Mechanical help...08 GMC Sierra

[MattCP97]

I just recently bought this truck, 2008 GMC Sierra 1500 4x4 5.3 gas, 235,000 miles. It has been running "ok" until just recently when I started to have a blinking engine light, and it started to run rough, especially in speeds greater than 60 mph.

Before having these issues though, I would like to state the engine codes I have been throwing and the problems, while it was still up to drivable standards. The oil pressure reads low to the point the oil light comes on when close or at an idle, but when I gas it or am going down the road, I read a perfect oil pressure. I figured that was a sensor, because I have been driving it for months like this. The codes are, P0307 (cylinder 7 misfire), P0455 (EVAP Emission control system leak), and P0449 (EVAP Emission Control System vent valve/solenoid circuit malfunction). It has always had a gas consumption issue, at first, I thought it may be because it was a jacked-up truck. I average 11-12mpg with a 12" lift on 285 tires. I just want to make sure I give all the information needed to help diagnose this.

I currently have this vehicle parked, but it is the best vehicle I have, (I know right.). I have researched to the bottom of every article and have not gotten any closer. I have ideas, but I need some help. Thank you in advance.

[Stealthee]

Flashing CEL is a major misfire. Could be caused by ignition issues or could be a plugged/faulty cat.

285 isn't your tire height it's the width. Would need the sidewall ratio as well as rim size to have an idea on tire size, but regardless the EVAP issues are more of a nuisance and less to do with how its running.

The oil pressure is concerning though. Oil pressure should go up when you gas it, but it being low enough to lock the oil light is not good news. Your bottom end likely needs attention. It's not if something happens but when it's going to happen.