Ignition Timing Trouble
 

The issue has to do with the trailing advancement with the distributor on a '83 with manual transmission. The timing for the leading spark plug is 0 degrees ATDC, which is at spec, but the adjustment for trailing timing stops at 15 degrees TDC where spec is at 20 degrees. Could this mean that the Ignition Control Module is failing, or is there a problem with the vacuum control line for the trailing advance on the distributor. The help is much appreciated.