4 liter rough idle when cold..............

[dpfurby]

I have the 4 liter engine in my Ranger. Starts up fine from a cold start-- starts at around 1100 RPM, then after a minute goes down to 600 RPM, but the idle is rough, but after it warms up then it idles fine.

Replaced and gapped new Motorcraft plugs.
Replaced plug wires with new Autolites.
New Throttle Position Sensor.
Cleaned the Mass Air Flow Sensor per the book.
Has new PCV valve.
Checked all vaacuum hoses-- no holes or tears.
My truck came without the EGR valve from the factory.
Has new upper intake manifold gasket, correctly torqued.
Throttle body was removed, cleaned with the Throttle Body Aerosol Cleaner.
Has a new coil pack and yes the engine does run a lot better.
Has new air filter.
Has new fuel filter.
I checked with vaacuum gauge and it is steady at idle-- 18 in. vaacuum.
Engine runs good, does not use any oil, just this idle rough when cold.
Gas mileage is reasonable between 16 and 20 mpg/ city/ highway.

Any ideas???

Thanks.

 

[RonD]

I would test the ECT sensor.
Engine Coolant Temp(ECT) sensor.

This is a fuel injected engine's Choke.
It is different than the 1 wire "sender" used for the dashboard temp gauge.

The ECT is a 2 wire sensor that is only used by the computer.
When the ECT shows a cold engine then computer will raise the idle and run engine rich with advanced timing until it warms up, 5 to 8 minutes depending on outside temp and driving conditions.
1 minute in not enough time for warm up, idle should go to about 1100 when first started, but then slowly drop down to about 600-700 after 5-8 minutes, it shouldn't just drop from 1100 to 600.
From your description it reads like the cold engine is running lean instead of rich, then when warmed up lean is normal

You can test ECT's voltage or OHMs
http://oldfuelinjection.com/index.php?p=28

 

[Fordisthebest30]

try cleaning out your idle air control valve. if its too bad, just buy a new one. theyre like 50 bucks and the plunger will stick and cause a rough idle.