Reader Question: My 1990 Ford F-150 4×4 Pickup with the 5.0 EFI begins to misfire and stumble only after reaching full operating temperature. It runs great when cold.
I pulled the trouble codes, and only got PFE/EVP/EVR and power steering pressure switch codes. I replaced the EGR position sensor, distributor mounted ignition module, and tested the MAP sensor and TPS, and they test okay.
When the truck starts to act up, I immediately pull off the road and check trouble codes and nothing new pops up. I am out of ideas.
I would rule out the basics for a misfire spark plug wires, spark plugs, cap, and rotor before I did much else here. It is common for a spark plug wire to break down and cause a miss when the engine and the wire are hot.
Might also want to check the EGR valve itself for vacuum leaks or excessive carbon deposits and replace as needed.
Also, make sure you have vacuum to the EGR valve.