TPS? or somthing else
Thread Starter
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Tuned
Joined: Aug 2007
Posts: 287
Likes: 1
TPS? or somthing else
1988 F250 351W - Reman. 4000 miles ago. All Sensors and everything else (almost) new at rebuild
Any ideas on how to pinpoint this annoying condition. The first startup of the day the engine works fabulous. Idle seems correct always going to about 800 RPM after coming to a stop. The first ride is in the mountains pulling a trailer about 12 miles. The truck sits for about 6 hours before the ride home. The second startup good - idle is initially normal at 800 RPM. Then somewhere in the first 5 minutes without fail the idle will start to hold at 1500 to 2000 RPM. Kicking the accelerator will not bring down the RPMs. Turning the engine off and then on will always fix the situation for the duration of the ride. Once while the idle was high, I stopped and pulled the IAC plug off. The idle dropped right off as expected. Replaced the IAC plug and the idle went back up. I keep wondering if the TPS is hanging? but why only on the second trip of the day?
I have a thought to run a few wires from the TPS to the cab, connect a digital voltmeter and check the TPS output when it's idling high to see if it's below 1.0 volts.
Any other thoughts would be great?
Any ideas on how to pinpoint this annoying condition. The first startup of the day the engine works fabulous. Idle seems correct always going to about 800 RPM after coming to a stop. The first ride is in the mountains pulling a trailer about 12 miles. The truck sits for about 6 hours before the ride home. The second startup good - idle is initially normal at 800 RPM. Then somewhere in the first 5 minutes without fail the idle will start to hold at 1500 to 2000 RPM. Kicking the accelerator will not bring down the RPMs. Turning the engine off and then on will always fix the situation for the duration of the ride. Once while the idle was high, I stopped and pulled the IAC plug off. The idle dropped right off as expected. Replaced the IAC plug and the idle went back up. I keep wondering if the TPS is hanging? but why only on the second trip of the day?
I have a thought to run a few wires from the TPS to the cab, connect a digital voltmeter and check the TPS output when it's idling high to see if it's below 1.0 volts. Any other thoughts would be great?
Cargo Master
Joined: Jan 2004
Posts: 2,512
Likes: 8
From: Cook Forest and Irwin PA
so the IAC is driving the idle up - this is good 'cuz its the only thing that can.
Now the next question is, is the IAC being driven, or stuck? You need a DVM with a duty cycle setting to find out what its saying, I dont have the spec with me, but in the service book there is a allowable spec for idle duty cycle.
If this is in range, time to think about stuck IACs. If its not in range, time to think about vacuum leaks or bad map sensors.
[edited to correct really bad script tags]
Now the next question is, is the IAC being driven, or stuck? You need a DVM with a duty cycle setting to find out what its saying, I dont have the spec with me, but in the service book there is a allowable spec for idle duty cycle.
If this is in range, time to think about stuck IACs. If its not in range, time to think about vacuum leaks or bad map sensors.
[edited to correct really bad script tags]
Thread Starter
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Tuned
Joined: Aug 2007
Posts: 287
Likes: 1
KOEO code was 11
CM code was 96 - maybe because I've been turning the engine off and on while driving to stop the high idle situation.
KOER code was 11.
I still have been using the original IAC that came with the truck - that's one engine component I never replaced during the rebuild.
The MAP was new from Ford a few months ago. I know there are no vacuum leaks verified by a secomd mechanic.
CM code was 96 - maybe because I've been turning the engine off and on while driving to stop the high idle situation.
KOER code was 11.
I still have been using the original IAC that came with the truck - that's one engine component I never replaced during the rebuild.
The MAP was new from Ford a few months ago. I know there are no vacuum leaks verified by a secomd mechanic.
Fleet Owner
Joined: Aug 2005
Posts: 23,666
Likes: 300
From: Easton,Ks
I do not know if this is what you are asking for but here goes.
The IAC is not opened like door half way, its either open or closed completely. The EEC sends many pulses per second to flutter the IAC open and closed. It flutters so fast that you should not notice any RPM fluctuation with a functional IAC. This fluttering is called “duty cycle,” when it’s calculated a percentage of on time. So when the IAC is off it is at 0%, when it is fully open it is at 100%, and a sliding scale of percentage for any RPM in-between.
The PCM monitors engine rpm and increases or decreases the IAC duty cycle in order to achieve the desired rpm.
A Duty Cycle Meter will display the ON or OFF periods in percent
Not all duty cycled solenoids are 12 volt solenoids.... some will burn out
very quickly with the application of 12 volts.
Let's start with one cycle... the time period is set by the device (in this
case it is the PCM) controlling the circuit. Just to make the math easy,
let's say the frequency of the circuit is 100Hz (100 cycles per second). One
cycle would last 10 milliseconds. At 0% duty cycle, the circuit is off all
the time. At 100% duty cycle, the circuit is full on. At 50% duty cycle, the
circuit would be turned on for one half of the cycle (in our example, 50
milliseconds) and off for the other half of the duty cycle.
More complications.... Some of these circuits are considered "full on" at
60% duty cycle... applying a steady 12 volts to one of these solenoids will
kill the poor little beastie quite suddenly. Others that do get cycled to
100% wont last long with a steady 12 volts applied but, in their normal
life, they will occasionally get 100% for very brief periods.
These types of circuits are extremely light duty.... It takes time for the
magnetic field to build and decay yet we still need the action to be rapid
to avoid a loping or porpoising effect (if one cycle lasts less time, it
becomes easier to position the pintle more accurately but we have less time
to build the magnetic field. The resulting weaker field can only move so
much pintle). Since these circuits are light duty, it doesn't take much to
"trap" the pintle.... very slight mechanical resistance can make these
controls fail.
The IAC is not opened like door half way, its either open or closed completely. The EEC sends many pulses per second to flutter the IAC open and closed. It flutters so fast that you should not notice any RPM fluctuation with a functional IAC. This fluttering is called “duty cycle,” when it’s calculated a percentage of on time. So when the IAC is off it is at 0%, when it is fully open it is at 100%, and a sliding scale of percentage for any RPM in-between.
The PCM monitors engine rpm and increases or decreases the IAC duty cycle in order to achieve the desired rpm.
A Duty Cycle Meter will display the ON or OFF periods in percent
Not all duty cycled solenoids are 12 volt solenoids.... some will burn out
very quickly with the application of 12 volts.
Let's start with one cycle... the time period is set by the device (in this
case it is the PCM) controlling the circuit. Just to make the math easy,
let's say the frequency of the circuit is 100Hz (100 cycles per second). One
cycle would last 10 milliseconds. At 0% duty cycle, the circuit is off all
the time. At 100% duty cycle, the circuit is full on. At 50% duty cycle, the
circuit would be turned on for one half of the cycle (in our example, 50
milliseconds) and off for the other half of the duty cycle.
More complications.... Some of these circuits are considered "full on" at
60% duty cycle... applying a steady 12 volts to one of these solenoids will
kill the poor little beastie quite suddenly. Others that do get cycled to
100% wont last long with a steady 12 volts applied but, in their normal
life, they will occasionally get 100% for very brief periods.
These types of circuits are extremely light duty.... It takes time for the
magnetic field to build and decay yet we still need the action to be rapid
to avoid a loping or porpoising effect (if one cycle lasts less time, it
becomes easier to position the pintle more accurately but we have less time
to build the magnetic field. The resulting weaker field can only move so
much pintle). Since these circuits are light duty, it doesn't take much to
"trap" the pintle.... very slight mechanical resistance can make these
controls fail.
Thread Starter
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Tuned
Joined: Aug 2007
Posts: 287
Likes: 1
Very informative.
After reading your post I think it's possible that I may have damaged the solenoid. I applied 12v battery directly to the solenoid when I had the IAC off during the rebuild. I found it very interesting that the pintle moved forward and back when the current was applied or removed and thought that the IAC was probably good. I had no idea of the theory behind the device and thus may have compromised it with my amateur testing method.
Thanks again.
After reading your post I think it's possible that I may have damaged the solenoid. I applied 12v battery directly to the solenoid when I had the IAC off during the rebuild. I found it very interesting that the pintle moved forward and back when the current was applied or removed and thought that the IAC was probably good. I had no idea of the theory behind the device and thus may have compromised it with my amateur testing method.
Thanks again.
