Bad miss, running rough

My 1998 Ranger XLT 4L auto 4x4 with 140,000 miles has has a miss that has been barely perceptible for many months. Last week it became really bad, and the truck was very jerky. It seeed to struggle reaching highway speed, but did eventually.

The miss was very intermittent, it would appear at all speeds, but worse under load, or decelerating.

I changed the fuel filter and added a can of Sea Foam to 1/2 a tank of gas and ran it 60 miles.

For part of the trip, about 15 miles, it ran pretty much properly at about 70 MPH, but when I started to play around with the speed, the problem came back.

My husband thought it was the #4 cylinder, so traded that spark plug and wire for a different one, and nothing changed.

He then tested the coil by pulling the plugs off, one by one, and when doing so on the passenger side it starts to run really rough, but removing the driver's side plugs, one at a time, really does not affect it and it runs fairly smoothly.

There is good hot spark jumping at least an inch from all 6 wires when they are pulled.

After that test, it is running super super rough, I would not even chance taking it on the highway. The only thing different from before the testing is he left the two spark plugs in the traded positions, he did not put them back where they were to start with, but the plug wires are back where they were before we started troubleshooting.

Any ideas as to what we should be looking for?

Ford used Waste Spark on the 4.0ls, so pulling spark plugs wires to test system won't work very well, not like it did on distributor system or COP system
One cylinder on drivers side and one cylinder on passenger side SHARE one coil in the Pack
So both spark at the same time, wired in Series

Coil pack wiring reflects the "Matched Pairs" for spark
[3 4]
[2 6]
[1 5]
Front

So 3 and 4 spark at the same time, share a coil
as do 2 and 6
and 1 and 5
Each matched pair has pistons at TDC when it sparks, one on compression stroke(powers engine) the other on exhaust stroke(wasted spark)

If you remove spark plug wire on one side 2 spark plugs will die, if you do it on the other side just 1 will die, which is what you described.
Pay attention to the 5 6 4 side of the coil pack, I often reverse wires on that side.
1 2 3 side I usually get right, lol.

4.0l OHV engine eats spark plugs, Motorcraft will last the longest, next is Autolite, any other brand works fine just won't last as long, and 4.0l needs a big gap, 0.054"
ONLY use Regular copper, or DOUBLE platinum spark plugs of any brand, Single platinum are a waste of money, 3 of the spark plugs will wear out just like Regular copper, so waste of money

I have had a spark plug start to go on my 4.0l, will do as you describe, then one day it just dies, solid misfire.

Handy tool is a Bluetooth OBD2 reader, $15-$40, uses your smarthphone or tablet with Bluetooth as the display, so wireless.
APPs are free or $5 for full app
Allows you to watch Live data while driving, and also read and clear codes
And it works on ANY VEHICLE sold in the US or Canada from 1996 and up, so not a Ford thing or even a US thing
So if you are going to own vehicles this tool is a good investment, like a volt meter or a vacuum gauge

Reading the codes, free at any auto parts store, will probably ID the cylinder that is misfiring, P030X, X = cylinder number, so P0304 would be misfire #4
This can be helpful so you can change spark plugs or check injectors specific to the problem cylinder


Clean MAF sensor once a year
After engine is fully warmed up let it idle and unplug the 2 wires on IAC Valve, Valve will close and idle should drop to 500rpm or engine may even stall, either is a GOOD thing, it means no vacuum leaks
If idle stays at 700 or higher then there is a leak