2007 CTS 2.8L - Rough Idle, Multiple Codes, Low MPG
 

Long time listener first time caller.....

Having an issue with my 2007 CTS 2.8L with 158K miles. I am trying to correct the rough idle and bad mpg. The rough idle/misfire is from idle RPM up to about 2000 RPM. I assume it is still there after 2k but harder to notice.

The history is about 2 weeks ago I noticed the idle becoming rough and the mpg dropped to 20. I had a set of plugs I had never installed so I installed them. The mpg went up to 22.5 and the idle improved.

After a week, the idle started to get rough again and the mpg dropped back down to 20. Then the check engine light came on. I went to the parts store and they pulled several codes. Some were labeled History, History MIL, History Current, and CONFIRMED.
U1016 Lost Communications with Engineer Control Module
P1793 Wheel Speed Sensor
P0300 Random Multiple Misfire Detected
P0303 Cylinder 3 Misfire Detected
P0523 Engine Oil Pressure Sensor Circuit High Voltage
P0522 Engine Oil Pressure Sensor Circuit Low Voltage
P0521 Engine Oil Pressure Sensor Range/Performance
P0303 Cyl 3 Misfire (CONFIRMED)

The P0303 was listed as confirmed so I targeted that first. I read a post that said low fuel pressure could cause a misfire. I check the pressure and it was 55psi as the post said it should be. I moved on to replacing the #3 cylinder coil pack that from what I found is located on the middle passenger side. I also replaced the air filter which was well overdue. The CEL went off immediately and the idle is better but still rough. After 50 miles I am at 17.5 mpg. After the tank get to 1/2 I plan to run a bottle of Techron fuel injector cleaner through it.

Other than more coil packs being potentially bad, I'm curious if there is something else I should be checking.

As far as the other codes, I replaced a wheel bearing at 151K miles so I believe that code may just be old. I have no idea how long those stay in there.

Any tips on what to do next would be appreciated. Thanks!