Constant, moderate pain in old root canal. swollen nearby lymph node, neck pain. dental x-ray clear.

See Endodontist : We cannot diagnose these types of situations over the internet. More is required than merely an x-ray to determine the cause of your symptoms. The swollen lymph node/neck pain could be totally independent of the tooth pain but still require treatment. See an Endodontist to evaluate tooth (root fracture?/ CBCT scan (3D x-ray)? extract tooth? If no dental infection,also consult with physician.

Get it removed: ALL root canals are chronically infected to some degree; many are very low grade and don't cause problems but constant pain and a swollen node suggest this has gotten to the point where the only viable option is to remove it (and it may not show up on X-ray). It is important to find a dentist who will remove the entire infected root. Then consider an implant. See https://tinyurl.com/ycm9yc9w

See specialist: An endodontist is your best bet regarding diagnosing the issue. Even though the radiograph appears clear, there may be residual debris or bacteria in the canals, or the restoration may even be leaking. All the best..

Failing root canal: Have tooth evaluated br an Endodontist, a root canal treatment specialist. There may be residual untreated auxiliary canals, a fractured root, improperly sealed restoration. Swollen node and neck pain may or may not bessociated. Time for an evaluation by a specialist.