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This engine uses a split intake runner for each cylinder. It has a long skinny runner for low RPM use and a short fat passage for high RPM use. The IMRC is a system to control the secondary "butterflies" in the intake manifold. When the engine is started the Powertrain Control Module (PCM) sends a command to the IMRC actuator to close the "butterflies". Above 3500 RPMs, the PCM takes the command away and the butterflies open, using spring pressure. The actuator has a position sensor inside that tells the PCM if the IRCM actuator reacted correctly. The P1518 code means that when the PCM commanded the butterflies closed, it didn't see them move. While watching the IMRC Linkage, start the engine and watch for movement. If there is no movement, remove the upper intake to gain access to the lower intake. Check the IMRC shafts for carbon or other reasons for the system to stick. Grab the shafts and try to move them manually.
Give me a few minutes and I will send you some pictures.
The upper intake is the top part of the engine.