Good call on the no cap and rotor, I forgot those cobalts have the ecotec motors that have coil packs like the Vortec motor.

 

my wife has 1, as soon as the warranty was up i started doing all the work on it, im getting to know the car very well

 

After changing the plugs,fuel filter and cleaning out the throttle body it still drops rpms while braking but the tune up did help the idle a bit. So I took it to the dealer and after 3 hours the tech comes out and says youve got a burnt valve on cylinder 4. Luckily its covered under the powertrain warranty. Now my concern is, do valves just burn sometimes or is there an issue somewhere else that causes them to burn? Dont wanna keep going back with burnt valves if theres another issue causing them to burn in first place. And would a burnt valve cause the rpms to drop while braking? Thanks

 

Wouldn't have guessed that. I'm just taking a stab at this because I'm not familiar with that motor and how it works exactly but on most motors if a valve is "burnt" I would simply assume that valve is not seating properly which is causing the cylinder to either (A) not achieve full combustion or (B) cause air to fuel ratio mixtures that are not making that engine happy. It may have been very subtle and that is why you only felt the idle die out when stopping because I would assume its simply on the decompression stroke thus the reason you never felt it during acceleration. As I said though, I'm only functioning on the premise that this is how most engines would respond, I cannot be sure with your motor.

 

i would say a burnt valve would make it idle low, im suprised it didnt run rougher