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Discussion Starter #1
A few months ago I bought a used 2008 Cobalt LT with 31,000 miles & automatic transmission. Just a couple weeks ago I noticed that right after starting the engine the car will jerk forward and make a "thunk" sound even through it's still in park. It only does it after the car has been sitting for at least a few hours; never having been just shut off. Any ideas? :sad:
 

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this "thunk" is how loud ?

when I start mine without foot on the brake, it will roll forward and make a noise,
but I would not even come close to call it a thunk.
I'll look at work cause I believe there was a tsb, something like,
customer complains that vehicle rolls forward when starting the car.
said something of this is considered normal operation for starting without
brakes applied.
just throwing that out there.
 

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Discussion Starter #3 (Edited)
The "thunk" & movement is different every time. Sometimes barely noticeable; just a slight "click" and little to no movement. Other times the car actually rocks forward and the "thunk" sounds like a REALLY hard shift. It's like the car is trying to move forward put can't b/c it's still in park.
 

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Discussion Starter #5 (Edited)
My driveway is perfectly flat. I do sometimes use the e-brake when parking, especially if parking on an incline.

After searching the entire internet I've got a couple thoughts:
- clutch/transmission plates are sticking
- transmission control module

Any other thoughts?
 

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Maybe you have a broken motor mount.
 

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Discussion Starter #7 (Edited)
Combined with the above problem, I've noticed is that the car seems to shift weird. There are times where I'll think "why did it shift there. I wouldn't have." When casually crusing down the road at 35-40-ish, it'll upshift again (when I wouldn't) and cause the rpms to go really low, like 1800. As soon as I give it a tiny bit of gas just to maintain speed it'll downshift. As soon as I lift off the gas to coast it'll upshift again... like it doesn't know which gear it wants to be in. It also seems sluggish to accelerate; I really have to stomp on the gas for it to downshift and accelerate.

Between the issue I first posted and the one described here, I really wonder if it could be the TCM? Is the TCM making the trans act goofy?
 

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Bry1105,

Do you plan on having your local GM dealership look into this concern? Depending on when your Cobalt was originally purchased, you may still be under your powertrain warranty. We can look into this further for you. If you are interested, please private message us your name, VIN, phone number, and the name of your GM dealership.

Erica Tiffany
Chevrolet Customer Care
 

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Discussion Starter #11
I would be ok with taking it to a dealer if I knew for sure the problem would be fixed. My concern is that they would be throwing parts at it guessing, which gets expensive for me.
 

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Levee
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I'm thinking a mount. When you start it, put your foot on the brake pedal, drop it into drive, let it shift into drive then go right to reverse, let it shift then back into drive. If you hear a nice clunk when doing that, you have a bad motor mount.
 

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Discussion Starter #13
Wow, everyone seems to think its a mount. My question is, how would the car actually move if its just a mount?

Would a TCM or control soleniod cause the odd shifting?
 

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hmm i might actually have a bad mount, hear a nice loud clunk sometimes when i shit into reverse
 

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I have good motor mounts on my 2007 LT sedan. Before I put new struts and strut mounts on my car, my car did this intermittently. It starts fine, but just every now and then it used to kind of jerk forward and make a slight erk sound.
 

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Discussion Starter #17 (Edited)
...so start with that. :smile1:
Good call. Simpler & cheaper. If it works, great. If not, then try something else.

I have good motor mounts on my 2007 LT sedan. Before I put new struts and strut mounts on my car, my car did this intermittently. It starts fine, but just every now and then it used to kind of jerk forward and make a slight erk sound.
So are you saying the motor mounts fixed the problem? What do the struts & strut mounts have anything to do with it?
 

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Levee
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A bad mount can cause a lurch as the motor rocks pretty bad when first staring and shifting into gear. The Park gear on Automatics are just trans locks. Why do you think when you put it in park and take your foot off the brakes it rolls slightly. That is normal lock "play", thus can be felt by bad mounts.
 

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I don't know much about the mechanical aspect of cars, but when I had my blazer it did something similar. Problem was a u joint... on the drive shaft I think? It took me like and hour to fix. It was 4 wheel drive though, so Idk if a cobalt even has one

2005 cobalt base model coupe
 

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Levee
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I don't know much about the mechanical aspect of cars, but when I had my blazer it did something similar. Problem was a u joint... on the drive shaft I think? It took me like and hour to fix. It was 4 wheel drive though, so Idk if a cobalt even has one

2005 cobalt base model coupe
U-joints are only on awd/rrwd vehicles. Anything that uses the rear axle to propel the vehicle.
 
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