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Random no/hard start


GhostKa78

New Member
Joined
Jan 7, 2013
Messages
9
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Location
New Hampshire
Vehicle Year
1999
Make / Model
Ford
Engine Size
3.0L
Transmission
Automatic
Hey Guys having trouble figuring out whats going on. I have a 99 Ranger 3.0L auto. The issue at hand right now is randomly the truck will not want to start. i have spark and fuel. but it seems that is does not want to "Run" i have to hold the pedal down after several attempts to start it. it will fire up but run rough then clean up and run fine. i can shut it off and then fire up fine. I need to figure this out fast, its my only ride till the big rig is up and rolling. this issue happens at any temp and only after sitting over night but does not happen often. :icon_confused:
 


RonD

Official TRS AI
TRS Technical Advisor
Joined
Jun 2, 2012
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25,363
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Location
canada
Vehicle Year
1994
Make / Model
Ford
Transmission
Manual
No CEL(check engine light)?

Reads like the PCM(computer) is flooding the engine, holding injectors open too long for idle setting.
When you hold down the gas pedal all the way you are in effect balancing the air/fuel mix to get it to start then the extra fuel is burning off, the rough running.

On the intake there is a Throttle Position Sensor(TPS), this tells the PCM what position your foot is on the gas pedal.
The TPS is just like a light dimmer or volume control, a variable resistor, if it has a bad spot it could be telling the PCM the Throttle is Wide Open when it is closed, so engine is flooding until you do open the throttle plate all the way, foot to the floor.

You can test the TPS with an OHM/Volt meter.
Ford TPS is supplied 5vDC by the PCM, it's return voltage when throttle is closed should be about 1vDC, and then 5vDC with throttle open wide.
There are 3 wires, ground, 5vDC and return(1vDC to 5vDC)
You can test it with an OHM meter, disconnected, or with a volt meter connected with key on engine off and move throttle thru its full range watching for jumps in OHM or Voltage.
There should be a nice slow increase or decrease as throttle is moved.

Since throttle sits closed most of it's life that where the TPS can wear the most.
 
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