I bought my truck with used engine installed. 54k on the 5.0 SCrew LKQ replacement. I had a CEL on cyl 3 and 5, took it too the dealership, and they replaced the coils. Problem got better but I could still feel it. Replaced the plugs last and reviewed the torque pro data.

In park and I rev it, I get a bunch of misfires on 3 and 5. But if I drive it, it runs good and on my spirited test drive which included two WOT romps I had 8 misfires on cyl 3 and 5 on cyl 5. What would cause a bunch of misfires on an engine with no load and hardly any while the engine was driven and floored a couple times?

 

Tried checking/replacing the plugs? Make sure they are gapped correctly

 

update: truck seemed to run fine, but i hooked up a trailer to it and it misfired all the way home. 127 misfires on cyl 3 and 80 on cyl 5 on a six mile trip. unhook the trailer, engine runs good with no misfires. Confirmed the next day when i returned the trailer. i ended up canceling a trip to pick up a car because of this. Any ideas why it would do this sometimes and not the other?

 

Because something is slightly off, and pushing the engine harder brings the problem out to play. Plug gap would definitely fit that kind of situation. Or a cracked plug.

 

The gap is .050, and the plugs are new. Could an injector cause these issues?

 

The porcelain could have cracked when you torqued the plugs down. I think the shop manual says to torque them to 11 ft/lbs if I recall correctly.

Those 2 cylinders giving you issues are not sequential in the firing order, and they're not physically near each other. So crank sensor or head gasket are likely out.

If you suspect injectors, you could swap with another cylinder and see if the misfire follows. It may not be a bad idea to do a compression test.