You need to understand how the ECU tests the EGR system!
With the engine at operating temperature the ECU monitors manifold vacuum via the Piezo pressure sensor in the EZL (electronic ignition) and when declerating will open the EGR valve, It is looking for a change in manifold vacuum corresponding with the operation of the valve.
If the vacuum line from the manifold to the EZL module is bad it won't see this pressure change and will set the light. Change the the vacuum line.
Good luck and please post your fix.
Mike