I have a 1994 Ford Ranger with 4.0 V6 Automatic. The problem is when it idles it idles low and surges up/down and pushes a very small amount of black smoke puffs and smells like gas. I have installed new coil, new wires, new plugs, new ect, new crankshaft position sensor, and new maf. It has new intake manifold, new injectors, new alternator and it still does the exact same thing. Nothing is phasing it. I ran the key on/engineoff test and got egr codes and a map sensor code, then ran the engine running test and got 998 and 126 code. The 998 code says that its basically a hard fault/ have no idea. I do know this is a donor engine out of another year not sure what year, but i know when i ordered the coolant temp sensor the only ones that would plug into it are the years 1990-1993. Now heres my other question, why would the computer give me a map sensor code when truck doesnt have a map sensor? It has MAF sensor, and why egr codes? The truck doesnt have egr and never came with it. I do know I've unhooked battery and replaced the maf and its like it doesnt even recognize it? Could it be the ecm/computer bad or needs flashed?

 

And btw, Im sure if the ecm/computer came out of an late '80s model truck, the truck wouldnt even run.

 

It's bad enough to try to determine the cause of a problem with the original equipment on the vehicle, but when some turkey goes ahead and starts switching engines and stuff we have no idea what we are working with.
That said, I would suggest that you pull the vacuum line off of the fuel pressure regulator and see if it is full with liquid gasoline , it should be dry. If it is , that would explain the excess fuel and idle problem because the diaphram in the regulator is ruptured and the regulator needs to be replaced.
The electronics in those years were not that complicated as are today's vehicles.
The info that I have available doesn't even show a 126 code for that vehicle.
Somebody went and switched stuff and managed to get it running, I think ,,,!
A hard fault can be anybody's guess because we don't know what was switched.
That year vehicle is not supposed to have a crank sensor, maybe you mislabaled the post, I don't know.
All the vehicles of those years have an EGR system and without being there to see it, maybe the people that switched the engine removed it or disabled it. Sorry for the long lecture, but there are a few more things you can check. Let us know if the fuel pressure reg was the problem.