First thing to do is drain/fill the engine oil if the dealer didn't do it. Leaving the oil diluted with gas is certainly not good for the engine.
Perform the ECU code readout again yourself with a portable scan tool to see if any fault codes are set; in particular a P0300. The tool can be purchased at most auto parts stores. Also most auto parts stores will perform the code readout free of charge. Post the actual codes here on the forum so that we may be able to help you further. If there is one or more fault codes set, they can help point to the malfunction.
There may be a major intake system vacuum leak if you're getting a P0300: To check the intake system for a vacuum leak, attach a vacuum gauge to a full vacuum source. With the engine fully warmed up, the reading at idle should be 18 - 20 InHg. At 3,000 RPM, it should be 21 InHg. If readings are under 18 InHg, check the intake manifold nuts to make sure they are tight. The gasket may have failed; spray a water mist at the gasket to see if the gauge reading changes. Also check the intake plenum bellows at the throttle valve and at the MAF for cracks or loose clamps.
Perform the ECU code readout again yourself with a portable scan tool to see if any fault codes are set; in particular a P0300. The tool can be purchased at most auto parts stores. Also most auto parts stores will perform the code readout free of charge. Post the actual codes here on the forum so that we may be able to help you further. If there is one or more fault codes set, they can help point to the malfunction.
There may be a major intake system vacuum leak if you're getting a P0300: To check the intake system for a vacuum leak, attach a vacuum gauge to a full vacuum source. With the engine fully warmed up, the reading at idle should be 18 - 20 InHg. At 3,000 RPM, it should be 21 InHg. If readings are under 18 InHg, check the intake manifold nuts to make sure they are tight. The gasket may have failed; spray a water mist at the gasket to see if the gauge reading changes. Also check the intake plenum bellows at the throttle valve and at the MAF for cracks or loose clamps.
