My friend’s 2001 DII has an odd Cylinder 1 misfire (P0301).

What’s unusual to us is that the engine runs smooth as silk when cold. The misfire happens and the truck starts to stumble only when the engine reaches normal operating temp. That seems odd, to us at least.

The truck has relatively new plugs and 8mm wires. If the coil was failing we’d expect misfires on the paired cylinder (Cylinder 6). My friend thinks it’s is a problem with the fuel injector. I’m encouraging him to try swapping in a different ignition wire for Cyl 1 (I have a couple extras) before going to the trouble of removing the upper intake to replace or try to clean the injector.

What do y’all think? Is it meaningful that the misfire appears only once the engine warms up?

 

In this economy, l'm sure gas stations are not changing their pump dispenser filters as often as they once did. Especially, cut rate stations, who typically purchase bargain basement poorly refined fuel, where it is even more important to change them regularly. Heck, during the covid years and after...we couldn't even get the correct filters..and had to eventually replace every injector in every gas engine vehicle in the fleet (15 plow and maintenance vehicles). Was just starting to get the correct filters before l retired last summer.

Replacing injectors is basically becoming a tune up requirement.

So, with all of that being said...you kind of get an idea where l would start looking...

 

I would start by swapping the spark plugs 1 to 3 unless he has relocated the coils and then I would start by swapping the wires, whichever is easiest.. Thermal expansion can cause ignition system failures. Very rare that injectors are bad and if they are bad it will show up in fuel trims and if it is bad enough to cause a misfire it will show up in the oxygen sensor voltage.

 

Have you taken a compression test? I believe sub 90 psi is about where a miss fire cel will be activated due to a weak cylinder.