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whats wrong with this picture


myold96

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my friend bought a 91 explorer that the p.o. spent a lot of money on the suspention but none on the engine, the heads were thrashed put new used heads on started truck up it ran beautiful smooth quiet lots of power so we shut it off went inside for the night next morning started up fine ,idles like crap,still plenty of power just bad idle. some days it will idle good some day not,changed brain box ,crank sensor,all the vac lines,air temp sensor, as well as any other sensor we could find,what could it be it has a 4.0 liter engine oh yes we also put in a new coil pack.
 


4x4junkie

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Did you pull any of the diagnostic codes before you started throwing all those parts at it?
 

RonD

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Fuel injected engines can't use regular "chokes", no jets.
So the computer operates in Open Loop(choke mode) when engine is cold, it raises the idle, runs injectors rich and advances the timing.
It knows the engine is cold by the ECT(engine coolant temp) sensor, this is a TWO WIRE sensor, that looks similar to the ONE WIRE sender used for the dash board temp gauge.
Easy sensor to test, read here:
http://oldfuelinjection.com/?p=28

Computer raises the idle using the IAC(idle air control) valve, this is a controlled vacuum leak, larger cylinder on the upper intake.
When cold, the engine should start and idle should go up to 1,500rpm or higher, then drop down to 1,000rpm, that's the computer testing the IAC valve, as engine warms up computer closes IAC until warm engine is at 750rpm.

If you start a cold engine and rpm doesn't go up to 1,500rpm and then down to 1,000rpm then IAC valve is stuck, easy to remove and clean this valve.
If rpm does go up but drops down to 750 then computer thinks engine is warm so check ECT sensor.
 

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