1st time poster here and new member. I purchased a 93 Ford Ranger XLT, V6 and 4.0 engine 6 years ago. 2 days ago as I was stopped at a light, the rpm shot up to about 3000 on its own then came back down. When I stepped on the gas the truck staggered and pulled and I could not go above 15 mph. Then the RPM dropped to zero but I could still drive. I changed the spark plugs today and the ignition coil. I can start it but the rpm still fluctuates even in park. Can anyone give me a general direction to go in? I've researched that maybe the fuel pump needs replacement but gas is going through the fuel line. The truck has not had any major repairs since I purchased it, just regular maintenance.

 

Welcome to the forum.

First thing is that air not fuel changes RPMs.
i.e. if engine is at 1,000rpm and you give it more fuel it will flood out, if you give it more air RPMs will go up.

On all fuel injected engines there is an Air Valve that the computer needs to use to set stable idle, can't use an idle screw because there are no Jets like on a carb.

Ford calls their Air Valve the IAC(idle air control) Valve

On the 4.0l OHV it will be on the upper intake where throttle plate/linkage is, so front part, it will look like a can on its side.
Inside is an electric motor attached to a valve that the computer controls, it is on an air passage that by-passes the throttle plate, so computer can increase the air(RPM goes up) or decrease the air(RPMs go down).

It will have 2 wires connected, motor is held closed by a spring, computer Pulses 12volts to the motor on IAC valve to open it, spring pressure closes it, so pulsed voltage can set it at different air flow amount.

So first test for a vacuum leak in the engine, and IAC valve working
Engine needs to be warmed up for this
After engine is warmed up and idling, unplug the wires on the IAC valve, idle should drop to 500 or engine make even stall, either is good it means IAC valve closed and you have no vacuum leaks.

If idle doesn't drop then either IAC Valve didn't close or you have a vacuum leak.

PCV valve needs to be changed ever few years, like the coolant, and it IS a vacuum leak but a planned one, if PCV valve sticks open that can cause wild swings in RPM, it should be closed at idle and open at higher RPMs.

Cleaning IAC Valve: How to: IAC valve cleaning thread (w/pics) | Ford Explorer and Ford Ranger Forums - Serious Explorations

Good thread with pictures
If you remove the IAC valve and then plug in its wires you can test it for opening and closing.
When you turn on the key the computer will open the IAC Valve all the way for starting the engine, this is why engine RPMs SHOULD surge on start up and then drop down as computer closes IAC Valve.
Then turn key off and IAC valve should close
If the valve part is sticking or not moving then replace IAC valve unit, they don't fail often so wrecking yard replacement is usually OK

 

I warmed the engine up and unplugged the wires. Like you said, the rpm dropped to 500. Not sure where to go to from here x__X

 

And with IAC valve plugged in the RPMs swing up and down at idle or just while driving?

Next stop would be TPS(throttle position sensor) on opposite side of throttle plate from the throttle cable(gas pedal cable)
Need a Volt meter set for DC volts
And a sewing pin
Ground the meters black probe

Turn key on, engine off

Top wire on TPS should have 5 volts, thats from the computer
Use sewing pin to pierce the top wire and test voltage with Red meter probe

If 4.8-5.1volts are seen then good voltage

Now use pin to pierce Center wire
The wire goes back to computer to tell it throttle position
With throttle closed you should have .69 to .99volts, under 1 volt
Now open throttle slowly, voltage should start to go up
What you are looking for is a nice steady increase, no jumping or dropping of voltage, that would indicate a short or "dead" spot.
At full throttle Center wire should have above 4.5volts

 

Thanks for this tip. I'll try it out sometime this week. Super busy with Christmas. I tried to run it again, but it starts stalling once i hit 1500 rpm.

 

That is a new symptom :)

Stalling like it is running out of gas?

Dirty fuel filter or low fuel pressure.
May need to rent/get a fuel pressure gauge

1993 Ranger should have above 20psi when off, 30-40psi with engine running.
Raise RPMs and it should hold above 30psi, not start to drop

 

Fuel pressure is fine. Dumb question, but when you say "At full throttle Center wire should have above 4.5volts", does that mean with engine running and the gas pedal being stepped on? Sorry, I'm not the best mechanic. If so, when I step on the gas pedal, it chokes at 1500 rpm and the voltage only goes up to 2.1.

 

Swapped out the the computer but same problem. Currently lost x___x

 

Test TPS with key on engine off

Didn't read like computer problem

 

Gonna do that tomorrow. Have a lifetime warranty on the current TPS, so gonna go ahead and swap it out for a new one.

 

No change with TPS, caving and going to see a mechanic tomorrow. GG