Good morning! I'm new to the forum and need a bit of help.
I've got a 1997 Dodge Dakota Sport, 3.9L, V6, WD, automatic, 287k miles. 2-3 months ago, I replaced the spark plugs and wires, oil change, general maintenance stuff.
Two weeks ago check engine light popped on with the codes P0300 and P0304, code reader said misfire on Cylinder 4. I also noticed it was very rough on idling. After much research, I've replaced the spark plug and made sure it was gapped properly, I've replaced the spark plug wires, replaced the distributor cap and rotor, checked that there is spark coming to that cylinder, checked the injectors, checked the ohms of the injectors, swapped the injectors from 4 to 6 and still getting the same error P0304.
Before I go nuts and tear apart half my engine, does anyone have any suggestions on what to check or been through this and know what to do? Any help would be greatly appreciated!

 

Failing crank senor? Are you getting injector pulse on #4? (you can rent a noid lite to test that from your local parts store.) Is the cylinder totally dead? Not firing at all? Considered doing a compression test?

 

See if the plug is showing rich or lean and/or use a noid light as suggested above.

Crank sensor is possible with a V6, but I'd be waiting for another even cylinder to misfire before blaming it.

oxygen sensor is how misfire codes are set. Link crank sensor, I'd expect more than 1 cylinder for it to be a sensor.

 

So after a few days to fiddle...
yes, cylinder 4 has compression.
We swapped the crank sensor and it seemed to have gotten worse. It idles fine in park and neutral, but when in drive, the engine gets rough. It was doing the same thing but now it's louder.
We've checked for vacuum leaks, but kinda don't know where to go from here...

 

What brand crank sensor? These trucks really don't care for aftermarket sensors in critical locations..... Yes, the mopar sensors are more expensive, but, they actually work right.

 

parts store sensors do not work at all on the V6. They are too slow to register the double window on the flexplate, they see it as a single long window.

 

The crank sensor I got was Duralast, and it sounds worse than it was. The Mopars were not in stock. I'll have to put the old one back in and order a mopar and try again and see what happens again.
I'll also get a tool to test the compression, we know we are getting compression, just not sure how much.
Also just found the air intake gasket is missing a chunk...so that's probably not helping things. So gotta replace that too.

 

If the gasket is pushed out above the #4 runner, that can make a lean misfire. If it's pushed out at the throttle body, not so much.

 

 

Dealer. Or, find the manufacturers part number, do a google search on that number. You will likely find many sources.