Engine High Idle + Some Shudder at High Speeds
In the last two weeks or so, my 2006 Cobalt LS 5-speed (all stock, no mods) has started having weird issues with the engine. I got the car in Jan. 2012 with ~125k miles and it now has ~137k miles. When I first got it I swapped the air filter for a K&N filter, changed oil, changed coolant, changed spark plugs (Autolite platinum), and even bought 4 new Michelin Hydroedges (90k mile tires). I even used some throttle body cleaner earlier in the year (along with a toothbrush) to make sure it wasn't gummed up or anything. I change oil regularly (every 4k miles), have recharged the K&N filter at least once, rotated tires a month ago, etc.
Ever since I got it, if I have the A/C compressor on, when I hit the clutch, it'll drop down below 500 RPMs for a second, shudder a little, then come back up to a normal idle, and be fine until I go again, then hit the clutch (due to stop-and-go rush hour traffic).
Now it seems to have lost some power while accelerating (whether slow or highway speeds, all gears). On some days when I start it in the morning (temperatures around 65-75F outside), it will idle between 900-1100 RPMs (not constant, fluctuates back & forth) for a couple of minutes and eventually settle back to a normal idle around 700RPMs. And the problem that only seemed to occur with the A/C on, *sometimes* happens when the A/C is off and has not been turned on since starting the car. Now to be clear: since I've owned the car it has NEVER thrown any codes, stalled due to the A/C or the current issues, failed to go into gear, shifted roughly, or dropped RPMs while accelerating.
I took the car to Midas to check the suspension/steering b/c I was concerned about that. The manager said all of the suspension/steering seemed fine. He did say I may eventually want to replace both front strut assemblies, but that it wasn't needed currently.
Then a few days later I took it back to Midas to also have them check on why the engine was behaving this way. He said the transmission looked fine. He did a test to see if it might be the clutch and said there was no problem there. He had a tech do a road test and found nothing there. He said they checked O2 sensor readings on both sides of the catalytic converter and all was well. He said at this point he would recommend seeing if I had AC Delco plugs since they're tested by GM to work specifically for their vehicles. I thought I had put in AC Delco plugs, but took one out and realized I had the Autolites. Last night I replaced all of the Autolite plugs with AC Delco Iridium plugs. This morning the issue of idling 900-1100 RPMs was still there and it still seemed to not accelerate as much as previously (i.e., a month or so ago). The Midas guy did say that if changing plugs did not correct the issue, his next recommendation would be for them to do a $70 injection cleaning that he said used chemicals better than the normal stuff you get at the stores (I guess industrial strength?)
SO, having said all that, any ideas on what could be causing this issue? Could it be that I need to give it another week or so to see if the problem clears up with the new plugs? Could it be a sensor issue? Something else? And how can I narrow down the list of potential issues?
I previously had a 2000 Saturn LS1 with the same model of 2.2L Ecotec and never had such issues, so I know this ain't normal.
2006 Cobalt LS, 5-speed, K&N Air Filter, otherwise stock -- can't afford an SS *sniff*
Last edited by 2006CobaltLS; 09-26-2012 at 07:11 AM.