My ranger has developed intermittent problems, what I mean by this is after a rain truck will start up but run extremely rough until warms up, then will go 1 hour or maybe a week with zero issues, other times it will start up with no problem but while driving it will buck then be smooth then buck. I checked the fuel pressure and is good, vacuum is great, 18.5in/hg steady, even when idling rough (like a misfire). I had a cel 171/174 lean bank 1 and 2, but cleaned the maf, throttle body and replaced the pcv, codes are gone and no new codes thrown.
No black or white smoke from the exhaust either. When driving and is bucking, I can floor it and it will get up and go no problem, but level off steady you will get whiplash, but not every time. I did a wot test to see if maybe I had a leaky FI, no start on wot while cranking, normal. I'm running out of ideas solve this mystery. Oh yeah, when it is idling rough, roms does not change, if I unplug the iacv, it stalls, I unplugged one at a time vacuum lines and engine would actually smooth out with a slightly higher rpm, unplug one spark wire at a time and would get worse.
This is everything that I can think of that I have tried so far, been doing this for a few months now.

Ty in advance, Reno

 

Welcome to the forum

What year is the Ranger 4.0l?

 

2000 automatic transmission, all stock

 

So a 4.0l OHV engine, 1990-2000 Rangers, (2001-2011 have the 4.0l SOHC engines)

You have covered most of the common issues, good work

At 23 years old, so should be getting its 3rd set of upstream O2 sensors, they only last 12 years, O2s are the ONLY sensors that have a time limit, 12 years or 100k miles and they start to run out of the chemicals that generate voltage
Could be the problem but not likely, so just a heads up

If you have a Code reader that can show "Live Data" check ECT(engine coolant temp) sensor temp of voltage, could be computer is operating engine under false temp readings
Cold start should show outside temp, i.e. 3volts or so, 60-80degF depends on your location, lol
Start engine
Within 5min it should show under 1volt, 180deg+
Under 150degF computer is using Choke Mode, which can cause issues if engine not cold, or rough running if it is cold

Also check TPS(throttle position sensor), it shows a %, 15-19% throttle closed, 90-92% at WOT
Key on engine off, watch the % change as you open throttle to WOT, should be steady, no sudden jumps or drops in %
Its a 5volt sensor, should be just under 1volt throttle closed, 1v is 20% of 5v
And 4.5v at WOT, which is 90% of 5volt
TPS is just a variable resistor so can get "dead spots" and computer responds to that like you suddenly took your foot off the gas pedal


4.0l OHV "eats" spark plugs, lol, Motorcraft or Autolite seem to last the longest, although I have had good luck with NGK as well, regular copper or Double Coating, never single coating
Big gap, 0.054

Have a look at the Coil Pack, it will show cracks when its time for a new one, but they tend to last basically forever, my 1994 is original
Spark plug wires last 10-12years or so

Can of Seafoam, or other injector cleaner, in the gas tank once a year smooths out my 1994 4.0l OHV, but its not like the issue you describe, just the odd misfires get more frequent as the year passes, lol
Dirty tips on Fuel injectors drip fuel instead of spraying it at lower RPM so poor air/fuel mix and misfires, but no issues when accelerating and injectors are more active

 

That "after a rain" business is where I'd look first.
Regarding FI cleaner, blokes on an oil related board I read swear by Chevron Techron Concentrate.
The secret ingredient (PEA) is said to work on injector nozzles at shut down. Use it before a stint of local driving.
The dosage is 1 oz. per gallon of fuel so the 20 oz. bottle is what I use in my 18 gallon tank.
When there's a 2 for 1 sale, I throw in two 12 oz.
There is another brand with high PEA but I can't remember the name.
And no, I don't shill for the company. I'm told this is the stuff Mercedes Benz dealers use.

 

It's about to rain here in Florida, city of water (Zephyrhills) so once sky clears I will use either for scan or torque and my phone to watch O2's I can also watch left and stft. When I read the part about the plugs I started thinking about how long it has been since I changed them, about 5 years, the truck sits more than being driven. I use it the most on rainy days or when I need my power chair due to my disability. But what would cause it to run the worst on cold start after a rain? Any other cold start it chugs and sputters but clears relatively quickly, and I will test that tps with my volt/ohm meter.

Thank you for your help, my luck it will turn out to be something simple, just overlooked, lol.

Reno

 

I believe that my tps is shot, I did connected it and to determine which line was power, signal and ground, then reconnected it turned key on to watch signal in volts, at first seemed normal then about 1/4 throttle volts started to bounce, not much at first, .1 to .25v then would bounce a whole volt i.e. at 2.5v drop to 1.25v then up to 3.1v then back to 2.5v. then I disconnected it again and started the engine, no difference in idle, nothing, let it warm up and stayed the same as usual. I reconnected it and started the engine again and took these snapshots of torque. I will need a O2 sensor as well, might as well change all 3.

Thank you everyone for your help, Reno