I recently purchased a '94 4.0 Ranger with 200k miles. It never idles high at start-up, whether it be cold, hot or anywhere in between. It struggles to find idle for maybe 1.5 seconds, then idles rather low. I have no tachometer to give specifics. I smell raw fuel out of the tailpipe until it warms up. The truck runs strong down the highway and will spin a tire with ease if you hammer the accelerator at a stop sign.

Here is what I have done to the truck:
Oil change 5w30 synthetic blend
Air filter
Fuel filter
Fuel system cleaner in 2 tanks of gas
Seafoam mist/fog through intake manifold
Patched cracks in air intake tube (with tape for now)
Cleaned throttle body
New IAC (idle air control valve)
New TPS (throttle position sensor)
New MAF (mass air flow sensor)
Checked and cleaned PCV valve
This truck does not have EGR

Receipts from previous owner (1 year ago: 2016):
Replaced computer (used)
Replaced starter (rebuilt)
Replaced fuel pump (new)
Replaced plugs and wires (new)
New brakes. New shocks.

No continuous OBD-1 codes. My CEL bulb doesn't work, but I have a scan tool.
Where do I go from here? I gave so little money for the truck that I don't mind throwing parts at it within reason (obviously).
I just want the thing to idle up at start-up like it's supposed to. If I jump in and drive it cold, it nearly dies at the stop sign at the end of my street.
I am new to RF, so if this thread already exists, sorry.

 

Welcome to the forum

Idle RPMs are controlled by the IAC Valve, computer varies voltage to IAC Valve to open it a little and close it a little, it is very precise should only vary +/- 5rpm

Cold engine
Remove IAC Valve from intake
Plug in it's wires if removed
Turn key on, computer will boot up

With RPMs at 0 computer will open IAC Valve all the way for start up, IAC Valve will have full battery Voltage, 12.5v approx.
Look at valve position
Unplug IAC wires
Valve should close all the way(no voltage)
Plug wires back in
Valve should open all the way(full voltage)

test the 2 wires, should have full battery voltage, test battery voltage first so you know what to expect
IAC's Red wire connects to fuel injectors, MAF, and other sensors, which all get 12volt with Key On
Computer controls the Ground wire on IAC, white/light blue stripe wire
Full Ground on key on, then it varies the Ground after startup to close IAC and set "target" idle.

How does computer know what "target" idle to use?
Computer uses the ECT(engine coolant temp) sensor, grey and green wires above thermostat housing on top of lower intake, to tell if engine is cold or warm, and temp sets "target" idle

ECT SENDER, used for dash board gauge, is next to the ECT sensor, different wire color, and probably just 1 wire on the SENDER, if gauge works then don't mess with this one

Sensors all have at least 2 wires

If your ECT sensor was telling computer engine is ALWAYS warmed up, then computer just "thinks" it is a quick restart, so no codes, just poor running until engine actually does warm up.

You can test ECT sensor with ohm meter, but it is not expensive to replace
You should test Grey wire for 5volts key on

Test IAC voltage and movement first, as thats the most common issue

New used to mean "tested and working"
New now means "you test it and we will replace it if there is a problem"

So don't assume any new part works and move on, you have to test it


It is troubling the CEL is not coming on
On the OBD connector look for the pink/green stripe wire, usually separate wire, connect volt meter or test light to Battery Positive and to this wire, turn on the key, you should see 12volt on meter or test light will light up.
This wire is the Ground for the CEL bulb in the dash, on boot up computer Grounds this wire to light up CEL

 

Thanks, RonD!

I tested the IAC movement and I see NO movement. It is closed all the time. There should be noticeable movement to the naked eye, correct?
The truck is cold so I assume it should open enough for me to notice.

I tested the voltage at the IAC plug as well, and it shows 12.1 volts which matches my battery voltage.
If I unplug the IAC while it is installed on the manifold (engine running), there is no change to the idle. And the plug voltage reads 13.8 which matches the alternator/regulator.

The truck actually idled better with the old, dirty IAC. Not correctly, but a little better. I think the plungers were deteriorated or stuck open a bit with gunk to allow more air.
Unfortunately, I gave the old one away to a guy with a 4.0 Explorer.

The new IAC is the cheapest one I found on Ebay. I think I will pick up a different one from the local parts store and see how that goes.
I will report back.
Thanks for sharing you knowledge with us folks here on RF.

 

IAC is dead, it opens all the way with key on engine off, no mistaking that movement

You are welcome

 

I bought a new IAC valve today from the parts store. Tested it before installation to verify movement. Bingo!
I installed it and have a nice idle now. Idles high at cold start, too.

 

Good work

Thanks for the update

 

If my truck was idling (poorly, but idling) with a non functioning IAC, that means I have vacuum leaks. Correct?

 

Maybe but it would depend on where the IAC Valve got stuck at before it quit working.

It's most common positions are closed, key off, and warm engine idle, 650-750
So if it died when engine was warmed up then that is where it would stay most likely.

But if its closed all the way then engine would not idle at all cold, and struggle to idle(500rpm) after it warmed up