manifold vacuum

I inherited this Explorer from my late father. The check engine light was on all the time, I asked him about it to which he said "it's nothing, runs fine, don't worry about it". He drove it for years that way. When I started driving it the check engine light made me crazy so I started looking...

Should I just replace the solenoid. I cannot get to the connectors to see if there is continuity. the Ranger just keeps chugging along with this problem, insufficient air flow.