Vehicle: 2014 Chevrolet Sonic, 1.8L 4 CYL, Automatic Transmission
Problem: The customer’s concern was that the MIL was on, the engine idled rough and there was a whistling noise coming from under the hood.
Details: The technician pulled two DTCs: P0171 (Fuel trim system lean) and P2097 (Post catalyst fuel trim system high limit). NOTE: P0171 sets when the ECM determines if the fuel control system is in a lean condition, based on the filtered long-term and short–term fuel trim metric. P2097 sets when the ECM determines if the post catalyst HO2S fuel control system has been unable to adapt to a lean exhaust gas condition for too long.
After reading about the same exact same problem in ALLDATA Community, the tech performed the initial test outlined in the post. First, he inspected the engine for any aftermarket equipment or modifications related to the intake or exhaust systems. Everything was original equipment and in good shape. Next, he checked to see if vacuum was present at the PCV orifice external port on the camshaft cover. There was vacuum present. He sprayed carburetor cleaner on the camshaft cover and the fuel trim instantly went negative. After removing the cover, he noticed a crack in the cover.
Confirmed Repair: The technician replaced the camshaft cover, cleared the codes and test-drove the vehicle. The new camshaft cover fixed the problem. NOTE: This is a common issue with these vehicles. We have also seen a cracked intake boot located between the mass air flow sensor and the intake manifold cause the same problem.