Jaya5602

Hi guys, Ive got a 2008 Cobalt LS automatic. While braking the rpms drop from around 900-1000 to 500ish and instantly goes back up as soon as the car comes to a stop. Its very noticeable as it seems the car wants to stall but it never actually does. No check engine light, lights dont dim or anything like that it just acts like it wants to stall. So while sitting at the light after this happens it idles rough but it has never died. It takes off from the light fine plenty of power no hesitation at all. Im looking for a few suggestions where to start looking so I dont have to pay dealership prices as the car is no longer under warranty. Thanks

PremiumBlend

Sounds like a vacuum leak to me.

Chevy lover for life

2008?

Should be still under warranty - right?

Take it to the dealer.

Jaya5602

Thats what I was thinking too. I sprayed all the vacuum lines I could find with carb cleaner while the engine was running but came up clean. So right after this I drove to the store and the check engine light comes on so i went to a parts store and they ran a diagnostic and it said something like miss fire to cylinder 4. Am I correct to assume this means change the plugs and wires and the fuel filter for good measure?

Jaya5602

Im pretty sure its a 3year 36000mile warranty and ive got 54000 on it. So that would be no warranty right?

PremiumBlend

54k miles, probably a good idea to do some basic maintenance... especially plugs. You could probably do cap and rotor too just to be on the safe side and DEFINITELY a fuel filter but this still doesn't explain your idle problem while braking. I'd go buy or rent a vacuum gauge and check your vacuum pressure. Something sounds a little fishy here, so if I were you I'd check with a gauge and not the carb cleaner method. Even though your engine should respond to the carb cleaner is may not, so you'll want to be sure using a gauge. Just my 2cents... that's where I'd start.

therewolf

Hi Jaya5602,

Welcome to the Chevy Forums.

I found out the hard way that those "100,000 mile" Iridium spark plugs GM is so happy about are only good for 50,000. At 57k they were getting ready to melt right off the side of my engine.

So in any event I'd say you're due for a new set of spark plugs, to say the least.

Jaya5602

Im going to change the plugs and fuel filter in the next few days when I get the time. Now a new problem just happened a few hours ago, coming to a light same thing, rpms drop while braking come back up after car stops but while sitting there with brakes firmly applied the car starts lunging forward like im hitting the gas and brakes at the same time. Ive read this can be caused by the a/c compressor kicking in but the a/c was off could this be more evidence of a vacuum leak?

therewolf

This is only a guess here, but I'd say the original problem is probably gradually getting worse.

2wd-K5

throttle postioning sensor causes the pulsating problem,if the plugs arent the problem, the best way to tell an ecotec needs plugs is a rough cold engine start and it shakes a bunch and when doing the plugs DONT FORGET THE DIELECTRIC GREASE! there is no cap and rotor and all the plug boots (no wires) attatch to the coil packs on top of the plugs, and plus gm doesnt offer a 36k mile powertrain warranty, thats the bumper-to-bumper you are thinking about, your warranty on the powertrain should be good to 100k miles as long as you kept up on your maintenence properly