I picked up a used '08 SS T/C a couple weeks ago for a new daily driver. It has 40K miles on it.

The issue is that on cold starts, it seems to run very rough and hunt for idle. Doesn't seem right to me. It drives fine, but the engine RPM really fluctuates when its first started, almost like it hunting around. Once its been run for a minute or so, it stabilizes out and runs smoothly.

Anyone experienced this or know what may be the cause??

 

spark plugs??

 

Haven't even checked them.

Are these engines known to go through plugs in 2 years/40K miles??

I'm an ASE Tech and I know GM LS engines very well, but these turbo Ecotecs are new to me.

 

Could be plugs, could be carboned up. Direct injection so no fuel past the intake valves, you might want to Seafoam it through the intake then change plugs.

 

Check plugs since thats pretty easy to do. If they look good, replace the front O2. GM Parts Direct has them for $60. Or take it back to the dealer since you just bought it

 

Could also be a leak in your charge pipes.

 

Do you know if its tuned? If you mess with the low values in the desired-air-load table, they'll go nuts in cold weather starts

 

^ECT SA table will cure that I have found

 

More details sir! (PM Me? )

 

Ygpm

 

I know the bracket that holds the charge pipe near the valve cover had already broken, someone zip tied it down. Checked the charge pipes briefly though, and didn't see any leaks.

 

The car is not tuned for that intake. That is why you have a rough idle.

 

I definitely don't think its tuned, feels stock and it only boosts about 14PSI.

Could the Hahn intake be wreaking havoc on my cold starts since the fuel/airflow tables are not compensated for the amount of air coming in?? Once its warmed up and in closed loop its fine, it probably because the wideband O2 compensates for the airflow difference by altering the fuel trims.

Ok, glad to get a definite answer. So looks like I should look into a tune in the near future.

I know some of these turbo cars are tuned very rich from the factory, do you know if a tune would improve the fuel economy at all?? It hasn't been that great (cold weather, winter fuel, ect.).

 

This one is tuned very lean from the factory at WOT, in the midrange anyway. From 2,000 rpms to 4,500 rpms it is commanded at 14.0-13.8 A/F.

 

Must be because its Direct Injected.

My friends stock STI is pig rich from the factory, it goes through fuel like a truck and the exhaust tip is always completely covered in carbon, along with the rear bumper.

 

DI makes a lot of soot. Our pipe will be black even at 14.0:1

I missed that you have the same intake as I, shifts the fuel trims 10-12%, you def need some tunage.

Even tuned, our intake causes the LTFT's to wander around -1.6 to around +2.3 best I can get in the weather changes around here. Flows well and runs great though.

 

My care is completely stock and it searches around for idle also. I had the intake vacuum line disconnected for a few weeks so I think that caused it. My care idles rough even at full temp. It's driving me crazy. It almost feels like the car is missing badly at times but only at idle.

 

Well, any vacuum leak will make a car idle roughly, depending on the size of the leak. If you had a line disconnected, I'd say that you were lucky it idled at all.

 

Plug the intake vacuum line back on, unhook your battery for a few minutes, and then hook it back up and start the car and let it re-learn idle.

Driving around with vacuum lines unhooked is an extremely bad idea, and even worse on a boosted car, where it can spell disaster if the A/F leans out badly enough.

 

To the original poster - is the car hard to start after a fuel fill up? It could be the evap purge solenoid is bad. Nothing to be terribly concerned about and a common failure in all cobalts, especially the SS. Symptoms include cold start idle seeking (idles high then low etc) and typically will take longer to start after filling up at the pump. Its a cheap part and easy to change, if the original owners warranty transfered to you just get it replaced at the dealer. Eventually it will pop a code and a CEL if it is the purge solenoid.

Check the same thing. However, the other much more severe option is a ring land cracked (seems to be happening a lot recently). Perform a full compression AND leak down test - it will tell you immediately if this is the case. This goes for the original poster as well. For both of you, if you are under warranty, just take it in - GM tech II tool can tell if the Solenoid is bad in about 10 seconds.