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More problems with the 2.9 idle dropping


Deercamp

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Jan 14, 2018
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Location
Pennsylvania
Vehicle Year
1995
Make / Model
Ford
Engine Size
5.0
Transmission
Automatic
Well my problems came back after running good for a month, My 1988 bronco ii 5 speed had a loose ground causing it to buck and surge. Well this month my bronco ii started running worse then ever it would start missing and would idle low at a stop sign then finally come to a complete stall. It got worse to the point where i couldent start my engine fuel pressure was at 0. I replaced the fuel pump relay and fuel pressure came back and i was able to start the truck. It drove good for a few days but today it's starting to do that idle surge again. but i still have power when driving. At a stop sign it will idle from 900 down to 500, If i give it gas it will shoot back up for a few seconds.
so i have been engaging the parking brake at stop and holding the accelerator so it idles at 1000. Any ideas this motor has been really frustrating it runs good until warm and idles like crap. Also before i replaced the fuel pump relay i had bad spark knock that went away. I always thought it was just knocking because it was cold. any help would be greatly appreciated
 


RonD

Official TRS AI
TRS Technical Advisor
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Jun 2, 2012
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Location
canada
Vehicle Year
1994
Make / Model
Ford
Transmission
Manual
Check IAC(idle air control) Valve

On all fuel injected engines an "air valve" is used to set idle RPMs, not a Ford thing.
An because of this you don't have to touch the gas pedal when starting a fuel injected engine.
Just crank over and engine should REV up to about 1,500rpm(IAC open all the way) then drop to 1,100rpm or so for cold idle(computer starts to close IAC Valve a bit)

When you turn on the key the computer will open the IAC Valve all the way for starting
Once engine starts computer starts to close IAC Valve to set "target" idle
Target idle is based on Coolant Temperature, from ECT(engine coolant temp) Sensor
Colder temp = higher idle, as engine/coolant warms up idle goes down

Pull off IAC valve from upper intake, 2 bolts
Plug its 2-wire connector back in if you unplugged it
Turn on the key-engine off
IAC valve should be open all the way
Unplug the 2-wire connector
IAC Valve should close all the way
Plug it back in and it should open all the way
Repeat this as many times as needed to see that it isn't sticking

Cleaning IAC Valve steps seen here: http://www.explorerforum.com/forums/index.php?threads/iac-valve-cleaning-thread-w-pics.84220/

IAC Valve gets 12volts(battery voltage) when key is turned on
Computer controls the GROUND wire on the IAC Valve, to move it to desired position to set idle RPMs
Key on-engine off computer provides full ground

Most alternators can produce 13.5 volts at idle, thats spec, if one of the 3 fields in an alternator fails then it reduces idle voltage, this can cause IAC Valve to close more than it should.
Failed field is often noticed from headlights dimming at idle, no that is NOT normal, lol, it means one of the fields in alternator is bad, battery still gets charged and everything will work, just not at idle RPMs.

This is a long shot but if you have a volt meter test voltage at battery, engine off and engine running.
 
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