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Discussion Starter · #1 ·
2021 Trailblazer serial 13575.

Every once in a while, at start-up and attempting to get rolling, the brakes would initiate a rapid (about 3 pulses per second) sequence that would cause the vehicle to stutter-start. In other words the car would shudder rather significantly during the first attempt to press on the accelerator pedal and get moving. At first I thought it was a brake pad hanging up on a rotor, then I thought it might be the electrically operated parking brake. Only one event and then everything would be fine. It did seem to be related to a cold start, but other than that, I could never establish a pattern for repeatability until I started trying some things.

The first thing I recognized is that I had somehow become a start-and-get-it-into-gear kind of operator. I noticed that during my attempts to get rolling, the tachometer and speedometer were still going through their start-up routine of going to a full sweep before coming to the correct reading. That's how quickly I was in gear! Holy crap dude, let it at least go through its start-up routine! The brake shudder was not always happening, but it seemed that changing my process to waiting for the tach to reach idle did in fact reduce the number of occurrences.

During my new wait periods, I noticed that during the initial engine start I was still pressing on the brake pedal. Electronic brake booster…hmmmm. I wonder…..? So, I adjusted my startup process once again. Now, removing pressure from the brake pedal after starting. Of course, reapplying pedal pressure to be able to shift park to drive.

It has now been several months and several hundred starting events and no shudder.
 

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2021Trailblazer RS AWD Scarlet Red Metallic
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Interesting. I've never had this happen, but I also let my car warm up for a minute or two before driving off. I developed this habit a long time ago after learning that it's always best to let the vehicle warm up after a cold start in order to allow the thermal expansion of the the engine and its internal parts. My dad used to hop in his cars and start driving before all the gauges and dash lights reset and it used to drive me crazy, lol. Most of what I'm saying only applies to a cold start, but it's still good to allow everything to reboot and reset before driving off.
 

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So I mean you solved your own problem and it's not a surprise given everything that's all so electronically connected in these cars. Also you should let the engine and tach idle down on these before taking off. It may not cause damage anytime soon but these are new vehicles but that said you should let the turbos circulate oil before taking off the idle down after startup is a good indicator for a when is long enough. If your looking to drive a lot some simplistic maintenance or routines could save you thousands down the road. GM labor rates for turbo replacements out of warranty are absurb.
 

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2021 Trailblazer serial 13575.

Every once in a while, at start-up and attempting to get rolling, the brakes would initiate a rapid (about 3 pulses per second) sequence that would cause the vehicle to stutter-start. In other words the car would shudder rather significantly during the first attempt to press on the accelerator pedal and get moving. At first I thought it was a brake pad hanging up on a rotor, then I thought it might be the electrically operated parking brake. Only one event and then everything would be fine. It did seem to be related to a cold start, but other than that, I could never establish a pattern for repeatability until I started trying some things.

The first thing I recognized is that I had somehow become a start-and-get-it-into-gear kind of operator. I noticed that during my attempts to get rolling, the tachometer and speedometer were still going through their start-up routine of going to a full sweep before coming to the correct reading. That's how quickly I was in gear! Holy crap dude, let it at least go through its start-up routine! The brake shudder was not always happening, but it seemed that changing my process to waiting for the tach to reach idle did in fact reduce the number of occurrences.

During my new wait periods, I noticed that during the initial engine start I was still pressing on the brake pedal. Electronic brake booster…hmmmm. I wonder…..? So, I adjusted my startup process once again. Now, removing pressure from the brake pedal after starting. Of course, reapplying pedal pressure to be able to shift park to drive.

It has now been several months and several hundred starting events and no shudder.
Reading your posted helped me solve my similar problem. My SUV was leaping forward, or reverse, on occasion, on start-up as soon as I put it into gear and lifted my foot off the brake. I bet it is because I do it to quickly as the vehicle is still in it's start-up routine. I haven't tried this yet but I'm betting the reason is as you stated. Thanks!
 

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Discussion Starter · #5 ·
Thanks @Raffy. The reason I posted was to help the next guy.
 

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Randy, thank you for posting this! I was wondering if this was something that I should be concerned about. Tried to explain it to my dealer who said it was considered "normal". I've been using the remote start more often to avoid such complications.
 

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I have this occasionally as well. Put the car into drive and go to "ease" off the brakes, and the car kinda jumps forward a little bit and then the brakes are SUPER sensitive and catch/release a couple times until I totally let my foot off the brakes.

It seems to happen significantly more and more dramatically when the car is a couple car lengths away from a building and facing it. Not sure if there's a possibility of some kind of sensor malfunctioning. The auto-emergency braking thinking it needs to kick in for some reason?
 

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I have this occasionally as well. Put the car into drive and go to "ease" off the brakes, and the car kinda jumps forward a little bit and then the brakes are SUPER sensitive and catch/release a couple times until I totally let my foot off the brakes.

It seems to happen significantly more and more dramatically when the car is a couple car lengths away from a building and facing it. Not sure if there's a possibility of some kind of sensor malfunctioning. The auto-emergency braking thinking it needs to kick in for some reason?
This?
Vehicle leaping forward

I've experienced it a few times since posting in this topic. For me it's kind of a jolt rather than leaping, but it does get your attention. It's very random and doesn't happen very often and letting the vehicle warm up and complete it's diagnostics doesn't prevent it. One thing I have noticed is that it only happens after a cold start, first time driving the vehicle that day.
 

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This?
Vehicle leaping forward

I've experienced it a few times since posting in this topic. For me it's kind of a jolt rather than leaping, but it does get your attention. It's very random and doesn't happen very often and letting the vehicle warm up and complete it's diagnostics doesn't prevent it. One thing I have noticed is that it only happens after a cold start, first time driving the vehicle that day.
Hmmm similar I would say. For me its more that it doesn't allow me to gently release the brakes. They go hyper sensitive which makes me jolt the vehicle a couple times because it's going full power brakes to no brakes off and on very rapidly. I haven't found a consistent way to replicate it, and mine seems to happen once per week or a little less, and not always on the first startup of the day. And it seems to always be fine when cold started if I reverse out of the garage, but it may happen if I'm going forward out of the garage. Reverse never seems to be a problem.
 

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Lot's of fun little gremlins for GM to figure out. None of these weird issues surprise me on a first year model that lacks "real" world experience.
 
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