I suspect the EGR valve is stuck open, and the service manual (page EC-582) also suggests this. You have 2 valves. The EGR valve is towards the back of cylinder head 1, and feeds gasses into the cooler. This is one I suspect is sticking. The EGR bypass valve is bolted to the side of of your intake elbow, under the Cummins badge.
Take it apart and clean it. Might was well clean the other valve while you're at it, as this whole proccess is a pain in the ass. If your valve is trashed, let me know. I removed my EGR and have no use for it, so I'll sell and ship it to you for dirt cheap.