I have a 1996 Ranger XL with the 4.0. It runs and drives great but randomly dies at stops. Sputters slightly and dies. It will not start right away, I have to sit with the key off for about a minute or so, then it starts right back up like nothing ever happened. Replaces plenty of things along the way and no resolve. Has new Plugs, wires, IAC, reman maf sensor, intake gaskets, and reman injectors. Occasionally, it’ll start running rough and go into open loop mode and the scan tool displays OL-FAULT. No other CEL codes at all.
Trucks been doing this since I bought it two years ago and I still haven’t figured it out. It runs great at idle and as long as you’re cruising, it has no issues but as soon as you come to a stop it just cuts out completely with no warning. Torque converter has already been ruled out. Any ideas?

 

So an automatic

If you shift to Neutral before coming to a stop does idle stay at 750-800 or does it still stall?

IAC Valve replacement needs to be Motorcraft or Hitatchi brand, other brands will cause what you have same as a bad IAC Valve
A 20 year old wrecking yard Factory IAC valve is a better bet than 3rd part new, lol

When you shift into gear the computer should bump up the idle by 50-75 RPMs same as turning on AC should

Temporary fix would be to adjust the Anti-diesel screw on the throttle linkage
Looks like an idle screw
Warm up engine and let it idle
Unplug IAC valve's 2 wire connector
Idle should drop to 600rpms or so, if engine stalls then turn Anti-diesl screw one full turn clockwise(tighten)
Restart engine
(CEL will be on because IAC is unplugged, it will go away once its plugged back in)
Should idle now, low but stable idle, if not turn the screw a bit more to get it to stable idle around 600-650

Put trans in gear, R or D
See if it stalls
If it doesn't then plug IAC Valve back in and idle should go up to 750-800

And after engine stalls when stopping, to restart faster press gas pedal down to the floor and HOLD IT DOWN all the way, crank engine for a 5 seconds then release gas pedal and it should start
This is called Clear Flooded Engine mode
When a warm engine stalls with key on the injectors keep pumping in fuel until RPMs are at 0 so it can Flood the engine, so you have to wait for fuel to evaporate to restart
Clear Flooded Engine shuts off injectors and allows max air flow(throttle wide open) to dry out the engine faster
When gas pedal is release injector start working again

I use Clear Flooded Engine every morning to start my high mile 4.0l
It gets the oil pump spinning so no Dry Starts
I crank for about 5 seconds then release the gas pedal, and no more "ticking" from dry lifters, lol

 

If I shift to neutral while I’m rolling to stop, it’s still stalls. Has an Motorcraft IAC valve in it. Still stalls if I put it in neutral before coming to a stop. Also makes no difference whether I unplug the IAC or the mad sensor but there are no vacuum leaks with a smoke test.

 

When you cold start the engine does it idle at 1,000-1,100rpms, then slowly drop down over the next few minutes to 750-800, it should, means IAC Valve is working
If it doesn't then IAC Valve is not working, or computer control is not working

Will the engine idle warm?
At 750rpms

Should be a red wire on IAC Valve, unplug wires and test red wire for 12v, key on, ground meter to engine or battery negative
Assuming it shows 12v, test both wires, use the other wire as the ground for meter
Should see 9v to 11v, key on, if not computer is not controlling/opening IAC valve

Computer has these 3 basic codes it can use for IAC Valve
P0505 Idle Control System Malfunction
P0506 Idle Control System RPM Lower Than Expected
P0507 Idle Control System RPM Higher Than Expected

So its ODD that you are not getting a code when engine stalls
When you unplugged the IAC valve and turned on the key did it set P0505?

What about when you unplugged the MAF sensor?
That will for sure set a code

Bluetooth code readers are $15 and well worth the money, I have used the ELM327 for many years and have no complaints

 

It cold starts and idles around 1500 and over the next minute or so drops to about 800-850. Scanner shows that the IAC valve is moving as it should. Sets an IAC valve circuit code if I unplug it, however nothing if I unplug the maf. It idles higher of i do unplug it. I have a foxwell scanner from Amazon that gives me the same functionalities as an expensive snap on scanner, so I’m able to read the live data from almost every sensor. The MAF is recognizing a change in airflow when revved, IAC responds as well. IAT Reads very close to the ambient temp outside. I even checked to see if the amperage of the heated o2 sensors changes. It stays pretty constant. I’ve tried another MAF and it makes no difference

 

Just noticed while looking over the LD again… Bank one sensor 2 has no voltage at all. Could that cause it? It’s also has no muffler so maybe not enough back pressure for the o2s?

 

No 4-stroke engine has Back pressure, and runs well, that's a MYTH started by people who installed the wrong headers and LOST power, lol

A properly designed exhaust manifold sucks out exhaust, creates a lower pressure at the exhaust valve ports, which boosts power
Read about: Scavenging exhaust

No Muffler won't effect engine operation or O2 feedback, just noise level, lol

O2 Sensor 2's are for Cat converters(downstream), to test if they are working, they should have a steady voltage of 0.7 to 0.8 after full warm up, they don't jump around as much as Sensor 1's(upstream)

There is often only one O2 sensor 2, Bank 1 sensor 2
Unless there are Dual exhausts, then there is Bank 2 sensor 2
So make sure your scanner is not confused and reading B2S2 which would be 0 volts instead of B1S2 which should be 0.7v+
Check B2S2 if it has voltage then scanner is making the mistake

 

The scanner recognizes that it has no B2S2 sensor. It doesn’t even show up on the data list unless I plug it into a vehicle with 4 o2s. B1S2 reads 0-0.001 volts at any given time. B1S1 and B2S1 are pretty erratic reading from 0.4-0.8v at idle. Keep in mind the cat is pretty much junk so I expect B2S1 not to be steady

 

I think you mean B1S2 O2

S1's are upstream O2s so should jump around alot

S2's should be steadier, but without working Cats they can be jumpy as well

O2 sensors generate their own voltage once they are heated above 600-650degF
0.1v is high oxygen
0.9v is low oxygen

O2s can only "see" oxygen, not fuel
After warm up, Computer calculates/sets open time on injectors and varies it to try and keep the upstream O2s at an average of 0.3v-0.5v

Cat converters burn up most of the oxygen in the exhaust when burning up emissions and unburned fuel(unburned fuel keeps the Cats HOT)
So Downstream O2s would have a steadier voltage of 0.7v - 0.8v low oxygen, after Cats
Computer runs engine Richer every few seconds to give the Cats unburned fuel to keep them HOT, so if the downstream O2 is showing a lower voltage than it should(no or bad Cat) then it runs engine richer all the time trying to get downstream O2 to a higher voltage
So MPG will go down a bit with bad or no Cats

Idle is a richer mix, so engine/cylinders don't overheat, computer basically ignores O2 feedback at idle AND at WOT(wide open throttle)
Best to test O2 voltages when cruising at a steady speed/RPM

STFT(short term fuel trims) are a %, computer calculates injector open time as STFT 0
-10% to +10% is normal
If it gets close to -20% then Rich codes are set
+20% then Lean codes are set
The % is the corrected open time from STFT 0, so -10% means computer closed injector 10% sooner than calculated to correct O2 voltage, +10% means computer kept injectors open 10% longer than STFT 0
Computer does this correction for each Bank based on O2 sensor feedback from that side of the V6 engine

Codes are not set if STFTs go up or down momentarily i.e. accelerating or decelerating, codes are set when STFTs are averaging higher or lower than 20%