startup idle issue as well where the car hunts around between about 600-800 RPM for several seconds before bringing the idle speed up. I've noticed that during this time, my LTFTs are very negative (around -20%). Not sure why the car would be pulling fuel at startup? From what I've researched, I believe this is during open loop, because after several more seconds the STFTs come in (entering closed loop), adding fuel. After the STFTs start adding fuel, the idle becomes normal.
-With the UPR catch can installed, my fuel trims do seem to change a bit. They tend to be more negative (pulling fuel due to rich condition) without it installed. Initially, Lund and I thought this was the culprit (possible vacuum leak), but the car exhibits the same issues regardless of the catch can being installed or not.
So, with all of that said, has anyone else experienced this? Does this sound like a tuning issue? Something else perhaps? I will post a datalog on here as soon as I can to help provide info. I'd appreciate any insight into this issue, so I can finally solve it. Thanks!
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