My 2009 LS Cobalt just started bogging under light throttle over the past month or so and seems to be slowly getting worse. It stumbles, almost to the point where it feels the engine shuts off, for about a second and then comes back and bucks for a little bit, say 3-4 seconds. Under heavy throttle, everything seems fine/no loss of power up to the red line. It seems to only happen shortly after I up-shift, give it light throttle (10% or less), and in between 2000---> 4500 rpms. It is also much worse when the engine is cool, making me think it is something in the fuel system (??) but seems strange then that problem goes away under heavy throttle. Running the A/C also seems to make it worse, but I`m not sure if that is just the drag from running the compressor. I`ve already tried changing spark plugs, oil, and the air filter, and it is not making any sounds or anything. Any thoughts?

 

Fuel filter?
Try a bottle of seafoam in the gas too.

 

I recommend Fuel filter change. Have you ever changed it or when was it changed last. That would be where I start first. After that the regular maintanence stuff. Plugs, air filter, throttle body cleaning, etc., oh and SeaFoam.

 

3 things you should do before you throw money at parts.

1)Scan for codes! Even if you don't have a check engine light that does not mean you don't have pending codes stored in the ecm. You can get this done for free at most automotive chain stores.

2) Purchase or borrow a diagnostic vacuum gauge and check what your vacuum looks like at idle. Any vacuum gauge can work for this test realistically though. Depending on your elevation, you should be sitting somewhere around 18-20inHG of vacuum at idle with the rpms relatively steady.

3) If the vacuum looks fine, purchase or borrow a diagnostic fuel pressure gauge that attaches to the shrader valve. Make sure the rail pressure is holding between 50-60psi with the key in the "accessory on" position and engine is off. With the engine running pressure should remain relatively close to the static reading.

If there are any codes, have the person wherever you went to have it scanned look into what the code could mean.

If test 2 fails then you obviously have a vacuum issue somewhere

If test 3 fails then you may very well have a fuel filter issue, fuel pressure regulator issue, or fuel pressure sensor issue

 

Thanks for the help guys. It just recently flipped the CEL, so I guess I will go get the code scanned.

 

clean your TB.

 

Any new developments, Hawk312? Hope you were able to get to the bottom of this all.
Sarah, GM Customer Service