UVCH
 

What are the symptoms that would lead to changing these? Do they wear out?

Typically the connector comes unhooked from the gasket, and ford sells a insert that helps hold them together. The symptoms for this would be a CEL, running rough(can be intermittent), and if resistance is affected it will fail a buzz test for one or more cylinders.

As far as the harness wearing out, when you have glow plugs that pull too many amps the harness and connectors can melt and make them impossible to separate from each other, and will ruin connector pins, as well as a small section of the engine wire harness. Symptoms would be a hard start, or running rough for a few minutes when cold.

A very common failure results from oil intrusion into the connectors. The oil will get between mating terminals (inside the connector) and cause resistance. Resistance equals heat. Once the terminals get warm, they loose tension on their mating terminal and begin to melt.

Usually the connector inside the valve cover gasket will come halfway unplugged and cause glow plug concerns and in extreme cases, IDM codes for injector circuit problems. Keep in mind that once an injector circuit is suspected to be open by the IDM that the IDM will kill the entire bank of injectors.

The 95-97 trucks seem to be less susceptible to connector separation.

Good call, Dave.

Mine just all of a sudden stoppped running on 4 cylinders... turned out to be burned out pins and a melted connector on the engine side of the harness that plugs into the VC gasket plug.

Replaced both harnesses inside the VC, VC gasket harness, glowplugs on that side, and the burned out connector on the outside for about $200