redFLcobalt

i have 28K miles on my 06 LS 5 speed cobalt its a stripper car i use for my highway commute. the last 10K miles i noticed a drop in fuel millage from 34 mpg to a worse of 28 mpg. also i noticed the idle surges in parking lot speeds when i push in the clutch. it drops down to 400 rpms and will surge over 1K rpms and eventually settle unless i give it gas. i ignored it for the most part. now all i do is drive 88 out of 90 miles 5 days a week on the highway. 70-75 mph. took the car to the dealership and they replaced the fuel injectors. some recall or something. the fuel milage seems like its better but the car is STILL surging at idle. its intermittent. somedays it does it, some days it doesnt. after they replaced the injectors i took it out and before i got out of the parking lot it started surging again. gave it back to them and they reflashed the pcm. still having issues. the dealership pulled the "well there are no codes so we dont think anything is wrong" i asked them to check the IAC and IAT sensors - they didnt know why that would cause any issues. i got frustrated and left.

anyone have any ideas?

Coblasts

Bad Idler pulley or something else on the drive belt over loading the engine at idle.
You could always pull it off for a minute and check everything.
Just a suggestion.

redFLcobalt

that would make sense if i was having problems with idle surge at all times in low gear. i dont. its intermittent. some days it does it, some days it doesnt.

Superdave420

Mine surged bad as well, exactly as you have described. anyway I just dealt with it, then I got a CEL about 3 weeks ago so I brought it in for service, they just told me that the "power train module needed reprogramming" whatever that means. lol but when I got the car back the surge is all gone, although my idle is now higher, it idles at a solid 1K now. leaves the line really nice now. I have no idea why the dealership had to "reprogram the power train module.." but the surging is gone now.