I recently swapped a 99 ranger 3.0 motor with a v vin into a 98 ranger with a u vin. The truck runs very rough and smokes badly, I thought changing the computer would resolve the issue but it wouldn' even start the truck with the v vin computer on it. So my question is do I need to swap wiring harnesses or will changing the intake resolve my issues?

 

No you should be fine, just different fuel injectors

3.0l V is flex fuel and has bigger fuel injectors to run ethanol

3.0l U is gas only and has regular injectors

All computers are programmed for the engine size, fuel pressure expected and fuel injector flow.
So if you install larger injectors then engine will get flooded out because computer is programmed for regular injectors.

So you need to match injectors to the computer you will be using

Since you ran the V computer with the V injectors that should have solved the problem, if it didn't then you had other issues before/after that

I would reinstall the 1998 computer in the 1998 Ranger and then swap injectors to the U models

The 1999 V computer would also want to "see" a Fuel Sensor hooked up, it told computer if there was gasoline or ethanol in fuel line and what % of either

 

Thank you sir, I'll swap the injectors tomorrow after work and reinstall the u vin computer and see if that resolves the issues. It makes sense though because it honesly just seems like it is getting flooded with gas.