Ok, good trouble shooting, so you changed the plugs & all the plug wires & even swapped out the #4 plug wire, just in case!!!!

Lets think about this a little more. seeing as how it's the #4 cylinder thats acting out & it's on the drivers side, I believe it's the bank that also gets the reverse polarity spark, from our waste spark system, so are you using the specified double platinum spark plugs?????

You say the old removed #4 plugs look normal, but if you put the old #4 plug in another cylinder, will it also cause a misfire in that cylinder????

More thoughts for pondering.

 

They Had A Cold Start Injector That Could Be Going Bad.

 

Pawpaw-
Yes, I used the spec'd Motorcraft plugs. Part # SP-500 (AGSF22FM)
No, I didn't try the plug in another cyl. Logical thinking tho. I reason I didn't, the factory manual says to reinstall used plugs only in the cyl that they came out of. Not sure why. I would think parts is parts, and what's the difference between a new plug and a good used plug with only 4-5K on it?
But, I know it has to be something other than plugs. Got the same Motorcraft part # plugs from different sources at different times, so 3 bad plugs in a row on the same cyl has to be 1/ zillion.

 

When we use double platinum replacement plugs, it doesn't matter which cyl they go in.

The OEM plugs had platinum only in the center electrode on the passenger side cyl bank, as it gets a normal polarity spark that eminates from the center electrode & only platinum on the ground electrode for the drivers side bank, as it gets a reverse polarity spark, that eminates from the ground electrode.

We can't get plugs made like that, so double platinums are specified as replacements, as it doesn't make any difference which cyl they go into.

All this so that the factory could save a few cents on plugs with platinum only on the needed electrode!!!!

So only if we're re-using the Factory plugs, does it matter which cyl bank they go back into!!!

Just wondering & trying to imagine what #4 cyl is doing to it's plug, that would kill it, or cause it to misfire in #4 cyl, yet have it's appearance & internal resistance look normal.

Would be interesting to see if using it in another cyl would cause it to misfire too!!!!

 

Hmmm....
Great explanation on the reasoning behind cyl (or bank) specific plugs. I seem to remember reading somewhere that one type of plug was used for the pass bank, one for the driver's. Never knew specifically why. Thanks!
I'm gonna look to verify that the AGSF22FM's are doubles...maybe they're not?

Ya, #4 doesn't like its plug...at least older than 4-5K. It's cool with new plugs.
This is a weird problem.
If I don't get it fixed before I have to swap another plug, I will try that one in #5 or 6.

 

Well, I looked...SP-500's and AGSF22FM's are definitely spec'd as replacements.
But, the owners guide says specifically to use AWSF-22PP's. So, next I will look to see if there are any differences between the AGSF's and the AWSF's (if they still make the AWSF's and they haven't been superceded).
I was also thinking about trying another brand double platinum and see if that matters.

 

Well, the AGSF's superceded the AWSF's and are fine wire double platinums.
I knew it couldn't be that easy!

 

Rockledge-
Since you had yours done, where did you send your oil for analysis?
Changed oil today, saved a sample.

 

If the plugs don't have any deposits on them & you say they don't, about the only thing I can think of, is that the plug in #4 cyl is running hot & has a fine ceramic crack, so look carefully with some magnification, at the center electrode ceramic insulator & the external insulator, if it looks ok inside.

If you find a crack in the insulator, it would suggest maybe that cyl is running lean for some reason. Maybe a fuel injector problem, or maybe a gaskt leak, or maybe a coolant circulation, or heat conduction problem.

Do you tighten your plugs to spec with a torque wrench????

 

I thought of the ceramic cracking too- can't see any cracks, but didn't look with a glass.
The time interval until "failure" is pretty constant too. After about 3- 3.5K the problem will start as a rough run for 3-6 secs at cold startup, esp if the truck hasn't been started for a couple days or more. Gradually (over another 500 to 1000 miles), the problem will worsen into a bad miss at idle and under any kind of load (and hot or cold) with a setting of the MIL (#4 misfire).
Ya, I always torque my plugs to specs, and torque anything else I can find a spec for, so we can safely rule out installation/ overtorquing problems.

 

Blackstone Laboratories - Gas Engines

 

Thanks, I ordered the kit.

 

To anyone who has a 4.0 that misses on a cold startup that goes away within about 20 seconds.... I was almost to the point of giving up trying to figure it out... kept replacing cyl 3 plug every few weeks only to have it start misfiring again on cold startups. It is a very good possibility that you have a cracked head. My truck never overheated, was not misabused, etc and I have found out that the 4.0 had this problem in the Rangers from early 90 models until they went to the ohc design in mid 2000.
It will develop a small crack in a head between the valves... the way I finally figured it out was when I parked the truck for the night I loosened the radiator cap and let the pressure off (be careful, it will spew hot coolant if you've just shut it off at operating temp). Leave the cap loose all night, then tighten it in the morning, start the truck.... if it starts with no misfire you have a cracked head or (less likely) head gasket leak. NO coolant was in my oil, either. When the motor cools the crack opens just enough to drip a few drops of the pressurized coolant into the cylinder affected. Upon talking to others I know that own Rangers, almost ALL the 4.0 owners of '98 - '00 models have had it happen. The coolant leak was so small that it always appeared to be full in the reservoir also... would add a little coolant to it maybe every couple mths or so, I don't think I ever used even a half gallon in the 6 mths it took me to figure it out ... and I did tear it down and that was the problem.... I bought new redesigned heads from Clearwater Cylinder Head Co. in Florida... something Ford SHOULD have done a long time ago.

 

Interesting story, that sorta goes along with a "coolant sipping" problem I had on my then brand new 99 4.0L, that wasn't run down & cleared up by the Factory & the Dealer utill 02.
Interestingly, in the process the Dealer & I learned from the Factory, that they had known "porous head casting" problems & they suspected that was my "coolant sipping" problem, but dye tests proved naught, yet it would use about a qt of coolant in about 5-6 weeks.

My new Rangers month old engine had been replaced with a factory crate engine, because of bad wrist pins, only to begin sipping coolant a month later, so they replaced the heads on the new engine & that didn't do it, so they relaced the head gaskets again & that didn't do it, so in 02, they replaced the head gaskets yet again, but this time with "revised" head gakets & that seems to have done the trick so far.

But it was a long struggle for the Factory & Dealer & my toungue was sore from having to constantly bite it & not fuss at them while they tried to run the problem down.

So it would seem to me that porus head castings & maybe the old head gasket design, might have been the root cause of many of the pushrod 4.0L problems?????

FWIW, the new head gasket P/N's are YU3Z-6051-AA & YU3Z-6051-BA.

More puzzle parts for pondering.

 

So, let's say there is a crack in the head/ bad gasket...some questions to ponder.
---A sniffer should detect the presence of combustion gasses in the coolant..yes?
---Would coolant (esp a miniscule amount) cause a plug failure?
---What precisely would a plug read look like, if this is the scenario? My plug read's normal.
---Why would a new plug abate the situation for 4-5K miles, and then the plug goes south, to the point of gross misfire (like running on 5 cyl w/ the MIL light on), only to be "fixed" immediately by installing a new plug?
(Next plug I have to pull, I'm gonna swap it into a different cyl. to see if the plug is really bad). Sheet, I really wish now I didn't toss the plug from the last swap.
---How about a leak down test?
The bottom line is, I don't think I'm gonna tear the top end down for this unless I absolutely know for sure it's the head or a gasket. I just don't understand the correlation between a cracked head and a plug misfire that is fixed, albeit relatively temporarily, with a new plug. In the absence of finding anything else I can try, and unless the problem gets appreciably worse, I probably will just swap plugs until I 86 the truck. With the size of my family the Ranger's a tad too small now anyway. Hell, even my crew cab SD's are getting too small! Maybe my next buy should be a Club Wagon or an X!
Well, I just hope to hell the head's not cracked, what a PITA.