Addressing check engine light for DTC’s P1391 and P1393: Glow Plug Circuit Low Input Bank 1 & 2.
Started by replacing all 8x Motorcraft glow plugs, but after checking resistance of each plug, found them all to be acceptable, low ohms. Went ahead and checked resistance of new glow plugs and installed all new.
Next, replaced both valve cover gaskets with units from Napa Auto Parts (p/n 600-3501) which integrates the wiring harness between the gasket and under valve cover harness (UVCH) to eliminate the 2 connectors under the valve cover.
For valve cover installation, there are tabs molded into the gasket to indicate the bottom orientation of the gasket.
It is important to manage the wiring under the valve cover to ensure wires do not get pinched and short circuited above or below the gasket.
A pinched injector wire causes a miss and can fry the glow plug relay and cause the truck to run terribly.
Replaced factory glow plug relay (GPR) with a White Rogers unit https://amzn.to/3rlKmB5, and eliminated the shunt by connecting the ring terminals to the GPR post.