Big Jerk

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sealandsky

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I'd like to tap in to the brain trust's vast pool of experience so I can lead my GMC dealer in the right direction (they can't lead themselves anywhere).

Vehicle: 2019 Yukon Denali - Callaway
Weather Conditions: Ambient temperature 20-50 degrees F
Vehicle Conditions: Not first run of the day, vehicle was at full operating temp (210 or so), shut down for 2-3 hours, re-start coolant temp 160-170
Fuel: BP 93 Octane

Situation: After driving to work and leaving my Yukon parked for a couple hours I'll restart to leave on an errand. After starting I'll drive out of my parking lot at idle or just above idle power. Either as I approach the street or after I turn on to the street and again apply very light power the engine will deliver a short hesitation and then a big jerk and surge of power - More power than I should be getting from throttle pedal position. In the old days before fuel injection I'd seen similar behavior from vapor lock in the fuel system. Is this what's happening? I'm also wondering if I'm getting bad fuel, it's happened over the last 4 tanks of gas and I buy my gas at the same station every time. One other thing I've noticed is a slight drop-off in power - seat of the pants feel is about 10-15%. I'm not getting a check engine light or any other unusual indications.After driving a couple hundred feet the issue clears up. I'll also add that when the issue occurs, it feels like I have no control over the throttle - in other words I'll get both less, then more power than I should receive with the selected pedal position.

Anybody have a theory that I can bring to my useless dealer?
 
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iamdub

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Sounds very much like the old school problem of gunk in the throttle body. Engine gets warm and the gunk melts so it's fine while driving. Park the car and the gunk settles to the bottom of the throttle body against the throttle blade and cools and solidifies, slightly sticking the blade in place. Hours later, the car is started and when the pedal is pressed, it's firmer and doesn't respond until it breaks that glued throttle blade free. It breaking free resulted in the throttle suddenly snapping open, making the vehicle surge. Once the "seal is broken", it would be fine until the car sat for a while again. Years ago, my then-girlfriend's Accord ('00 with cable throttle body) would have a "stiff pedal then surge" in the mornings. I told her to press the throttle pedal to unstick it before starting it until I could get to her house that weekend to fix it. Just needed to clean the TB and cap off the vent hose that routed oily air from the valve cover to the intake tube so it wouldn't gunk up again. In the past, a cable-driven TB would transfer this stuck feeling to the pedal and you'd feel it. Now, with drive-by-wire, you won't feel it. You would press the press the pedal and it'd feel like it always does, but the servo that drives the throttle blade would be what's bearing the brunt. The computer is trying make the servo open the throttle the amount that coincides with how much input it sees from the pedal. The servo motor finally snaps the blade free and you get your surge. You never felt the resistance or the "snap", just the result of a throttle being instantly cranked open.


These direct-injected engines are generally quite clean on the intake side. But, since yours in under boost, maybe it gets a little more oil mist blown through the intake?


As for diagnostics, the dealer could compare the inputs from the TAC module (the "pedal input") and throttle position sensor. If the throttle is sticking, there would be a split second where the TPS is reporting a lower throttle angle than what the TAC is commanding. They would just have to hook up their scanner to your Denali while it's cool and have it start recording the data from startup, to that first or first few throttle pressings.


You lead on that yours doesn't do it at first start. But maybe since the revs are higher at first start during the warm-up, it's already unsticking the throttle so it's not noticed then.


Those are my first thoughts. Other guesses are:

1) Your TB is just jacked up (faulty spot in the rheostat?) and it takes the PCM a little bit to adjust for it.

2) You have a leak in your intake tract and the PCM has adjusted for it at idle, but has to correct when under throttle

3) Dirty MAF confusing the PCM
 

iamdub

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could it be the transmission hunting a gear as you roll up to the stop/turn and then finding 1st/2nd?

Good guess. With so many gear options these things have, it probably does shift a time or two even at those low speeds. Confused shifting is a classic GM trans programming issue and these things can change with temperature. So yeah, you might be onto something.
 

wjburken

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Good guess. With so many gear options these things have, it probably does shift a time or two even at those low speeds. Confused shifting is a classic GM trans programming issue and these things can change with temperature. So yeah, you might be onto something.
I think some very good possibilities have been shared. One more thought and it is definitely a long shot, but it might be something to do with the accelerator position sensor. Might put a scanner in it and track accelerator position and see if it’s changing in sync with your foot. Again, a long shot, but I’ve had that issue on older vehicles.
 

gat0r

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how many miles do you have on it?

id think more related to possibly the trans or torque converter


will be interested to hear what dealer eventually finds.



eta:
have you had it in for service recently prior to these issues?

could also be the dealer was an idiot & did a recall update, followed by OEM tune on your ECM... thus wiping out the somewhat limited tuning that callaway does.
 

Larryjb

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If you have access to a scantool that can record live data, try to record what happens during the big jerk.
 
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sealandsky

sealandsky

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could it be the transmission hunting a gear as you roll up to the stop/turn and then finding 1st/2nd?
I don't think so - This is all happening in 1st and my foot doesn't even need to be on the gas pedal. Besides, I've never had the 10 speed "hunt" for a gear, it's pretty much flawless.
 

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