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

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Old 02-22-2014
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92 4.0 idles tough

My dad has a 92 ranger 4.0, it starts then runs rough like it has a big cam. If you touch the gas it dies. After about 5-10 min of running rough it revs up and idles fine. Any ideas?
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Old 02-22-2014
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When you first start a cold engine it needs to run rich with a higher idle.
With fuel injection(EFI) you can't use a regular "choke plate", like on a carb, because there are no jets.
So EFI systems use an electronic "choke" run by the computer.
On the 4.0l near the front of the lower intake manifold, by thermostat housing, is the ECT(engine coolant temp) SENSOR, it has TWO wires.
The capitals were used because there is a similar looking SENDER, with 1 wire that is for the dash board temp gauge, both of these are located near each other.

The ECT sensor tells the computer the temperature of the engine, if cold the computer runs the engine rich and opens IAC(idle air control) valve more to raise the idle to about 1,000RPMs

If idle is high on cold start then ECT and IAC valve are working, if this is not the case then either could be at fault.
IAC valve can be removed and cleaned.
ECT sensor can be checked with Volt and/or OHM meter.
ECT sensor is a simple resistance sensor, OHMs are high when cold and then get low when warm, so test cold then test warm.

If idle is high on cold start then I would clean the MAF sensor, this is the main Fuel/Air mix sensor.
And if it has be very cold weather in your area I would test the IAT(intake air temp) sensor, same test as ECT sensor, if incoming air is very cold then computer needs to run engine even richer, but I would think this sensor acting up would effect engine after it warmed up as well.
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Old 03-04-2014
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It doesn't have a high idle. Its running rough as if it has a giant cam. It doesn't matter if its summer or winter he has to squirt a little ether in it to make it start. Fuel pump runs constantly when ignition is on KOEO. We've replaced the fuel pressure regulator, fuel pump, fuel pump relay, even tried a different ecm. Im outta stuff to replace. Any suggestions. There isn't a check engine light. It has correct fuel pressure. We're thinking its gotta be electrical. I'll try to post a link to a video to give you a better idea.
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Old 03-05-2014
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Reads like computer is bad with those two issues

Ford PCM(computer) only runs fuel pump for 2 seconds when key is turned on.
Computer does this by grounding the Fuel Pump Relay.
If computer is holding the ground ON then it is broken.

So "fuel pump running all the time" means something is wrong.
There is a connector on passenger side near firewall, it is there to read engine problem codes, OBD I, but it also can be used to ground the fuel pump relay using a short jumper wire, make sure someone has not put a jumper on that.

No high idle on cold start means PCM doesn't know or doesn't care that the engine is cold.
The ECT sensor, TWO WIRES, can be checked to determine if the computer doesn't know(ECT sensor is bad) or doesn't care(computer is bad)
This is the reason for the rough idle cold and why he needs extra fuel(ether) to start it cold.

Computer will add extra fuel for cold engine start and idle, this is "choke mode" for EFI cold starts.

The computer sends the ECT sensor 5voltsDC on one wire
The other wire will be a lower voltage being sent back to computer, 4 volts would mean 32degF(freezing), 3 volts would mean engine is at 70degF.
.6 volts would be 200degF, normal warmed up operating temp for engine.
So voltage goes down as engine warms up.

Set up Voltmeter for DC voltage
ground the Black probe.
Use a sewing pin to pierce one wire on the ECT sensor
Test its voltage, if it is 5volts then remove pin and pierce the other wire
You want the wire that is under 5volts
If both are the same voltage replace sensor.

If engine is cold and the lower voltage wire is below 2 volts then replace sensor.
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