Ok guys, I have been trying to figure out what is causing this for far too long. My brother is a mechanic, and we have had many discussions regarding it, I have done myriad of repairs/maintenance since the issue started, and I need some assistance.

Description of the issue. When I accelerate, or throttle the engine in neutral above 2500 rpms, no issues. As soon as I decelerate, the engine begins to misfire. This can last anywhere from a few seconds to over a minute, depending on how quickly and how many RPMs I throttle the engine to. In neutral, a fast acceleration to 4K rpms, and a fast deceleration will sometimes cause the engine to completely stall. I get two diagnostic codes, P0171, and a cylinder 1 misfire code, which I think was P0301. I have checked for vacuum leaks, and repaired several. Intake manifold gasket, throttle body assembly gasket both done. I can find no more vacuum leaks. Plugs and wires have been done twice since it started happening. Valve cover gasket was done for other reasons, but no oil leaks from that since. TPS done, PCV done, I am losing my mind trying to track this down. I even did the fuel injector in cylinder 1 today, even though it tested fine. no change. MAF sensor has been cleaned and seems to be reading fine. OBD2 readings definitely indicate excessive positive fuel trim. I see the trim jump up to over 30 sometimes, and the trim is always high during the misfire. I am not sure which happens first, but the misfire goes away in concert with the trim dropping back too 2-3%. O2 sensor has been replaced with no change in behavior. Like I said, I am going crazy trying to figure out what this is. Even if I get a new truck someday, I will keep this truck until I figure out what is causing this.

Thanks for your help.

 

Welcome to the forum

One of the benefits of fuel injection is 0 fuel use when decelerating, with carburetors the high vacuum when decelerating sucked extra fuel in from idler jets, this is why on long downhill grades it was better to shift to Neutral and coast instead of using the engine as a brake, MPG-wise

Fuel injection computers have a fuel management routine that will turn off fuel injectors if RPMs are above 1,500 and TPS is showing under 1volt(closed throttle), i.e. coasting downhill in gear
Computer will inject some fuel now and then to keep Cat Converter hot, but it ain't much
And when RPMs fall under 1,500 computer will restart injectors at idle levels

So your description reads like a cylinder is FIRING when it shouldn't, not a misfire, so causing a jerkyness when decelerating, cylinder(s) are firing when they shouldn't be.
Long shot would be to check the vacuum line on the FPR(fuel pressure regulator), remove it and check for gasoline inside it, under high vacuum(deceleration) a slight leak in FPR could cause gasoline to be sucked into engine, causing cylinders to FIRE randomly

Stalling
The computer reads engine RPM at all times and it uses the IAC(idle air control) valve to set idle levels, the IAC Valve can be cleaned and tested
Remove it from the upper intake and plug in its 2 wire connector
Turn key on, IAC Valve will open all the way at that time for engine Startup
Now unplug the 2 wire connector and IAC Valve will close, it doesn't move all that much but does move.
Plug it back in
Repeat as much as you need to to determine IAC valve is working.

Clean it if it looks dirty inside

With IAC Valve back in place warm up the engine to operating temp
With engine idling unplug the IAC Valve 2 wire connector
RPMs should drop to about 500, or engine may even stall, either is GOOD
If RPMs do not drop then it could be a couple of things
Vacuum leak is usual cause, but check "idle screw" first

"Idle screw", as you should know fuel injection can't use an "idle screw", no jets, which is why they use an IAC Valve, there is an Anti-diesel screw on the throttle linkage the "looks like" an "idle screw"
If in above test the idle RPMs don't drop down with IAC Valve unplugged then someone may have mis-adjusted the Anti-diesel screw
Locate and turn the screw Counter clockwise with engine idling, RPMs should start to drop, if not then turn screw back to where it was, this is not your problem

If idle does start to drop then continue to turn screw until engine is barely idling, 500-600RPM, its now set again
This will also lower the TPS "closed throttle" voltage, you can test it with sewing needle and volt meter
Engine off, key on, pierce the center wire on TPS with the needle
Test its voltage, should be .69v to .99v, under 1 volt DC
Open throttle all the way, should now read 4.5v to 4.9v, at 4.5v or higher Computer will set WOT(wide open throttle) parameters
WOT will turn off AC compressor, for best power, computer will ignore O2 sensors and dump max fuel into the engine for best power, so important to get 4.5v at WOT
Adjust TPS accordingly


1995 and up Rangers are using the newer EEC-V computers which control Spark as well as fuel
If everything tests OK and engine still stalls and "misfires" you most likely have a computer problem, its not common but does happen
There is no way to test for it, if you couldn't read codes or reading codes took a few tries that would confirm computer issue

 

Ok, so after everything I've replaced, which is just about every part and hose that could be related to air and fuel, it turned out to be an exhaust leak. The flange between the header and the catalytic converter pipe was leaking. When I let off the gas, exhaust pressure would go down, and the cat would cool, drawing air in to the leak. this normally wouldn't do anything, except the placement of the O2 sensor makes this leak upstream from it, and it detected O2. Given the O2 sensor does not differentiate between O2 being drawn into a leak, and extra O2 in the cylinders, it would adjust the fuel trim accordingly. Since the extra O2 was not in the cylinders, and it was dumping extra fuel in there, it would flood the cylinders, causing a misfire. Took it to an exhaust guy who got it all fixed up for me, and BAM!, no more misfire on deceleration.

 

Good work

Thanks for the fix info

Never heard of an exhaust leak after upstream O2 causing that issue, have to remember that