Bet this will stump you

So my 91 ranger 2wd is having an idling issue where at first start it will chug and die unless I rev the motor then it will run fine for a while then stumble and drop rpm, which is corrected by reving it up again and then it will idle fine again.

The more interesting problem is only when I'm moving (could be going 1mph or 85) is it acts almost like it has cruise control by keeping its Rpms the same or slowly increasing them, even if I push the clutch in and rev it up it will hold its rpm until I stop, the moment I stop the Rpms rise and fall normally.. Any ideas?

Pull off IAC valve and clean it

IAC(idle air control) valve allows the computer to set the idle, fuel injected engines do not have "jets" like a carb so an idle screw will not work.

IAC valve works by letting more or less air into the engine, like the throttle plate does.
The IAC valve has a Step motor on it, a step motor is an electric motor that can be controlled in "steps", they will have at least 100 "steps", the computer can "tell" the IAC valve to go to "step 20", for example, and motor will turn a precise amount.
The step motor opens and closes the air valve, more air is a higher idle less air a lower idle.
If the valve is dirty it may stick or air flow may vary.

With engine warmed up and idling, unplug the IAC valve wires, engine idle should drop to about 500rpms, it may even stall, either is OK at this time.
It means you do not have a vacuum leak.
If idle doesn't drop to at least 600 or lower then you could have a vacuum leak OR someone has messed around with the TPS(throttle position sensor) adjustment screw, thinking it is an idle screw.
This screw is located on the throttle linkage and "looks" like an idle screw.
If you adjust this and idle starts dropping then adjust it until idle is about 500, and you will know there is no vacuum leak.

Plug IAC valve wire back in and idle should go up to 700(manual) or 800(auto)

The IAC valve can only make engine run up to about 2,000rpm when fully opened, I think, so if your surges are going above that level then it may be something else.

An engine is basically a big air pump, restricting the air flowing in is the ONLY real control we have.
Adding more fuel won't increase RPM much and less fuel won't decrease RPM much, either can stall the engine, because of flooding or lack of fuel but RPM will remain constant until that happens.
RPM is set by air flow or lack of it

I cleaned the iac when the problem first happened and it changed nothing any other ideas?

nesralyrrej said:

I cleaned the iac when the problem first happened and it changed nothing any other ideas?

Click to expand...

"With engine warmed up and idling, unplug the IAC valve wires, engine idle should drop to about 500rpms, it may even stall, either is OK at this time.
It means you do not have a vacuum leak.
If idle doesn't drop to at least 600 or lower then you could have a vacuum leak OR someone has messed around with the TPS(throttle position sensor) adjustment screw, thinking it is an idle screw.
This screw is located on the throttle linkage and "looks" like an idle screw.
If you adjust this and idle starts dropping then adjust it until idle is about 500, and you will know there is no vacuum leak."

Did you follow the above instructions so someone can help you eliminate possible problems?..Just because the IAC is clean dont mean it cant be bad.

You need to go through the process of elimination to find the problem.

nesralyrrej said:

I cleaned the iac when the problem first happened and it changed nothing any other ideas?

Click to expand...

Yes, you do need to do some tests of the IAC valve, and other systems that are involved with air flow into the engine.

The computer can react very fast to RPM changes using the IAC valve.
'91 would be using a CKP(crank position) sensor, the computer uses this sensor to tell what the RPM is.

So if the drop in rpm is not recovered from fairly quickly I would also suspect IAC valve, but because of other symptoms you could be having the start of a computer failure.

Some auto parts stores have the older OBD I readers, and they will read the computer for free, codes are important but most important is testing if the computer can communicate with the reader.
If OBD I reader can't "talk" with computer then you are looking at a failing computer

So when I unplug the iac there is no change up or down in idle, i can't find any vacuum leaks. I cleaned the maf and went to the junk yard and picked up the best looking iac I could find, we'll I got two really, one that looked the best was off of a v6 and I'm not sure if it will fit.

So what should I do next? It no longer holds higher rpms while driving but when first starting it will sputter and die and when I'm sitting and idleing it will just start to miss badly and I can notice a lack of power while driving after that. In case it helps when driving all of the time i notice 3/4 of the wad on the petal has more power then all the way down. Thanks for any help

The IAC valve is either dead or computer control for IAC valve is.
Is Idle RPM higher when engine is cold?

So what were the changes when you installed the new(used) IAC Valve?

Rpms seem the same when cold maybe A slight increase no change from new iac valve, replaced the eec recently as we'll as the icm

Start up routine for ECC(computer) is fairly straight forward.
When key is turned on EEC gets power
It then powers 3 startup sensors and checks return voltage.
ECT(engine coolant temp) gets 5volts
IAT(intake air temp) gets 5volts
TPS(throttle position sensor) gets 5volts

ECT is a TWO WIRE sensor, only used by the EEC.
FYI, there is a 1 wire sender used for the dashboard temp gauge that looks similar.
The ECT sensor data(cold or warm) will cause the EEC to run engine rich with high idle(choke mode) if cold, or normal lean mix with low idle if warmed up.
This sensor can be tested with ohm meter, High ohms cold, ohms drop as ECT sensor warms up.
Volt meter connected to the two wires hooked up to ECT and key on, voltage will go down as ECT sensor warms up, 3volts at about 70degF, .5volts at 200degF(operating temp)

Without correct ECT sensor data the EEC will run in "Limp mode", CEL(check engine light) should come on if ECT data doesn't change or is out of range.

So ECT sensor is very important, it is the main sensor to set EEC in correct mode for startup and running.

IAT sensor data causes slightly rich/lean mixes based on outside air temp, cooler air is denser than warmer air so requires more fuel to be added, it is similar to O2 sensor in that it can cause EEC to run engine too rich or too lean if it is reporting the wrong data to EEC, so if MPG drops(rich) or engine is pinging(lean) then IAT(or O2) could be the issue.
IAT sensor can be tested the same way as the ECT sensor and will have similar ohm and voltage readings with cold and warm air.

TPS sensor gets the 5volts and if throttle is closed EEC sees .6-.9volts return voltage.
EEC "learns" TPS voltage so there are no exact voltages, under 1 volt closed and above 4.5 volts Wide Open are the specs to check.
But more important is the stability of the voltage when opening and closing the throttle, voltage should increase steadily as throttle is opened, no jumping around, and decease steadily as throttle is closed.
If there is a "dead spot" or "short spot" on the TPS then EEC thinks you have taken your foot off the gas pedal or you have "Floored it" when you hit that position on the TPS, since throttle plate position hasn't really changed engine doesn't die or run up to high RPMs but the fuel mix does change, leaning out or gets richer, so change in performance is noticeable at that problem spot on TPS.

Cold idle should be 1,000rpms +, colder outside temp = higher than 1,000rpm, but 1,000rpm would be minimum cold engine start idle.
It will then drop gradually over the first 5-10minutes to 700-800rpms as engine warms up.
If this isn't happening then either the IAC valve is not getting signal from EEC or EEC is not sending the correct signal to IAC valve, this could be because ECT sensor is reporting the wrong temp to EEC.

Oh man this next set of testing is gonna be fun.. I'll report my findings shortly

If your idle is not 1,000+ RPMs on cold start then there is only 2 things to check, assuming new IAC Valve, wiring for IAC, and ECT sensor

Replaced the iac with the iac off of either or a v6 or v8 and it runs better at startup but when idling it still stumbles

Fuel filter, my 99 ranger was the same way when I 1st got it, changed the fuel filter and add some prestone fuel treatment worked fine after that

couldn't hurt, going to replace the coils, plugs, and wires soon as well, will post any updates.

so howd it turn out?