1998 Dakota 3.9L V6 Magnum (Not 100%sure the transmission)

Ok... I am totally stuck.
I have dropped $700 on this thing to try and figure out what is wrong. No light. No codes. No hints as to how to fix what is wrong with it. Right now, I'm basically throwing money at it and hoping it fixes it, because it refuses to give me a code so I will at least have a starting point.
Replacement parts so far... in order:
Ignition Coil and Wire
Distributor Cap and Rotor
Crankshaft Position Sensor
Idle Air Control Valve
Throttle Position Sensor
Fuel Pump
MAP Sensor

When we go to start it (when it actually decides it wants to start), the RPMs go up to around 1200 and then down to 300. Sometimes it'll spudder and die. Other time, it'll pick back up to around 800. If it dies, it'll start right back up. What causes this?

In the last few months, it has had intermittent starting issues. Id go to start it and it would crank and crank, but wouldn't turn over. We couldn't figure out what was going on, since it would start most of the time, but would screw up every now and then and wouldn't start no matter how many times we cranked it when it messed up. Last month, as I was driving to work, it started missing at the stop light about a mile before work and it wound up dying when I pulled it into my normal parking spot at work. When I got off work, I started it and it ran just fine, until I got about a mile from work. It started backfiring on the way up a hill and died at the top. I had to pull it into a parking lot to get it out of the road. My old man started giving it a once over and he noticed that the coil wire had been melted almost in half in a few areas. The wire for it wasn't in stock, so we used one that was for a different car that wound up working, but it backfired and died about 100 feet from my driveway. We replaced everything that could possibly make it fire. I drove it to work for about a week, but it ran different. It had power, but it seemed like it was dogged down a bit. well, it lasted about a week and I went to go somewhere that Friday and It wouldn't start. Still no codes. We were back to square one.
In and effort to get it to run, we changed the crankshaft position sensor. IAC, TPS, and then got a hint that it was probably a fuel pump problem. So we changed the fuel pump. Voila!!! it ran! Finally! But it now has an issue where it revs up to 1200 on startup and then goes down to 300 and dies sometimes, but when it doesn't die, it picks back up to 800 RPMs and idles there. we replaced the MAP sensor, but that didn't help. How do I fix that?

 

Get a genuine mopar IAC. Clean the throttle body REALLY well when you replace it. Leave the battery disconnected while you play. When you have it all back together, turn the key to On, count to ten, (don't touch anything) THEN start the truck. It may take it a bit to figure out idle, but it should happen fairly quick. See if that doesn't improve it's attitude at all.

 

IAC, TPS and especially the Crankshaft sensor need to be Mopar.

 

arrrgh…. if it cranks it "turns over" "turns over" has NOTHING to do with whether it actually starts or fires or tries to start.

 

What brand sensors dod you use?
Yes it factors EVERYTHING!
Answered your own question.

Never buy parts store sensors for these trucks. They are known to be picky.

 

A lot of them were parts store sensors. One was bought from Amazon.
I didn't know they were that picky. Thank you for telling me. Wait... What could it do if it is a crappier brand? Like earlier, it started and revved up to like 1700 and then down to 1200 and idles at 900 for like two minutes. I hit the brake to try and back it out of my driveway and it died. I couldn't get it to start back up. My old man went out like five minutes later and started it right up. He said it had issues idling and died a couple of times, but after it evened out, it ran great. We thought it needed some injectors cleaned, so we ran some seafoam through it and it's idle evened out a lot. Maybe that's just trash working it's way out? I'm scared to try and take it anywhere, because I think it's going to leave me stranded.
I'm just now learning some about cars. (That should show by my screwing up on basic terminology. This makes that last guys comment kind of unwarranted.) Mostly because I've taken my vehicles to mechanics around here and one of them charged me a few hundred dollars for a simple part I could have changed in less than an hour myself and it took him 3. 😒.
I took my Blazer to an inspection site later and they tried to tell me that the suspension needed all kinds of work when the guy I bought it from had just replaced it all. I knew the guy very well and him and his dad ran an honest business. He was telling the truth. So I kind of try to avoid mechanics around here.

 

Get a crankshaft position sensor from the dealership. The 3.9 V6 chews up and spits out the parts stores ones. Went through 2 parts store before i got a Mopar and cleaned up nicely. Dieing in reverse is a tell-tale sign that the sensor is confused. I'm surprised you're not getting a misfire code.

It has to do with how the 5 degree notches are placed on the V6 versus the V8. There are 9 slots for 6 cylinders instead of 8 slots for 8 cylinders. Unfortunately, a parts store sensor can see two of the 5 degree slots as a single 15 degree slot, throwing it into a detonating misfire, loss of fuel sync, and shutdown.

Usually it's more prevalent above 2500 RPM. But i experienced it occurring when put in reverse (but not forward) as well. My hypothesis is that the input shaft and converter load changes in reverse, pushing the converter forward and mis-aligning the slots from the sensor.