I also had trouble with the P1491 code cleaned out the passages and checked out the EGR valve which seemed OK but the code kept coming up which I eventually found to be caused by the linear pot in the egr valve which was failing intermittantly. to check it properly you need a voltage source and an analog meter. Coil is terminals 6+ and 4- 6.1 ohms resistance. Starts moving the plunger at 4 volts and fully opens at 8 volts. On a new EGR I got the following readings. retracted 1-2 1155 ohms. 1-3 3362 ohms. 2-3 4230 ohms. Extended 1-2 3086 ohms. 1-3 1413 ohms and 2-3 4222 ohms. The readings should vary smoothly as the plunger moves back and forth otherwise the pot is no good and the egr valve needs replacing. Hope this helps some of the people that have been struggling with this particular code.