That is fast idle solenoid, it is in circuit with IP timing advance and its purpose is to have faster, lower emissions warm up of cold engine. The part is getting hard to come by, especially at reasonable cost. If you can't find used or rig up a return spring, they cost about $150, last time I saw one.
It is nice to have fully operational, but is not really needed to use truck. One of mine has been non functional for years, truck runs fine, just slower, smokier warm up.
Edit; Also possible solenoid is fine, and the temp sensor on back of head that runs that circuit is faulty. If this is the case, it should be fixed, otherwise timing advance will be stuck on even if fast idle solinoid removed. If sensor is working, you should be able to hear timing back off a few minutes after cold start.