About 3 weeks ago my 1999 Ranger 3.0, 135,000 miles, (also recently had alternator problems solved in another thread) started running rough at all RPM only at times after the truck has been driven a while. It felt like a dead miss. The truck had just been serviced at the dealer a couple of days before -- I found some new oil in the air intake tube near the throttle body so I cleaned it out well, thinking the tech had overfilled the tube. So I cleaned the MAF and throttle body and changed the dirty air filter. It ran better (haven't seen any oil there since), but was still running rough at idle and while driving at all RPMs. It will be running fine then start running rough like an electrical miss out of nowhere. Then at times it will then smooth out and be fine again like turning on a switch.

About 2 weeks ago when it began running rough again after driving about 10 minutes the CEL came on for the first time ever. I went straight to Auto Zone and got the code checked -- they said "spark plugs". I asked what cylinder and how did he know for sure the plug and not the wires or coil. It was cylinder #2, the spark plugs are easy, so I got a new set of double platinum plugs. The old plugs looked worn but had no obvious problems. The wires were under lifetime warranty so he gave me a set. I changed it all, and yes it ran better.

But the problem, same as before, returned the next day with a CEL. Again to Auto Zone - the guy says "spark plug, cylinder #2. I tell him it's all new so I bought a new coil. It seemed fine for several days (meanwhile I got the charging problem fixed with a brand new alternator.) A couple of days ago the problem returns -- out of nowhere after driving a while it starts running rough while driving. My daughter was driving it, pulls over, calls to tell me, says it started running rough again and was sitting there idling rough. I had her to shut it off and restart it. Then it was fine. The CEL came on but I haven't checked it yet. My money is on Cylinder #2 spark plug.

Other than those intermittent times it runs smooth and strong. I have 2 possible thoughts -- EGR sticking? MAF acting up?

Sorry to be so long but I want everything I know out there. Any ideas, guys?

 

UPDATE: Just returned from Auto Zone. Same code -- #2 spark plug. Told the guy everything was new. He says, "It could still be the plug." The same plug, same cylinder 3X in a row? Don't think so......

 

EGR is run by the DPFE sensor, if you still have the metal one, replace it with the plastic one, the lines running to it as well.
If the EGR is sticking shut then it will ping under acceleration, if it's stuck open, then you will get rough running, but that would throw up a different code.
The way to test it is after the engine is warmed up, pull the line of the EGR valve, hook up a hand held vacuum pump and open the EGR valve, with the vacuum pump opening the EGR valve a little bit the engine will run really rough or even stall.
When the vacuum is released and if the engine smooth's out, then the EGR valve itself is OK.
If you remove the valve and it's really dirty you may want to replace it any rate. They can be cleaned, but it's a PITA and they are cheap for a new one.
What was the actual code from Auto Zone ?

I wouldn't think it's the MAF sensor at this point, a faulty sensor would affect all cylinders across the board not just #2.
Plus you said that the plugs looked normal, with a faulty MAF sensor, the plugs wouldn't look normal.

I'm thinking a faulty fuel injector on #2, possibly just a dirty connection.
It may not be opening enough and letting in the right amount of fuel.

Inspect and clean the connection and run it for a few days.
If that doesn't clear it up, then swap the injector around on another cylinder and see if the problem moves to that cylinder.
You'll have to go back to Auto Zone for another reading.
Maybe time to get you own code reader ?

It also maybe a fuel pump problem too, not enough pressure to the engine, but it would effect the whole engine, not just #2.

 

If Auto Zone says it's #2, then switching a plug around with a working one from another cylinder is the obvious thing to do.

 

It would be good to get exact code for this, i.e. P0302 means misfire #2 but doesn't mean it is a spark plug issue, just #2 is not adding power to crankshaft spin when it should be.

There are ignition/spark codes as well, these are feedback issues from coil pack.

You can get your own Bluetooth OBD2 reader that works on ANY car/truck sold in North America since 1995, so not a Ford thing.
And works with any smart phone or tablet that has Bluetooth
Cost is $15-$25

And you can watch live data while driving down the road, which can be very helpful

Spark plugs don't tend to become intermittent when they fail, but air/fuel mix can drowned a failing plug, idle and accelerating is a Richer mix

You could have lower compression in #2 which will cause misfires.
Shops can use a laptop now to do a compression check on any engine with OBD2
Takes less than 10 minutes to do
I would call around and see what it would cost
Compression is either good or bad, so once that is taken off the table you can look at spark and fuel.
But if it is a compression issue you will be spinning your wheels and wasting time and money on things that won't work long term