my brother has a 99 Ranger 4.0 automatic 2 wheel drive. It has developed a misfire that is changing cylinders. It started on #6 then moved to #3 then to #4. this is according to the codes, Wires were replaced and the plugs are about a year old. He is changing them tomorrow. It will run fine with no miss for about 20 minutes then starts missing and running rough. Let it set for a few hours and it will run fine for another 20 minutes or so. Any help would be appreciated.
Thanks

 

Odds are it is running out of gas or running out of spark. You might have plugged injectors, low fuel pressure, semi-plugged fuel filter or a failing coil(s) or pickup.
If you get the symptoms from the OBD diagnostic, you might be able to put it into the 'contribution' test where the ECM cuts pulses to individual cylinders to see the resultant rpm drop. If it doesn't drop, it is not contributing much...
Even a fuel pump relay that is dying could cut power to the pump, and thus delivery volume / pressure.
Given the variety, I'd expect it is an 'individual' component, rather than a common component, such as an intake manifold leak. A coil pack that has failng windings will generally not move from cylinder to cylinder, but one that had bad ground could.

When it misses, is it solid or still intermittent? That would point to different causes.
tom

 

^+1, I like tomw's ideas. Lots of things belong on the random misfire suspect list.

Post All trouble code Numbers, as they can offer up good trouble shooting clues.

With the vintage of this puppy, bring it up to date on All due past & present scheduled maintenance replacement parts & this includes the air & fuel filters PCV valve, both upstream O2 sensors if they have over 100K miles, as their switching speed & range slows with age, plugs & wires gapped to spec. The specified Motorcraft plugs & wires are highly recommended, as they are of good quality & designed for the double work load our waste spark ignition system puts on them.

The random misfire coming about after a 20 min drive sorta sounds like an ignition electrical problem thats heat sensitive.

If no joy after getting the scheduled maintenance items installed, our coil pack is known to crack & cause the kind of random miss problems your having. So raise the hood at night & look for a fireworks display around the coilpack. Maybe try spritzing it with a spray bottle of water to see if you can make things worse. Its also known to crack underneath & cause breakdown, so unbolt, lift & insulate the coilpack to see if that'll clear the misfire. If so, spritz its under side with water & look for arcs & sparks to confirm breakdown there.
If no external break down found, but you come to suspect a weak spark from internal coil pack problems, most autoparts stores can & will do a no cost bench test for weak output.

Also maybe consider having a read here on diagnosing random misfires & let us know how your trouble shoot goes. Diagnose Engine Misfire
EDIT: Good misfire video from Wells. http://www.wellsve.com/video_mode6misfire.html

 

The plug on #4 was the problem. Not sure if it was damaged on installation or from manufacturer. There were no external signs of damage.

 

OK, good find, fix, feedback & to hear all now seems well. I once developed a bad random miss that was Much worse with humidity on my then new 99 Ranger. With that clue the Dealer found a cracked insulator on #6 spark plug using a "wet down test".

BTW, we need to use the specified Motorcraft, or Autolite plug wires & finewire iridium enhanced center electrode, with platinum pad sidewire design plugs, or at least double platinum plugs, in our waste spark ignition system. They are designed to take the Double work load that system puts on them by being fired on Every rotation of the crankshaft. Lesser quality wires or plugs will work for a while, but wire insulation & or center conductor can't take the double work load & the plug electrodes will suffer spark gap erosion really fast & misfires will soon return!!!