I want to preface this by saying I have been browsing forums like this for years but this my first ever post- any ideas are much appreciated.

BACKGROUND:
I bought a 2001 ford ranger with the 3.0 gas engine (~150k mi) about a month and a half ago and have had some pretty bad misfire problems ever since. The CEL came on soon after I bought the truck (go figure the guy must have unplugged the battery right before I test drove) and the code was for a misfire in cyn 3. I initially started with plugs - replaced them all (all pretty worn out, gap was also horribly off). Cleared the code, car ran much better, still had some rough idle, but much better. Couple days later, cyn 3 misfire again- much worse almost stalled the car (blinking CEL)- so I went and replaced the wires. They were all pretty worn but cyn 3's in particular had a loose connector at the spark plug. Same deal- better for a day then more misfires in no 3. I read on the forums that a coil pack was a likely culprit as they are prone to cracking so I replaced it too- same thing.

At this point I was fairly confident that the electrical system was fine so I started looking further- read that the O2 sensors (they were definitely original) could be an issue which seemed to make sense, replaced both of them- same pattern. I did the same thing for the camshaft syncro and camshaft position sensor (this also eliminated a fair amount of squeaking from the engine- camshaft syncro bearings were shot). All of this kept the same pattern, better for a few days then it would start running really rough and eventually misfire. It was around this time that I started to get cyn 4 misfires as well. EDIT: (3 and 4 are the closest to the firewall).

Here I turned to the fuel system- new gas cap (old was broken), new fuel filter, and finally bit the bullet and bought new injectors. I also replaced the female electrical connectors off the wiring harness for each injector- someone had gone through and broken off the locking tabs on each one and zip tied them in place instead. When I installed the injectors I also checked the lower intake manifold bolts- all torqued to spec. New upper intake manifold gasket and all bolts torqued to spec on upper intake. Same pattern though the misfires seemed to be less and less severe.

This whole time I had been looking for vacuum leaks and had found none. I finally gave up and took it to a local mechanic without clearing the CEL codes, he looked at it for a week, smoke test, injector test etc. and eventually concluded that there was nothing wrong with it because once the codes were cleared it ran fine for a while. (not very helpful)

Some more reading and testing led me to the IAC valve- this made a HUGE improvement, it FINALLY ran smooth for about a day then it began running rough again... New PCV valve and elbow also installed. Finally this morning, I installed a new EGR valve and EGR solenoid after finding a forum somewhere where a guy had a similar situation and it turned out that the EGR wasn't fully closing. I got really excited because it was the best the car had ever ran for like 2 hours then it went back to misfiring and running rough. For other non-related issues, I have also replaced the starter, alternator, battery, thermostat, idler pulley, tensioner pulley, and belt.

CURRENT SYMPTOMS:
So here is the weird part. When I have a misfire code, if I clear it, in 10 or so minutes the engine smooths out and idles much better. Then eventually it starts running rough again (vibrating the car), which eventually develops into a misfire of cyn 3 or 4. That is what I do not understand, why does the problem go away after the code is cleared and why is it so intermittent? One mechanic I trust suggested over the phone that it might be valve seats or something related, but it seems to me that it would be a much more regular occurrence if that was the case? To me this seems like a PCM or ECU issue where the computer is getting hung up on something, adjusting the fuel trims and causing a misfire then 'forgetting' about it once the code is gone. I should also mention that the car drives great on the highway or in town (fuel trims <3%) the ONLY time it has a problem is at idle (stopping at a light makes fuel trims jump up to >10-12%) I do need to point out that everything I have done so far has been done in a parking lot with a harbor freight toolset- I away from home on a long term work assignment ending in a week and a half (this would have been a lot easier in my shop back home...)

Any help is greatly appreciated - I am at a loss on this one!

 

Welcome to the forum

3 and 6 are closet to the firewall, not 3 and 4
Ford V6 numbering
3 6
2 5
1 4
front
of engine

But 3 and 4 do have one thing in common, they share one of the coil's in the coil pack
The V6 coil pack only has 3 coils inside, each coil sparks 2 cylinders at the same time, called a Waste Spark system, this was actually the very very first spark system on gasoline engines, uses crank shaft timing not cam timing like a distributor

Anyway 3/4, 2/6, and 1/5 each share one coil
Your coil pack wiring should look like this
3 4
2 6
1 5

That 5 6 4 side always gets mixed up, lol

Can also be
1 5
2 6
3 4

Just depends on how the coil is mounted on the engine

I would try a new coil pack, since that's the only thing 3 and 4 have in common
You can test coils but since the very first ignition coil you can only test if it "bad", not if its good, so unless its a total failure it can test OK but still not be good, lol
This has to do with heat and the rapid use a coil must endure

The 2004-2006 3.0ls did have valve seat issues, so your 2001 is not in that group
And exhaust valves in general are often the cause of misfires as any engine gets older, they just wear out and leak, so lower compression and misfires on that cylinder
Google: exhaust valve dollar bill test
If you want to see if its an exhaust valve issue WHEN it misfiring

Resetting doesn't really tell you much, you are not changing anything in the computer, just clearing the codes and idle(IAC valve) memory

O2 sensors can only see Oxygen in the exhaust
When there is a misfire NO OXYGEN is burned up with the gasoline, so that oxygen is dumped into the exhaust system
O2 reports higher oxygen levels
To the computer high oxygen level means Lean fuel mix
So computer responds with higher Fuel trim numbers

Fuel trims are the Open Time for each fuel injector
Computer calculates 0 fuel trim on the fly, so its a "floating 0" based on RPMs, throttle position, engine load and air weight
+10% fuel trim would mean the computer is opening fuel injectors 10% longer than it calculated based on O2 sensor feedback
-10% fuel trim would mean the computer is closing fuel injectors 10% sooner than it calculated

-15 to +15 fuel trims are not unusual, over 20 either way for extended periods will set Lean or Rich codes, engine is never running lean or rich that's not what those codes mean, not at all, just FYI

Change the coil pack and go from there

 

What method are you using to clear the codes? Have you tried an ECM hard reset? Has a compression test been done? Verify the spark plug wire firing order? A lot of people will change spark plugs one at a time so they don't get wires mixed up, but what if the guy before you got them mixed up. lol

 

Thank you both for the comments.
RonD- you are correct on the order of the cylinders- I was mistaken. A coilpack was my thought as well- I have already replaced it with a brand new motorcraft unit- was one of the first things I replaced. Ive also tried switching the 3 and 4 cyn wires on the coil pack (back when I was having only cyn 3 misfires) which resulted in no change. Dollar bill test is a good idea- question on that though. I do have a couple small exhaust leaks, one before the cat and one about 2 ft after- wont that affect the vacuum you are looking for in that test? I am unable to fix those on the road... need a grinder...

Grumpa- I am using a bluedriver Bluetooth code reader. Its quite a nice unit for what im doing- that is also where I am getting the fuel trims. I have 'technically' tried the ECM reset (20 mins with neg battery cable unplugged) as I have been working on other things- weird part is it seems to make a big difference. After the battery is unplugged for a while, the car runs great for a few days then eventually goes back to running rough. I ruled out a compression issue because the misfire and rough idle are so intermittent- if I lost compression I would have thought it would never be able to run smooth right? lol good point on the wires- the guy before me did have them mixed up but they were on the same waste spark cycle so it didnt ultimately matter- one of the first things I checked tho.