I've been struggling with an EGR valve issue for a few years now. I've replaced the EGR valve, the solenoid and all the tubing. We've also checked for blockages where the EGR valve is attached and in the aluminum tube that runs to the manifold. All components function according to the two manuals I have, so at this point I'm at a loss of what to do next. Does anyone have any more suggestions on how to troubleshoot, or what could be going wrong?