[Dodgewoman]

2003 dodge ram 1500 5.9l v8 (190k miles)
So changed spark plugs, thermostat, temp sensor, wires. The truck overheated this past summer and we changed all of the above within the summer. This fall we started having a misfire which has gotten progressively worse. Cleaned everything with seafoam changed all fluids and filters check spark plugs cleaned and gapped but still having the issue of misfire. Long story short the new thermostat is a performance thermostat, which keeps the engine cooler. I'm starting to wonder is the thermostat is causing the misfire or is it possible that it's the fuel injector. -dodgewoman

 

[Richard66]

First I would change back to stock thermostat to see if this fixes it. If not swap #5 coil pack with another like #1 and see if the code follows, if it does probably a bad coil pack, if not could be an injector or the dreaded lifter failure.

 

[142504]

Usually, if a lifter has failed, you'll hear the clicking pretty clearly, and there's no mention of noise. And the '03 5.9's aren't prone to that problem the way the 5.7's seem to be.
So if the engine is not making any weird noises a dodgy coil pack(s) or injector(s) is more likely
You don't say if the truck is throwing any codes. If it is, and the code indicates a misfire on a specific cylinder, swapping coil packs to see if the misfire follows is the way to go with that, like @Richard66 says and if it's not that, then an injector issue is the next most likely thing, and if it's not that, then it would be time to do a compression test on that cylinder and compare it to compression on others.
If the misfire is hopping around and not restricted to specific cylinder(s), it could be a number of things including a vacuum leak or something not right with the fuel supply pressure or injector control or crank positioning sensor etc etc.

If you installed a lower temp thermostat as a corrective for an overheating problem, I don't think that's wise, though I see no reason it would cause a worsening misfire. The engines are designed to run best in a temperature range that the OEM thermostat provides. Running deliberately cooler in a competition setting is something different, of course.

If you were overheating with the stock thermostat, assuming that the thermostat was working as it should, there will be another problem you should want to track down. A radiator full of bugs would be the most obvious and easiest to fix, or perhaps there's something restricting radiator flow internally. Or the cooling fan isn't working properly. Or the water pump is compromised somehow. Worst case would be a head gasket leak, which could also cause a misfire as it gets worse.

Let us know what you find out.