'98 Ranger SCab, 4.0L OHV, 143k miles.

This has happened several times over the last few years. Engine begins to run rough and throws a P0304 code. First couple times I replaced the #4 spark plug, problem solved. Happened last year so I replaced all plugs and wires, problem solved.

3 days ago, it throws the code again! Replaced the plug, problem solved. This is recurring after only a few thousand miles. Plugs are Autolite APP103 Platinums.

Any idea on what is causing a problem like this? Maybe the coil? The last plug came out looking good. Inner porcelain was still white and the electrode clean but wet.

Anyone else experience something like this? Any ideas?

 

I'm curious if something is actually frying the plug. Next time it happens, try simply swapping two plugs rather than putting a new plug in it and see if the problem moves to the cylinder that you moved the #4 plug to, or goes away temporarily again.

There are several things that could cause this code however. Among them are:

Faulty spark plug or wire
Faulty coil (pack)
Faulty oxygen sensor(s)
Faulty fuel injector
Burned exhaust valve
Faulty catalytic converter(s)
Running out of fuel
Poor compression
Defective computer

I tend to suspect the coil pack. It's possibly producing a weak spark on #4 and when you get some miles on the plug it's got just enough crap on it to build up a little electrical resistance and the coil can't quite fire through it.

This is just a guess however...

 

You prob have a cracked head. I work at a dealership and have seen this many times. If you have access to a bore scope pressure test the cooling system for a couple of hours and look for coolant in the cylinder.

 

I have had the exact same problem. At first I didn't know what it was. I just replaced the entire ignition system. I replaced all O2 sensors, plugs wires, coil pack, injectors. And nothing. Then I called a friend and asked him if he knew. He said to look at the cylinder prior to #4 fire. which is #1. Wha lah. It was a plug that wasn't completing the circuit. I get a P0304 again and I tried the same trick. But nothing. Runs really rough under load or gas pedal down. especially uphill. I am thinking a cracked head or a ring not seating all the way. Whatever it is I am at a loss.

 

This is an old post but I'll comment, the problem still kicks up every several thousand miles. My wife only puts about 8k a year on this thing so it might occur once or maybe twice a year. I'm leaning towards cracked head, now. I've been told by several that is likely the problem. I'm thinking screw it! My coolant levels are fine and we're not experiencing one iota of other issue from the truck so I'm gonna throw $6 at it once/twice a year and call it good until something significant occurs.