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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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Rough Idle Question

A while ago I noticed that my truck would idle extremely rough when cold. When it warmed up it started idling smoothly. I also noticed that it was idling very quickly, like 1200-1500 rpm at stop signs and lights. I have also gotten CEL with codes for #3 cylinder misfire and lack of upstream hot air. I replaced the ignition coil, the O2 sensor, the mass air flow sensor (the whole assembly), air filter and fuel filter. It is idling at a more normal speed but the same day I replaced the fuel filter a CEL came on. I haven't had a chance to get the code read. I reset the code by disconnecting the battery for a bit to see if it was a fluke. It also appears that the majority or the rough idle when cold has gone away. Just wondering if anyone has anymore ideas as to what to check next. I am running some seafoam through the fuel system now.
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It would help to know which 4.0L engine you have.
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4.0L OHV engine
141,000 miles
Plugs and wires are about 20k miles old
Thinking I will replace the IAC on Monday.
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After engine is warmed up unplug the two wires on IAC Valve, it will close all the way
RPMs should drop to 500 or engine may even stall, either is GOOD, it means no vacuum leaks in intake side

If RPMs do not drop then you have a leak

If you want to test IAC Valve then with cold(or warm) engine remove it from intake.
Plug in its two wires connector
Turn on key(engine off)
IAC Valve should now be all the way open for starting engine
Unplug the two wire connector, IAC Valve should close all the way, no power
Plug it back in and it should open all the way
Repeat as you see fit to make sure it is not sticking

All fuel injected engines use IAC Valve, that's Fords name for it but all use same air valve setup to set idle.
With key on IAC Valve opens all the way for startup
Without touching gas pedal all engines should REV up on startup and then RPMs will drop to "target" idle as computer closes IAC Valve.
"Target" idle RPMs are set by coolant temp, and preprogrammed at factory, if engine/coolant is cold idle might be 1,100+, the colder the temp the higher the idle.
Computer also runs CHOKE mode on cold engine, Richer fuel mix, higher idle and change in spark timing, same as Carb engine needed, except you can't use a choke plate with fuel injection or an idle screw.
The Coolant temp comes from ECT sensor on the engine
There are two coolant temp devices
ECT(engine coolant temp) sensor, used only by the computer
ECT SENDER, used only by dash board temp gauge

They are different electrically so not interchangeable, but look similar and are often located next to eachother

If ECT sensor it telling computer coolant is 140degF and it is really 60degF(cold) then engine will run poorly until it actually warms up, needs more Choke

4.0l OHV eats spark plugs, Motorcraft are best bet, Autolite next best
Only use regular copper OR Double Platinum, never single platinum, waste of money

Cold engine gets Rich mix so needs Strong spark, which is why 4.0l uses 0.054 spark plug gap, big gaps makes stronger spark
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I replaced the PCV and noticed an immediate increase in power. I haven't noticed the rough idle when it's cold yet. I also had to replace the Bank 2 Sensor 1 O2 sensor since the code came up on the CEL. I'm replacing the ECT as well. Will let you know if anything else pops up.
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