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89 2.9 ranger runs rough at cold start


tsimmons21

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Feb 10, 2011
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Location
Dallas, Texas
Vehicle Year
1989
Make / Model
Ford
Engine Size
2.9 soon to be a 302
Transmission
Automatic
i have a cold start issue, after I drive it for about 3 miles it idles just fine. if I just start it up and let it idle after about a minute it will die. I've replace the iac, map sensor, checked vacuum but it is so erratic at start up I have no idea what it means, I've checked the plugs, they look maybe carbon fouled I know its running really rich at start up to. fuel pressure is good. could this be a simple coolant sensor? spark plug issue? maybe timing? i'm lost and it has so many codes in it I don't know what code to go by.

is there a way to attach videos to this so I can upload a video of it?
 


RonD

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canada
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1994
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Ford
Transmission
Manual
First, cold engine startup should go like this.
Crank engine(foot OFF the gas pedal)
engine starts and RPMs go up to 1,500+, because IAC valve is fully opened
RPMs then drop to approx. 1,100, cold idle set by temperature of coolant, as read by ECT sensor.
As coolant warms up idle will slowly go down
within 5 to 10minutes engine will be fully warmed up to 190-200degF, t-stat should be 192/195degF
idle should now be 650(manual trans), 750(automatic).


Fuel injected engines can't use a Choke Plate because they have no jets, so they use a computer software routine as the Choke which is required when starting any cold engine.
The computer will set a high idle, run a richer mix and advance the spark timing in "choke mode"

The first thing the computer does when you turn on the key is to check the ECT sensor.
ECT sensor is a TWO wire sensor located near the thermostat housing.
There is also a ONE wire sender that will be nearby, that is the temp sender for the dash board gauge, these look similar but are not shared.

The ECT sensor is a simple resistance sensor, low resistance when cold, high resistance when hot.
Computer sends it 5volts on one wire when on.
If computer gets back 3volts on the other wire then engine is cold, under 1volt and engine is warmed up.

Computer bases if engine is cold or warm on this one sensor.
You can check the voltage on both wires cold and then hot and see if sensor is working as it should.
But if the cold start info above doesn't match what you are seeing at cold start, i.e. high idle, then ECT sensor could be the issue, assuming IAC Valve is working.
 
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