Fast idle when cold
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Joined: Apr 2005
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From: Tennessee
Fast idle when cold
I've got an '89 F150 4x4 with a 302. When first starting a cold engine the idle is very high (no tach so I can't say how high) and the engine will die if I tap the accelerator to slow it down. If I let it idle for a few minutes or just take off at high idle, the idle is about right once it warms up. It has 143,000 miles on it. I used to do mechanic work for a living, even worked at a Ford dealership, but changed careers in the early 80s. Vehicles had carbs then and very few sensors. Anyone got an idea of where to start? The truck sat up for a while before I bought it. Could this be something stuck or carbon?
Last edited by tnh2oman; Jun 16, 2005 at 11:31 PM.
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Joined: Jul 2004
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From: Easton, Pa.
Here's how things work.
At inital ignition turn on, the EEC looks at the temperatire related values from the Coolant and Air Charge sensors.
This tells the EEC the engine is cold, to open up the Idle Air controller, set ignition timing and increae the amount of fuel injected.
The high idle is dependent on how cold the engine is and the air temp coming in and can raise the idle quite high in cold winter temp.
When this happens, the EEC does a timing function that will slowly brings the idle down over several minutes.
In more normal temps, and a fault free system, the idle will go upwards of 1000-1200 rpm for possibly a minute then come down.
The point the engine fully returns to low idle of about 750 depends on how fast the oxygen sensors heat to their operating temp of about 650* min and begin to output to the EEC. At this point they control the fuel and Air bypass for idle conditions.
Problem areas: You can see that a sticky IAC, dirty throttle body operation, faulty coolant sensor or ACT in single or combination would cause cold start and idle problems. Faults in these areas can also cause other operating problems such as excessive fuel use, hesitation etc. on a fully warmed engine.
To add a bit more to the operation, the IAC is a spring loaded solenoid that is "pulsed" by the EEC for precision control of air bypass prividing a very stable idle.
The idle speed is set in the EEC program. The sensors provide the set points that modify the base program that in turn operate the IAC to control idle to change that rpm value as needed. The reference point for the whole operation is the PIP sensor in the distributor.
The TPS (throttle position sensor) provides the same operation in electronic form that an accelerator pump does on a carb plus much more.
It tells the EEC how far the throttle is open (how much air to expect), how fast the driver wants to accelerate (rate if tps movement), when to go open loop for fuel (wot), when to engage the EGR system and when to update the memory for changing system conditions.
This is also the same essential operation that the present trucks up to 04 do with the exception of Mass Air that your truck does not have.
The difference in the systems is those engines without Mass Air are called speed density systems such as your's that uses a MAP sensor to also control the fuel by sensing the intake manifold vaccum level..
All you may need to do is some cleaning of the IAC and the throttle body to clear the problem.
I provided this amount of information so a search by others will be helpfull in understanding the system with out getting to deep into it's operation.
At inital ignition turn on, the EEC looks at the temperatire related values from the Coolant and Air Charge sensors.
This tells the EEC the engine is cold, to open up the Idle Air controller, set ignition timing and increae the amount of fuel injected.
The high idle is dependent on how cold the engine is and the air temp coming in and can raise the idle quite high in cold winter temp.
When this happens, the EEC does a timing function that will slowly brings the idle down over several minutes.
In more normal temps, and a fault free system, the idle will go upwards of 1000-1200 rpm for possibly a minute then come down.
The point the engine fully returns to low idle of about 750 depends on how fast the oxygen sensors heat to their operating temp of about 650* min and begin to output to the EEC. At this point they control the fuel and Air bypass for idle conditions.
Problem areas: You can see that a sticky IAC, dirty throttle body operation, faulty coolant sensor or ACT in single or combination would cause cold start and idle problems. Faults in these areas can also cause other operating problems such as excessive fuel use, hesitation etc. on a fully warmed engine.
To add a bit more to the operation, the IAC is a spring loaded solenoid that is "pulsed" by the EEC for precision control of air bypass prividing a very stable idle.
The idle speed is set in the EEC program. The sensors provide the set points that modify the base program that in turn operate the IAC to control idle to change that rpm value as needed. The reference point for the whole operation is the PIP sensor in the distributor.
The TPS (throttle position sensor) provides the same operation in electronic form that an accelerator pump does on a carb plus much more.
It tells the EEC how far the throttle is open (how much air to expect), how fast the driver wants to accelerate (rate if tps movement), when to go open loop for fuel (wot), when to engage the EGR system and when to update the memory for changing system conditions.
This is also the same essential operation that the present trucks up to 04 do with the exception of Mass Air that your truck does not have.
The difference in the systems is those engines without Mass Air are called speed density systems such as your's that uses a MAP sensor to also control the fuel by sensing the intake manifold vaccum level..
All you may need to do is some cleaning of the IAC and the throttle body to clear the problem.
I provided this amount of information so a search by others will be helpfull in understanding the system with out getting to deep into it's operation.
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Joined: Apr 2005
Posts: 51
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From: Tennessee
Thanks for all the info. I plan to clean the IAC and throttle body this week. My son is driving the truck now while his Explorer is in the body shop so it'll probably be late this week. I'll let you know how things turn out. I don't have a problem rebuilding carburators but I'm definitely a novice when it comes to fuel injection and electronics. Your info will be very useful. Thanks again.
Thread Starter
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Member
Joined: Apr 2005
Posts: 51
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From: Tennessee
I finally got a chance to work on the truck yesterday. I removed the IAC and cleaned it with Deep Creep. It had some carbon buildup. I also held the butterflies in the throttle body open and sprayed Deep Creep in it and cleaned it the best I could without removing it. The truck idles fine now and seems to run smoother. Thanks for the advice. 