Big Jerk

[sealandsky]

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?

 

[dubyagee01]

Does it do it with the traction control disabled?

 

[iamdub]

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

 

[randeez]

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

 

[iamdub]

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]

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]

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]

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

 

[sealandsky]

Does it do it with the traction control disabled?

Yes, no change.

 

[sealandsky]

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.

 

[sealandsky]

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.

My last dealer visit was in January so they could damage my dashboard. The visit prior was for them to remove the ECM and send it to Callaway for software update. The vehicle has run great until the last couple weeks when this issue started. I feel the issue is definitely engine related, it will hold RPM's even with my foot off the gas pedal - It's kind of disconcerting.

 

[sealandsky]

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.

Almost 10K miles btw.

 

[sealandsky]

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

I don't have the scan tool, but having one hooked up would be ideal!

 

[hcvone]

Hi Rich, so it ran ok when the computer was retured from Callaway, and then started out of the blue? Have you added any gas dry to the tank? It sounds like bad gas i.e. water in the fuel. Did this happen a day or two after refueling? Does you state have 93 or just 91? Yea I would be pissed, I only have half the miles you do, but not one issue so far

 

[wildcatgoal]

Had this on a 2011 Silverado. It would also race RPMs when stopped randomly. Replaced the throttle body. Issue gone.

 

[sealandsky]

Hi Rich, so it ran ok when the computer was retured from Callaway, and then started out of the blue? Have you added any gas dry to the tank? It sounds like bad gas i.e. water in the fuel. Did this happen a day or two after refueling? Does you state have 93 or just 91? Yea I would be pissed, I only have half the miles you do, but not one issue so far

Yeah, everything was awesome, running like a champ and then one day it's like my Yukon caught the coronavirus. Slight loss of WOT power (I can feel it in the shifts - Less of a "bang" and more like a stock Yukon) and then of course the surging and jerking mentioned above. I might try some "gas dry" and also will pull off my intake this coming weekend to check the throttle body for gunk. It's time to put in a new air filter anyway...

 

[sealandsky]

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

So yesterday I watched as the car accelerated all on it's own (foot off gas pedal) to 1800 rpm in 1st gear, then the rpm's fell back to idle speed and the car shifted to 2nd.

 

[sealandsky]

Yeah, everything was awesome, running like a champ and then one day it's like my Yukon caught the coronavirus. Slight loss of WOT power (I can feel it in the shifts - Less of a "bang" and more like a stock Yukon) and then of course the surging and jerking mentioned above. I might try some "gas dry" and also will pull off my intake this coming weekend to check the throttle body for gunk. It's time to put in a new air filter anyway...

And yes - I'm running BP 93 octane and almost always buy it from the same gas station. I might mix it up and find a different one for a couple tanks.

 

[sealandsky]

I'm following up on my original post and also saying thank you to "Wildcatgoal" who suggested it might have something to do with the throttle body. I finally got some time to dig in to things and decided that the throttle body being dirty was a good bet - In spite of having only 12,000 miles on my Yukon. I bought a can CRC MAF cleaner and also CRC throttle body cleaner and first hosed down the MAF so it could dry for a while while I accessed the throttle body. Once I had a view down the bore I saw a fair amount of black gunk, so suspicions confirmed I removed the throttle body for a good cleaning. Half a can of CRC later aling with rubbing with a rag, toothbrush and finally polished with some very fine steel wool it appeared to be as clean as it could get. I read online that you aren't supposed to move the butterfly as it could mess up the calibration, but if I didn't move it I could not have cleaned it very well. The faint ring around the inlet bore is a machining mark. After reassembling everything I took it for a drive and can say that this solved my problem! This leads me to think that my next purchase needs to be a "catch can" as I'm clearly sucking a whole bunch of crap from my breathers. Or else, every 10K miles I need to do the cleaning.

 

[dsh1106]

@sealandsky

Thanks for posting this, I just did this to my 2017 trying to cure some hesitation and low speed drive ability response issues. It appears this maybe the fix, never thought at 24k the throttle body would be this dirty,

Thanks again
Scott