Ive been chasing this problem since I bought the truck. Ive also been searching this forum and have listened with a stethoscope on the passenger side injectors and they sound good, can't get to the driver side injectors to listen to them. At an idle the truck sounds great and smooth, rev it up and sounds nice, quick and crispy. But, if you barely crack the throttle while in park, say hold just under 2k rpm the engine will stumble and sound like its flooding and lose rpm. same while driving where you are barely opening the throttle especially around 30-45 and set cruise it will get really rough. but go up to 65 or 70mph and its nice and smooth. Ive replaced the plugs and set gap to spec, new plug wires, new coil, checked fuel pressure and within spec and stayed steady when stumbling. ran around 500 miles with Lucas fuel injector cleaner, new fuel filter, tried running 87 and 91 octane fuel, gets crappy fuel mileage either way and no change in the problem. service regularly, has good power when you go wide open throttle, ????? Please help, Also no check engine light or codes other than the ones i already know about with are ABS front speed sensors, and driver air bag.

 

Now as i have it outside warming up, i can hear a miss at idle... injectors? burnt valve?

 

Welcome to the forum

Assuming 1990-1997 4.0l OHV?

Behind alternator on lower intake is the Fuel Pressure Regulator(FPR), it has the Fuel Return line and also a vacuum line, check that vacuum line for gasoline.
If FPR diaphragm starts to leak fuel can be sucked into the intake at higher vacuum levels causing low MPG and misfires, lower levels like when accelerating would run fine.

Clean MAF sensor, should be done once every 2 years or so on the 4.0l, very easy to do

Also if O2 sensors haven't been changed recently, change them.
O2 sensors use a chemical reaction, like battery does, so they do wear out, just like a battery does.
O2s are usually good for 100k miles but if engine is running Rich then that shortens their life.
O2 sensor plug ins are universal, so if you do change them do it ONE AT A TIME, there is a passenger side and drivers side O2, computer needs to know which is which, and since plug in is the same for either side they can get reversed, and that would make for very poor running and MPG.

 

Thank you for the reply and the info.Yes its a 1995 Ranger XLT auto 4.0L V6 Im going to check the vacuum line on the FPR right now. How many O2's does this thing have total?

 

1995 and up V6 will have 3, 2 "upstream" near exhaust manifolds, drivers side and passenger side
1 "downstream" after Cat Converter, it is used to test if Cat is working, it couldn't cause engine problems, only the upstream O2 sensors can.
Downstream also "sees" cleaner exhaust so tend to last 300k+ miles

All 3 are the same type of O2 sensor so interchangeable, about the only difference when ordering O2s is the length of the lead, the connector wire, that plugs into the engine wiring harness, thats what the different part numbers are for

 

Well I seem to have fixed it, I found a broken vacuum line that comes off the FPR and goes down to the vacuum canister on passenger side of radiator, down about halfway. Runs smooth at all RPM. hope to get better mpg.

 

What you describe may be the Vacuum Reservoir, black ball usually, it holds vacuum in reserve to run the cab Vents, i.e. defrost, panel, floor controls

FPR = fuel pressure regulator, it can share the vacuum line from intake to Reservoir, with a "T" connector

 

Vacuum line fixed the idling rough problem but the main problem is back now that i have drove the truck again. At an idle, in park, rev the engine just barely below 2000 rpm and engine will stumble and lose rpm. set cruise at 30 or 40 and it will do the same, stumble and hesitate. but if you set cruise at higher speed where the engine is working harder, its runs smooth.

Do you still think this could only be the O2's? I did check the vac line to the FPR, no fuel in line. fuel pressure was 33-35 at idle.

 

I was able to test the O2's with a scanner. The front 2 seem to be working fine. The rear is dead. I bought a new one and installed, still dead. all connectors are good. Do i possibly need the new harness that goes over transmission out to the o2's or are there some known problems here. How would i test resistance for a broken wire?

 

I was able to test the pin 91 (signal return) from ecm plug in to the rear 02 sensor plug and had continuity on the orange wire. But, from the ECM to the other wires, no continuity. Im not sure if i used the wrong pin out numbers, but 2 of them were not matching up with the color format of the wires. does anyone know for sure which ecm pins go to the rear o2 sensor for sure.????? and what color of wires i should be seeing at the rear o2 plug to match and be able to test continuity? Thank you for any help!!