Hello everyone. I hope someone can help me out. I'm getting tired of the same problem about every 6 months. Plug #4 keeps going bad. Its been doing this for years. It starts with rough idle when first starting up that clears up quickly then eventually it leads to a misfire when accelerating. Every time this happens I replace plug #4 and its good to go. Just recently (roughly 6 months ago) did a full tune up and all new injectors. About a week or 2 ago it started its rough idle at start up and just the other day it started misfiring again. What would keep causing this??

 

The forum needs some more background info to make informed guesses about where best to look next in your trouble shoot.
Scan the computer with your ELM scantool & post All code Number clues.
You have performed a plug read....right???? If so, post what kind of deposits are on the #4 plug that keeps failing, or if it's failing because of ceramic cracks/chips tell us which ceramic is failing. Also tell us how the Other plugs read.
What made you focus on the #4 fuel injector, then replace All injectors????
Define "Full Tune Up" for us & what mfgr parts have been used.
Did this problem begin suddenly years ago after some event, or slowly over time????
Is this problem on your 99 4.0L Ranger, if so how many miles on this puppy???

 

Currently no codes but a have been fighting a lean code for years. I ignored it for a while because it ran fine other than the misfire on #4 every once and a while. Started out a couple years ago checking for vacuum leaks that lead to new intake gaskets and also had MAF sensor cleaned but still had lean code so I ignored it for a little longer. Not sure what you mean by plug read but according to my mechanic some of the plugs looked like it was running lean with #4 always being the worst. So that lead to doing the injectors with new fuel filter and new plugs and wires. Since then the lean code hasn't came back but its starting to misfire again. I always use motorcraft parts. And yes I'm talking about my ranger. It has just under 160K on it.

 

OK, good feedback.
Seeing as how this puppy has 160K miles on it, lazy switching O2 sensors belong on the suspect list for the lean code.
Since your running a oiled cotton gauze air filter, they're known to pass more dirt & if over oiled it & the dirt can migrate to & bake onto the heated MAF sensor element & corrupt its PID input to the computer about true airflow into the engine & it doesn't take much, all a vicious circle.
Yes your mechanic did a plug read. It's for deposits showing on the internal insulator, or the lack there-of can indicate a lean condition if the internal insulator looks to be really clean, cooked, blistered, eroded, chalky white, without any deposits. This read provides clues about whats going on in each cylinder.
So, since the plug read suggests all plugs are indicating a lean air/fuel ratio, use your ELM scantool, or the like, to monitor fuel trim on both cyl banks & if the air/fuel ratio is lean it'll show in the fuel trim readings. you can also monitor the upstream O2 sensors switching range & speed, to see if they're lazy. If you don't have a scantool that'll do that & you like turning your own wrenches, consider investing in the low cost ELM scantool discussed here. https://www.ford-trucks.com/forums/1...-scantool.html
Perform a fuel pressure check & volume over time test, to make sure the pump is doing its thing. PSI = 64 +/-8 & volume should be 1/2 pint/15seconds. Low fuel pump pressure or flow can cause a lean condition. EDIT: Also with 160K the PCV valve should be replaced & check its rubber hose & connections for splits, dryrot cracks, loose fit, as its down stream of & not monitored by the MAF sensor, if it is leaking vacuum from worn parts, weak spring, deposits causing it to stick open, leaking fittings, it'll corrupt fuel trim lean.
More thoughts for consideration, let us know what you find.

 

All that was checked out already. MAF cleaned multiple times and also installed a new one. Trim levels showed it being lean on both banks. Got better after doing injectors. Still a little lean but not out of range to set off CEL. Fuel pressure/flow was good to the fuel rail. PCV valve was replaced after intake gaskets were done. hoses looked good. Lean code hasn't came back since doing injectors. But the misfire is returning. Someone on the ford ranger forum said its possible that the head is cracked by cylinder #4 which is why that plug keeps going bad.

 

OK, more good feedback.
Have you checked both upstream/before cat converter O2 sensors switching range & speed, or replaced them with like OEM design sensors???? With 160K on them they've lived 2 lifetimes & with age their switching range & speed get slow, such that their PID feedback to the computer about the exhaust O2 content is slow & behind where it needs to be for the computer to be able to adjust fuel injector squirt time fast enough to keep fuel trim within bounds. This scenario fits your input that fuel trim is "Still a little lean but not out of range to set off the CEL."
When you had everything apart to replace the intake manifold gaskets, was the lower intake manifold cleaned & if so, what did the deposit load & level look like, especially in the #4 cyl runner????
I'd consider replacing the K&N air filter with the specified Motorcraft paper filter, so you know the new MAF sensor is staying clean, if it isn't already contaminated.
The Ranger factory air intake system isn't restrictive & the OEM air filter doesn't strangle engine breathing.
Might be helpful if you'd post the Numbers you got on your tests for fuel pressure, flow rate, O2 sensor switching range & frequency/speed, STFT & LTFT.
EDIT: Is it sipping coolant over time????

 

O2 sensors have never been changed since I bought the truck with 60K on it. So they probably do need changed. I'm not sure what the readings were on them and I also cant remember what all the other readings were for the other things when we had it hooked up to his big fancy snap on scan tool. But as far as the lean code I'm not too worried about it for now since its not setting off the CEL. I would just like to figure out why plug #4 keeps going bad. Someone said its possible there is a crack in the head between the exhaust and intake port on #4. And yes its loosing coolant over time. the bottle keeps drying up but I never have any overheating problems that I know of. I never see any white smoke out of the exhaust or anything in the oil so IDK where its going. I occasionally smell coolant when I'm being hard on it on the trails.

 

OK, so its sipping coolant, so have you seen any signs of coolant contamination in the engine oil, like a chocolate milkshake like sludge on the oil fill cap, or on the dipstick, or have you had a UOA=Used Oil Analysis performed by a lab like Blackstone, to see if signs of coolant are showing up in the oil & if so is it enough to be causing excessive engine wear, by elevated wear metal numbers detected in the UOA????
This is something you need to run down right now, without further delay.
Thus a problem with the head castings, or head gasket around the #4 plug is worth putting on the suspect list.
I had a coolant sipping problem on my then new 99 4.0L that took Ford & the Dealer about a year to run down. Ford said that they were aware of porous head castings & swapped out both heads under warranty, but the problem persisted, so they replaced both head gaskets again, but this time Ford had the Dealer do it with "Revised" head gaskets in 02 & its been coolant leak tight since. So head castings & head gasket are good suspects.
You might consider performing a cylinder blow down test on #4 & see if it'll cause bubbles in the coolant at the radiator cap. Or idle it with the radiator cap removed & look for exhaust bubbles in the coolant or a strong exhaust smell at the radiator.
When you say #4 plug goes bad, whats wrong with it to call it "bad"???
The inexpensive ELM scantool running FORScan or the like software to your viewing device could let you back into the techs world for a few bucks & shorten up a trouble shoot, if you like turning your own wrenches.

 

Nothing visible in the oil. I will look into the UOA and check out the coolant. I'm not sure what to say about plug #4 being bad. In previous times when it acted up it would set off the CEL for a misfire on cylinder #4 so I have my mechanic look at it and he would just change the plug and that would fix it. So now when the truck starts to idle rough after start up and then turns into a miss when driving I change #4 and its good to go again for a while. I don't even wait for the CEL anymore because its always the same.

 

If you have any of the old #4 plugs, have a close look at the internal & external ceramics for fine line cracks & let us know what you find.
A lean air/fuel mixture will cause the heads & plugs to run hot.
Are you using the Specified Heat Range Motorcraft plugs & torqueing them in to spec??? Have you been using any anti-sieze compound on the plug threads???
Has & is the cooling system & coolant been in good condition, changed on time such that its been without rust, or internal deposits, so you've been & now are getting good coolant flow to the heads. If coolant has been neglected, it can turn acidic & get after the head gaskets over time & cause leaks. It can also get after the water pump impeller blades & etch them away & cause poor circulation. Also neglect, or using hard water can cause scale, sludge & rust deposits that'll cause poor heat conduction woes in the heads, all a vicious circle.
Right now, with it sipping coolant, focus on the head gasket & passenger side head around #4 cyl.
Since your Tech said All plugs looked to have been running lean, I'd also seriously consider opting for the ELM scantool, so you can monitor fuel trim to make sure its right & staying that way.
I agree, if the upstream/before cat O2 sensors have never been replaced, its way past time to do it. The plugs, wires, fuel filter replacements are part of our scheduled maintenance replacement parts & were a good idea to do to get All past & present scheduled maintenance items up to date. I'd still seriously consider installing the specified Motorcraft air filter, so you know the MAF sensor is staying clean.
More thoughts for pondering, keep us posted on your trouble shoot findings.

 

Since its sipping coolant, Don't replace the O2 sensors until the coolant loss problem is put right, lest the coolant wipe out the new O2 sensors too.

 

Sounds like a bad head gasket to me.
Go to Oreilly's and borrow their block checker. You will have to buy the fluid.
Warm up the engine to operating temperature and check the coolant for combustion gases using the block checker.
When the engine warms up, the head and cylinder block expand due to the heat, now usually the head takes up that expansion and contains the combustion gases, but some times the pressure leaks past the gasket into the cooling passages,

 

Ok guys thanks for the input and all the info. Ill get all that checked out soon. This weekend he's going to do state inspection for me and we're going to replace the shocks with bilstein 4600s. Kind of excited to see how they work. Maybe we'll check out those things this weekend too while its in the shop. Thanks again PAW PAW for all your info.
Love this forum

 

Ok, let us know how the trouble shoot goes.
If you find the passenger side/cyl bank-1 head gasket is suspect & your going to replace it, I dug out my Dealer/Ford warranty replacement head gasket receipt & it shows the Ford "Revised" head gaskets, that have been weep/leak free for 13 years now, to be as follows, (1) YU3Z-6051-AA gasket cyl head, (1) YU3Z-6051-BA gasket cyl head.
To replace the cyl head gaskets, we also have to replace the head bolts & since your in there, consider replacing both head gaskets, upper & lower intake manifold gaskets & their Ford part numbers were as follows.
Head bolts (16) E6TZ-6065-A
Intake Manifold Gasket (1) F57Z-9439-AA
Intake Gasket (1) F57Z-9E436-AA
Cover Gasket (2) F87Z-6584-AA

 

Ok thanks again for the great info.