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Discussion Starter #1
I am having an idle issue. When I first start the truck, the idle goes to about 2000 rpm, then drops to about 900 rpm. When I blip the throttle, it settles at around 1500 rpm, and never comes back down. It's rock steady at 1500 rpm, when I put it in drive, holding the brake, it's rock steady at 1000 rpm. This is clearly too high.

I have done a lot of testing. My results are:

Codes:

KOEO
81 Air management 2 circuit failure (AM2/TAD). (I have no Thermactor system)

82 Air management 1 circuit failure (AM1/TAB). (I have no Thermactor system)

33 (KAM) EGR valve opening not detected. (This is a wiring problem that I haven't found yet. I verified that the new Motorcraft valve works properly with my vacuum pump, and it has a new EVP also.)

95 (KAM) Fuel pump secondary circuit failure. The EEC senses infinite resistance to ground from the fuel pump on the Fuel Pump Monitor circuit. (I just replaced a bad fuel pump last week with a shiny new Motorcraft pump)

KOER
44 Thermactor Air Injection system inoperative (Right side). (I have no Thermactor system)

77 System failed to recognize brief WOT during Dynamic Response Test. (This was user error)

I can find no vacuum leaks. I plugged each line at the tree, with no change in idle. I also listened all over the engine, to no avail. Most of my gaskets are new as of the engine swap this summer.

FPR tests good. Checked it out after my fuel pump swap.

The TPS tests good, both for voltage and internal resistance.

If I disconnect the IAC when truck is idling at 1500 rpm the idle drops to 900 rpm. If I disconnect the IAC, and then start the truck, the idles sits at 650 rpm. The IAC is a new Motorcraft part. I bought it because I did not know how old the old one was. It tests good according to Haynes, with internal resistance of 10 ohms. However, the PCM side of the IAC connector reads 12.08 volts with the key on. According to Haynes "It should be approximately 10.5 volts. This indicates that the BPA-ISC [IAC] valve is receiving the proper signal from the PCM."

Is 12.08 volts within the realm of "approximately 10.5 volts"? If not, does that automatically mean that the PCM is FUBAR?

As always, thanks.

Will
 

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Discussion Starter #3
However, the PCM side of the IAC connector reads 12.08 volts with the key on. According to Haynes "It should be approximately 10.5 volts. This indicates that the BPA-ISC [IAC] valve is receiving the proper signal from the PCM."

Sounds like someone messed with the throttle stop screw. With the IAC unplugged, the idle should drop to the point that the engine NEARLY stalls, and then after a few seconds, sputters out. (That's just a ROUGH method.)

should drop to ~600RPM with the IAC unplugged
So you guys don't think it has anything to do with the fact that the IAC is receiving 12.08 volts from the PCM with the key on?

(That's just a ROUGH method.)
Think I should give the rough method a shot? I'm pretty sure someone has messed with the screw in the past, the dab of orange paint on it is broken, but I don't want to mask any other problems.

Will
 

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Satyr of the Midwest
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Point of clarification: the IAC always has Vpwr (+12V) on one side. That's done so the PCM can use pulse width modulation (PWM) on the other side to augment plunger position. This type of signal is NOT readable with a traditional (D)VOM; you'd need an oscilloscope to see the pulse width to know for sure what it is. Sometimes you can get lucky and approximate the duty cycle with a voltmeter, but it's really not a good measurement.

Also, with the key on and the engine off, the PCM holds duty cycle @ 100%, i.e. it grounds the side opposite Vpwr so the valve is 100% open to aid starting.
 

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I stoped using the Haynes manual years ago, worthless POS IMO
Battery voltage is 12.6 - 14.7 where does 10.5 come from :twak

Unplug the IAB
Adjust the screw to aprox 600RPM
check the TPS voltage and adjust it to aprox 0.8 volts
plug the IAB back in
 

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Discussion Starter #6
If I disconnect the IAC, and then start the truck, the idles sits at 650 rpm.
Unplug the IAB
Adjust the screw to aprox 600RPM
check the TPS voltage and adjust it to aprox 0.8 volts
plug the IAB back in
It's already at about 650 with IAC disconnected at startup (see my quote above)

Point of clarification: the IAC always has Vpwr (+12V) on one side. That's done so the PCM can use pulse width modulation (PWM) on the other side to augment plunger position. This type of signal is NOT readable with a traditional (D)VOM; you'd need an oscilloscope to see the pulse width to know for sure what it is. Sometimes you can get lucky and approximate the duty cycle with a voltmeter, but it's really not a good measurement.

Also, with the key on and the engine off, the PCM holds duty cycle @ 100%, i.e. it grounds the side opposite Vpwr so the valve is 100% open to aid starting.
Thanks for the clarification. I agree with Fireguy, where does the 10.5 volts come from?

Anyway, I guess I will just use the screw to bring the idle down even further. The thing I can't figure out is why it stays around 1500 after the first time I touch the throttle. I'm getting tired of riding my brakes as I idle through the neighborhood.
 

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Satyr of the Midwest
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The only reasons I can think it'd do that is either a loopy TPS, sticky throttle bores/linkage, or an IAC with broken internals (dashpot, return spring, etc).

When you tested the TPS, was it with the key on, and did you sweep through the entire range, closed to WOT? Did the voltage increase smoothly? What's the voltage when the engine's idling? Have you cleaned the throttle bores and linkages lately? SeaFoam is great for that, btw.

Also, I'm with Ryan on a lot of the diagnostic routines in the Haynes. However, the 10.5V test (KOEO) just verifies that the PCM is commanding the IAC 100% open: Vpwr on one side, ground on the other. The reason that it's not specifying ~12V is because 10.5V is the published number Ford came up with; it accounts for variants such as low battery, bad ground connections, poor wiring, corrosion in the connector, or a high-resistance connection between the DVOM and the connector & wiring. After all of that is taken into account, dropping a couple of volts isn't all that much.
 

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Discussion Starter #8
The only reasons I can think it'd do that is either a loopy TPS, sticky throttle bores/linkage, or an IAC with broken internals (dashpot, return spring, etc).

When you tested the TPS, was it with the key on, and did you sweep through the entire range, closed to WOT? Did the voltage increase smoothly? What's the voltage when the engine's idling? Have you cleaned the throttle bores and linkages lately? SeaFoam is great for that, btw.

Shouldn't be a "broken" TPS, it did it with the old one, and the new one I just bought from Ford.

Throttle plates return to closed, but the idle doesn't go to where it should.

Everything is very clean, I just put the engine back together a few thousand miles ago.

I will double check the TPS, though. I haven't checked it at idle, but I'll try it out.

Thanks,
Will
 

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Discussion Starter #9
When you tested the TPS, was it with the key on, and did you sweep through the entire range, closed to WOT? Did the voltage increase smoothly? What's the voltage when the engine's idling?
I just retested the TPS. Here is my "TPS Report." (Get it?)

Engine Not Running, Key On:
Idle Position .915 volts, WOT 4.62 volts

Engine Running, IAC disconnected:
Idle position .929

Engine Running IAC connected:
Idle position .926

After I blipped the throttle, with the IAC connected and engine "stuck" at 1500 RPM, 1.011 volts.

I think the TPS is doing what it's supposed to. :scratchhe

Any more ideas?
 

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the TPS sensor is totally independent of the IAC. Idle air BYPASSES the throttle plate, so the TPS has no idea what the IAC is doing. Im curious why your TPS voltage went up .8 volts when you blipped the throttle. Sounds like the plate isnt closing properly.

Did you have the correct cover sheet on the TPS?
 

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Satyr of the Midwest
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Ah, it's staying above one volt after you blip the throttle! There's why the idle won't come back down. It's got to come back to its original voltage, or at least below one volt. Two possibilities: something's causing it to hang open (linkage or otherwise), or the EEC's own internal voltage regulator isn't keeping the +5V line constant. The increased engine speed momentarily raises alternator output; perhaps the regulator isn't compensating, resulting in a higher TP voltage returned. Most of the time, I set mine for ~0.87V or so @ idle, FWIW.

Can you try rotating the TPS a little, so the voltage is reduced slightly?



Oh, and I believe you have my stapler. :D
 

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Discussion Starter #12


the TPS sensor is totally independent of the IAC. Idle air BYPASSES the throttle plate, so the TPS has no idea what the IAC is doing. Im curious why your TPS voltage went up .8 volts when you blipped the throttle. Sounds like the plate isnt closing properly.
This is really driving me crazy. The plate seems ok, visually, and the throttle arm (sorry I don't know what that's called) is resting against the idle screw. I keep thinking it has to be something either mechanical (stuck linkage cable, etc.) or vacuum. I can't find vacuum, and it's so steady that I kind of discount it that way, too. The mechanical thing seems out also, since I can see the plate closing. AAArrrgghhhh. :banghead

Ah, it's staying above one volt after you blip the throttle! There's why the idle won't come back down. It's got to come back to its original voltage, or at least below one volt. Two possibilities: something's causing it to hang open (linkage or otherwise), or the EEC's own internal voltage regulator isn't keeping the +5V line constant. The increased engine speed momentarily raises alternator output; perhaps the regulator isn't compensating, resulting in a higher TP voltage returned. Most of the time, I set mine for ~0.87V or so @ idle, FWIW.

Can you try rotating the TPS a little, so the voltage is reduced slightly?
I just tested the constant voltage, and it remained at 5.08 volts regardless of engine speed, throttle, etc. So I guess it isn't the EEC regulator. When I installed the TPS, I had it cranked as far as I could to get it below one volt at idle.

I wish I could find a more detailed procedure for setting the base idle. I would use Gacknar's or Fireguy's, but it is already idling at 650 with IAC disconnected. I might give Steve83's a shot, though......

Will
 

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I just tested the constant voltage, and it remained at 5.08 volts regardless of engine speed, throttle, etc. So I guess it isn't the EEC regulator. When I installed the TPS, I had it cranked as far as I could to get it below one volt at idle.
The TPS sensor is just a variable resistor (potentiometer...whatever) that is dependent on the mechanical input from the throttle linkage. There is no way it can rise .08 volts (since you verified the 5 volt regulating circuit was good) unless the mechanical linkage changes (assuming the TPS also sweeps linearly).

Next time you have the TPS reading .9XX at idle, move the throttle manually until it reads 1.011...I bet the resulting RPM increase will match your complaint...the 1500rpm.
EDIT: I just looked at Steves graph, excellent info. If your throttle plate is open ~8 degrees (Im interpolating here...) it looks like the TPS would read 1.011 or thereabouts. Now think about an 8 degree opening of the throttle butterflies...that would easily cause RPM to increase to 1500 right?

My vote is mechanical linkage problems. Carbon on the throttle plates?
 

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Discussion Starter #14
I'm leaning more and more toward y'all's idea of mechanical problems. Here's why: I just went out and tried to back out the idle screw with the IAC disconnected. Guess what...it wouldn't drop below 600 RPM and there was a sizeable gap between the set screw and the thing (what is that thing?) that rests on it. Soooo, I disconnected the throttle cable and the TV cable, and dorked with it by hand. There is a definite spot where is seems to bump and/or hang up. If 8* would make that much difference this might indeed be my problem.

I have to go pick my dad up at the airport right now, but I'm going to take the TB apart tomorrow and see what I can find.

As always, thanks for all the great info, and I've learned a lot. I'll check back in with my results.

Will
 

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If 8* would make that much difference this might indeed be my problem.
Im revoting carbon buildup. It could stick to the walls, and cause the hangup, and could make a gap between the walls and the plate, preventing it from closing all the way.

You wont see it from the front because all the EGR/PCV gasses come in from the back.

If, when it is reading 1.011 volts, you push it closed with your hand and it subsequently reads .9XX....you definitely have a mechanical problem.
 

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Satyr of the Midwest
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Steve, thanks for the reminder, I forgot about the diode's characteristic switching voltage somehow! :doh0715: :duh

Yes, for an Si diode, 0.6-0.7V is the voltage drop across a conducting (forward-bias) diode. For those taking notes, the diode exists as a protective device for the PCM. As for the VOM being faulty, it wouldn't be the first time I've experienced one with an internal problem, but I agree anything with that much error needs to be destroyed. The "fudge factor" is arguable, but assuming all of the connections to be 100% perfect is just being idealistic. Regardless, I still don't trust Ford's own publications to be perfectly correct.

Back to wwatsonh's problem: I think a new throttle body would be a great idea if the cleaning doesn't provide absolution.
 

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I have a similar problem that occurred after I cleaned the TB. TPS is good and adjusted correctly, idle screw is adjusted properly. Pulled codes, nothing, I checked the IAC and it was bad, replaced it and determined the new IAC is working, but there is absolutely no difference if I unplug it. It still idles as about 950 rpms in neutral and constantly roams between 950 and 1200. When in gear, rpms are stable at 650-675 rpms if i am not moving, but is i let off the gas while moving it will idle at about 1100 rpms. Before I ran into the problem, it would idle at 650-675 rpms in and out of gear.

I wouldn't have any problems with this, but during the winter I let the truck idle at least 10 minutes before I leave so I don't freeze my balls off. I have noticed a considerable drop in fuel economy now that it idles so much higher. Any suggestions for solving this problem. I have pulled codes weekly and have had 111's for the past 6 months (before the problem started).
 

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Discussion Starter #18
33 (KAM) EGR valve opening not detected. (This is a wiring problem that I haven't found yet. I verified that the new Motorcraft valve works properly with my vacuum pump, and it has a new EVP also.)
I did, but I don't know about bobjoe.
 

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Discussion Starter #20
I'm officially a bonehead

:doh0715:

Well, I went to the junkyard today, and bought a "new" throttle body from an '88 F150 5.8. $15 later, I got it home, and compared the two. UUmmmm, one of these things is not like the other.

"Old" one (came with the engine, out of a '95 F250):



And "new" one (from the '88 F150 at the j/y):



Think those holes in the throttle plate might be my "vacuum leak"? Anyway, I put in the '88 throttle body after cleaning it, and the truck purrs like a kitten.

Moral of the story? Use the right t-body for the year, I guess.

As always, thanks for everyone's input and help.

Will
 
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