My daughters 190E 2.6 (engine's probably close to the 300E) had a check engine light on, and the smog check guy found the tube that connects to the EGR valve was plugged. The tube is about 1/2" OD and goes from the valve around the back of the head and connects under the intake manifold. He disconnected the tube at the valve and started the engine and felt for vacuum on the tube, and there wasn't any. He took his air hose and gave the tube a couple of good blasts and the engine sputtered a bit and the tube was cleared, he cleared the code and the car passed the smog test. I'm sure the plastic bag wouldn't have anything to do with this scenario, but this might help Cap'n Carageous.