I'm new to the forums so hopefully this is in the correct spot. I bought my ranger about 6 months ago and recently have been experiencing a misfire at idle. I used a code reader and found it to be p0304 and then followed up by doing the full ignition system as the truck has 168k miles and I was not aware of it ever being replaced. After this I still have the misfire and thought another possibility could be a dirty injector. I ran 3 tanks of Lucas Oil fuel system cleaner and it did not improve the misfire at all. I doubt the injector is completely ruined as it clears up when I drive and the rpm stays above 1600 or so. I have checked vacuum lines for leaks and have yet to find one that could be the issue. I also tested the IAC by unplugging it during idle and the engine fell in rpm dramatically so it seems to be working. I checked my MAF and it appears to be fairly clean. My truck is a 2002 3.0 auto and 2wd. This is my first truck so I would love to fix it and any help would be greatly appreciated.

 

I would do a compression test, I think your 2002 3.0 falls into the years that had bad valve seats.

 

This is probably a stupid question but if that is the issue what would be necessary to fix this?

 

Welcome to the forum

+1 ^^^ if you don't want to waste money on maybes then compression test would be the next step, actually the first step when there is a steady misfire, lol, but new spark plugs are not a waste of money

Cold engine
Pull out all 6 spark plugs first
Test each cylinder and write down results for each
That will give you an average for engine, compression gauge and battery voltage, which all change the PSI number for a compression test

If #4, or other cylinders, are lower(15-20psi lower) than the average, then do a WET test
Use a straw to add oil to that cylinder, dip straw in bottle of oil and put finger on the top, transfer oil to spark plug hole, you want about a teaspoon of oil, 1/2 straw is fine

retest the cylinder, PSI will go up, always, but if it does not go up above the average then you have a "burnt" exhaust valve, its leaking
If so you need to pull the heads and do a "valve job" or replace with rebuilt heads(already had a valve job, lol)

You can do the work and replace the injectors, or just swap #4 injector with #5 and see if misfire code changes to P0305, but I would do the compression test regardless because that can be the ROOT problem of why a new spark plug or injector makes engine run better for a bit, and then goes back to the misfire

 

So say the average is 150psi for ease of explanation. If any cylinder is 135psi or below then do the wet test. But with the wet test even the cylinder with lowest compression should achieve higher than 150psi? but if it does not achieve this number or higher then it is an exhaust valve leak. Ignore the numbers as I just used something easy so I can understand the process.