we have recently purchased this truck, which has all records from dealership, the previous owners took really great care of this vehicle and the records show it, we went to take truck on a drive and noticed it was idleing and running rough so went a few blocks down to have it read, after getting error code, I choose to start with a new spark plug at number 3 cylinder, which I will try in have a picture of, if the pic does not show, the middle of the plug is cracked. after replacing plug the truck ran great and after driving it the check engine light went of and is still running fine, my question is what would cause the plug to fail like this?

also this vehicle has had a couple of fuel injector cleanings thru the years

 

Welcome to the forum


Wrong spark plug.

4.0l OHV engine needs Motorcraft or Autolite spark plug, it "eats" other brands, not that other brands are bad, 4.0l OHV combustion chamber design just wears them out fast.

Gap is also fairly wide at 0.054, so make sure to gap spark plugs before installing, new plugs will for sure have smaller gap.

Motorcraft SP500 is spec'ed
Or
Autolite APP103

These are double platinum, Ford uses a waste spark system, so single platinum will wear out the same as copper/nickle so either go cheap, copper/nickle, or go double platinum


My original spark plug wires lasted 18 years, well 16 really because I ran a few years with some idle misfires, lol.

Ford Dealers run sales all the time on Motorcraft parts, call and ask when spark plug wires will go on sale, usually cheaper than even discount auto parts.
And they will often give you the sale price anytime, so worth a free phone call.
Same for the spark plugs
The Motorcraft wires are worth the money


My 1994 4.0l OHV has original injectors, I run a can of Seafoam in the gas tank once a year, seems to work well at keeping the tips clean, fuel system has never been "cleaned"


4.0l OHV is a very reliable engine, it only has one weak spot, it does not like overheating
So keep the cooling system in good working order, change coolant every 2 years.
If engine starts to overheat pull over and stop, it will save you hundreds of dollars
DO NOT try to make it home or anywhere else, let engine cool off then drive it a bit more and shut it off and let it cool down, repeat as needed, remember you are making over $100/hour for this time delay in the money you will save.

If you overheat the 4.0l engine you WILL crack a head, for sure, first time and every time, no "do overs", lol
There is a weak spot in the head casting, between valve seats, if head gets too hot the metal expands too much and you get a crack.

 

Thank you for your great response
so I have finally got around to looking at other plugs they are all motocraft, why would someone change out # 3 cylinder to something else

 

It is not usual these days for spark plugs to crack, don't really remember that in the old days but had to change spark plugs more often back then any way.

So someone may have had a cracked/bad spark plug and swapped it out with what they could get.