1994 4.0 Rough idle, stalling - Ranger-Forums - The Ultimate Ford Ranger Resource


4.0L OHV & SOHC V6 Tech General discussion of 4.0L OHV and SOHC V6 Ford Ranger engines.

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Old 06-02-2016
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1994 4.0 Rough idle, stalling

1994 4.0 4x4 5-speed.




Truck began a sudden onset of stalling, sputtering, running rich with black smoke, etc while driving 60-65 mph. The Check Engine Light flashes on/off intermittently while the stalling and stuttering is going on. However at times the truck will seemingly run and idle just fine before reverting back to stalling, sputtering, running rich, smoking etc. There is no consistent pattern that I can tell. All of the symptoms occur while either driving or idling in the driveway.


Other symptoms have also included at times the interior lights flickering/fading with the battery gauge needle also flicking side to side along with the lights as the truck sputters, stalls etc.


Local auto parts store tested the battery/alternator and advised "there were no problems with the electrical system".


Yesterday we tested codes by using a copper wire with the key on and the engine running. When first performing the test the truck decided to run perfectly, during this period I repeatedly got the code 538 (3-1-1-5-3-8-5-3-8). Later tests the truck began to act up with all of the above symptoms and I received a 412 code as the CEL blinked weakly.




My service manual has 3 different 538 codes, and since I followed my service manual exactly for the test I tend to believe it is telling me the 3rd listing below involving the CID circuit failure. My Dynamic Response test consisted of me performing hard left/right turns on the steering wheel with hard press of the brake. No mention of pressing the gas was listed in my manual, although when I did press the gas (shot in the dark) in one test I still received the 538 code.


538 - Insufficient change in RPM/operator error in Dynamic Response Check
538 - Invalid Cylinder Balance Test due to throttle movement during test.
538 - Invalid Cylinder Balance Test due to CID circuit failure.




Trouble code 412 states "Unable to control RPM during High RPM Self-test". I assume this has everything to do with the truck barely running at that particular time.



Any thoughts, suggestions, or knowledge shared would be greatly appreciated. Thanks!

Last edited by rifleman85; 06-05-2016 at 12:09 AM.
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Old 06-02-2016
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Welcome to the forum.

CID is cylinder identification

On a '94 4.0l this is done using the Crank Position(CKP) sensor and the EDIS-6 module.

CKP sensor reads a tone wheel behind the main crank pulley
Sensor is at 10:00-11:00 on the front of the engine, close to the tone wheel
Tone wheel looks like a gear, 36 teeth minus 1 tooth, one tooth every 10degs then a missing tooth that IDs cylinder #1 at TDC.
The CKP sensor sends a pulse each time a tooth passes by, then a gap when #1 TDC passes by, this signal goes to the EDIS-6 module

EDIS-6 module runs the spark and is hooked to the coil pack
My '94 4.0l has EDIS module on the front side of rad support drivers side, you can see it looking up under the bumper.

EDIS module sends the CKP signal to the computer, computer uses that signal to time the fuel injectors, computer also sends EDIS module spark advance info from driver input, like Throttle position(TPS)

Good read here on Fords EDIS: Ford EDIS technical information

I would first check the tone wheel and CKP sensor for mud or ??
And make sure CKP is not loose.
CKP wires run up by the water pump, thermostat and into the main harness under upper intake manifold.


I would also test your Throttle position sensor(TPS)
It is just a variable resistor like a light dimmer or volume control
It gets 5volts from computer on upper wire
Lower wire is a Ground
Center wire sends .7volts to 4.8volts back to the computer to tell it throttle plate position.
.69 to .99v, under 1 volt, on center wire when throttle is closed is spec
Above 4.5volts when throttle is wide open(WOT)

Because this is a moving part it can get worn out, dead spots or shorts

Set volt meter to DC volts
Use a sewing pin to pierce the center wire
Turn on the key
Hook meter to a good ground and to the sewing pin
You should see .69-.99v
Now open throttle slowly and watch voltage increase
you are looking for sudden changes, dropping or jumping up of the voltage, replace TPS if you see this
At WOT you should see above 4.5v
And slowly close throttle watching voltage.


And on a 4.0l ALWAYS clean the MAF sensor anytime there is a problem.


After engine is warmed up and idling, unplug the wires on the IAC(idle air control) Valve
It will close and idle should drop down to 500rpm or engine may even stall, either is good, it means no vacuum leaks.
If idle stays above 700 then there is a leak

Last edited by RonD; 06-02-2016 at 04:24 PM.
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Old 06-05-2016
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Thank you very much for the info!

The CKP was not loose or gunked up bad, I suppose it still could be neither of those things and the sensor still be bad?

We did test the TPS sensor with the volt meter just as you described above and there were no jumps, the readings on the meter were very consistent idling up and down. With no throttle we showed 1.06 consistently, with the throttle wide open we showed 4.6 consistently. I assume those readings are close enough to what you listed above and aren't worrisome?

We have looked at the MAF sensor and there appears to be some blisters on the heater wires.

I have not yet specifically looked at the IAC valve and checked for vacuum leaks as you mentioned above.

The vacuum line for the Fuel Pressure Regulator was also checked and no fuel was seen to be in the line at all. (I saw you mention looking for this in another thread and thought I'd give this a try as well)

It was noticed today that a hose running from the Evaporative Emissions Control canister to the intake had completely rotted in half and was not connected to the canister in any way. We replaced this with a new hose and test drove the truck. The truck started and ran fine the first 3-4 miles before the CEL light came on, then within another mile or so the truck began trying to shut off and spit, sputtered, etc with the CEL light going on and off sporadically. We hoped finding the hose rotted in half was the culprit, but replacing it certainly didn't hurt.

So all of the problems originally described are still happening, and there still seems to be no consistent pattern. Although the truck does seem to run fine for a few minutes/miles once it has set an extended period of time before it starts acting up.

Last edited by rifleman85; 06-06-2016 at 09:54 AM.
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Old 06-05-2016
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CKP failure is a "no start", no rough running just no running at all

Engine computers have 2 main "programs"
Open Loop and Closed Loop

Open loop is when an engine is cold or a main sensor like O2 or MAF is out of it's set range.
Reason for Open Loop when engine is cold is because O2 sensors can't work until they are above 650degF, this is also why O2's are Heated.
In Open loop computer will use Factory tables for Air/Fuel mix, based on MAF air flow and RPM.
Once engine warms up the O2 sensors are used to give computer feedback about Oxygen levels in the exhaust, too much oxygen means Lean burn, too little oxygen is Rich burn.
So this is a Closed Loop, MAF>>>Computer>>>O2>>>MAF>>>Computer>>>O2>>>ect... ...

Open Loop, MAF>>>computer>>>............no Feedback

ECT(engine coolant temp) sensor is what the computer uses to see how warm engine is.
This is a TWO wire Sensor, right next to the ONE wire Sender used for dash board temp gauge.

ECT sensors are not too expensive, $8-$15, and you can test them in water being heated on the stove.

What can happen is that a failing ECT sensor could be telling computer engine is warmer than it is when cold, so computer is switching over to Closed Loop too soon and colder O2 sensors are causing bad air/fuel mix.
Just guessing though.

But the OK running after first starting and then poor running after a few minutes seems to point to something that has to do with temp.

Does it run OK after it warms up and you turn it off and restart?

Last edited by RonD; 06-05-2016 at 02:46 AM.
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Old 06-05-2016
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No the truck doesn't run any better after warming up, turning off, then restarting. It stays the same. The only time it ever seems to have a moment of running fine is after it has set overnight etc. Even then, it's not guaranteed to run right.

I checked the IAC valve today and the truck shut off as soon as I unplugged it. The truck was running rough at this moment already so it didn't need any help shutting off, but it certainly seemed to shut off like a switch when I unplugged it. So that was "good".

I also held the gas down and attempted to start the truck to see about leaky injectors. The truck turned over of course but was not actually firing. All seemed normal there.

What about the MAF sensor? There is a literal blister on one of the wires that runs between the little prongs. Obviously that's not suppose to be there. Also, we used a volt meter to perform different tests that we ran across online. The tests consisted of piercing the different wires in the harness with a pin and then using the volt meter to test between the wires and the battery posts. When doing these tests the volts raised and lowered steadily as we increased/decreased the throttle. However, the level of volts when there was no throttle being given often varied from 1.7 to 3.4 and was very inconsistent. Based on the blistered wire between the posts, and the inconsistent volt readings we are thinking the MAF could be a prime suspect. Thoughts on that?

What about Oxygen sensors? What is the likely hood of one of them being the problem here?

Last edited by rifleman85; 06-06-2016 at 09:56 AM.
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Yes, Replace the MAF.

MAF is similar in voltage output as the TPS(throttle position sensor), 0v to 5volt range
And should be consistent at consistent RPM, no jumping around
Low RPM = low air flow, under 1,000RPM should be just under 1volt, at 6,000RPM above 4volts.

O2 sensors "fine tune" the air/fuel mix, they can cause "pinging" if engine is running too Lean or lower MPG if running to Rich, but unlikely to cause your described problem.

MAF is most likely problem if it looks like you said.
You can unplug the MAF sensor and drive truck that way, computer will switch to RPM only based air/fuel mix, no optimal performance but should be steady.
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Old 07-15-2016
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Very overdue update here: The MAF was replaced and the truck now runs great again. Thanks again for all of the information, I have a much better understanding of the sensors etc under the hood.
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Old 07-15-2016
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Good work

And thanks for the update and fix info
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