thoughts on high-idle

[Pix3L8]

Hey guys, so since my last post, I got my 2.9 put back together, in the truck, and it's running like a champ, purring like a kitten. You see the only issue that I'm having, which as I understand, is a 2.9 "thing" is that my idle won't go down no matter what I do to it. If I start the truck and bring it up to temp the high idle shuts off and it drops from about 1500/1700 down to roughly 1000 and just sits there. when I rebuilt the motor I replaced the MAP, IAC, TPS, and o2 Sensor. SOOOOOOOO....it cant be a sensor, right? my thoughts are that it may be my o2 sensor because a few years ago I had a high idle issue like this that a new sensor fixed. The last piece of the puzzle, when I tweaked my timing when I first got her running I set the timing at 10deg with the spout unplugged and the engine idles down with the connector out....suspiciously right about at 800 or so, right were it should be, but it runs rough. So, gentlemen, I'm stumped, could it be a timing issue, maybe a faulty or not correctly grounded o2 sensor, who knows maybe the truck is just haunted. I love to hear your thoughts on this.

Oh, and one more thing, after it's warmed up and off "high idle" if I rev it up at all the RPMs are very slow to fall and it seems to hover around 1500 for a long time before it comes back to 1000, some times it just won't and I have to shut the truck down.

 

[rusty ol ranger]

Vacuum leak.

Or faulty new IAC

What year is it? If its 86/87 id look hard at the EGR valve/solenoid.

 

[Pix3L8]

It’s an 89, it is throwing a lean code for the o2 sensor and now it’s throwing a code for the tps under voltage but I checked and it’s at .8v-.9v which should be in spec

 

[Dirtman]

In the future, dont throw a bunch of new parts on to try and solve a problem. Aftermarket electronics like sensors and IAC valves are a crapshoot. So not only are you unsure what the problem is, but have to face the very real possibility that one or more of the parts you replaced is faulty.

 

[Pix3L8]

In the future, dont throw a bunch of new parts on to try and solve a problem. Aftermarket electronics like sensors and IAC valves are a crapshoot. So not only are you unsure what the problem is, but have to face the very real possibility that one or more of the parts you replaced is faulty.

I totally agree with you, I got carried away on rock auto and now I’m in a pickle

 

[Dirtman]

I'd check for a vacuum leak first. Youtube has a ton of DIY leak test videos. I prefer to use a homemade smoke machine.

 

[Pix3L8]

I’ll try the cigar trick when I get home. The only way that I could get the idle down to about 900-1000 was by retarding the timing to tdc base timing and about 10 deg with the spout plugged in

 

[RonD]

With SPOUT plugged in????


Spark should be 20deg
10-12 with SPOUT unplugged

Reset spark timing back to where it should be

After engine is fully warmed up................
Unplug IAC Valves 2 wire connector
RPMs should drop to 500 or engine may even stall, either is GOOD it means no vacuum leaks, if that happens then skip to **

If RPMs do not drop then it "may" be a vacuum leak but one more thing to do FIRST
On the throttle linkage is an "anti-diesel" screw, often confused with an "idle" screw, but of course it can't be because there can be not idle screw on a fuel injected engine, there are no idler Jets!

Anyway, find the screw and turn it counter clockwise while IAC Valve is still unplugged and engine is idling high.
If RPMs start to drop keep turning it until RPMs are under 600 or engine is barely running
Then its set, someone adjust it and didn't know what it was for

If turning the screw does NOT lower RPMs turn it back to where it was, you have a vacuum leak OR wrong IAC Valve

You can only use Motorcraft or Hitachi IAC Valves on Fords, any other brand will give you intermittent issues or what you have now


**If unplugging the IAC Valve DOES drop RPMs to 500 or engine stalls, then no vacuum leaks and anti-diesel screw is OK
But the Computer or IAC valve is not
The computer is telling IAC Valve to set high idle, or IAC Valve is misinterpreting voltage from computer(wrong brand IAC Valve)

What are the two codes?

TPS under voltage doesn't actually mean TPS is under voltage, lol, well in the way you think it does, most codes don't translate into english very well
It means TPS voltage is below what it should be with MAP sensor showing lower vacuum in intake

The MAP sensor tells computer the vacuum level in the intake, when you open the throttle the vacuum will drop, so TPS voltage and that drop should match up within a certain range, this is the LOAD on the engine
So it could mean TPS voltage is at 2volts and should be at 3v with MAPs vacuum reading

 

[Pix3L8]

With SPOUT plugged in????


Spark should be 20deg
10-12 with SPOUT unplugged

Reset spark timing back to where it should be

After engine is fully warmed up................
Unplug IAC Valves 2 wire connector
RPMs should drop to 500 or engine may even stall, either is GOOD it means no vacuum leaks, if that happens then skip to **

If RPMs do not drop then it "may" be a vacuum leak but one more thing to do FIRST
On the throttle linkage is an "anti-diesel" screw, often confused with an "idle" screw, but of course it can't be because there can be not idle screw on a fuel injected engine, there are no idler Jets!

Anyway, find the screw and turn it counter clockwise while IAC Valve is still unplugged and engine is idling high.
If RPMs start to drop keep turning it until RPMs are under 600 or engine is barely running
Then its set, someone adjust it and didn't know what it was for

If turning the screw does NOT lower RPMs turn it back to where it was, you have a vacuum leak OR wrong IAC Valve

You can only use Motorcraft or Hitachi IAC Valves on Fords, any other brand will give you intermittent issues or what you have now


**If unplugging the IAC Valve DOES drop RPMs to 500 or engine stalls, then no vacuum leaks and anti-diesel screw is OK
But the Computer or IAC valve is not
The computer is telling IAC Valve to set high idle, or IAC Valve is misinterpreting voltage from computer(wrong brand IAC Valve)

What are the two codes?

TPS under voltage doesn't actually mean TPS is under voltage, lol, well in the way you think it does, most codes don't translate into english very well
It means TPS voltage is below what it should be with MAP sensor showing lower vacuum in intake

The MAP sensor tells computer the vacuum level in the intake, when you open the throttle the vacuum will drop, so TPS voltage and that drop should match up within a certain range, this is the LOAD on the engine
So it could mean TPS voltage is at 2volts and should be at 3v with MAPs vacuum reading

Ok so I tried your test with the IAC, initially it dropped the rpms but let me stress it only did this ONCE. I ran some errands and came back. Now when I unplug it I get no reaction. I put on the old IAC, same result. The plug going to the IAC gets 12.7V KOEO and 14.34V KOER. I also unplugged all vacuum lines except the MAP and the fuel pressure regulator. Both of those lines are new and I have checked are not leaking. about a minute after I unplugged the IAC the most recent time and plugged it back in the engine dropped the rpms so low it stalled itself out. I does seem to do this but it can take minutes for there to be a noticeable effect. The first time earlier today when I did this the rpms dropped immediately after pulling the plug. I have literally swapped the old sensors in, swapped my computers, swapped throttle bodies. I’m at a complete loss here
Codes: 10 and 11c which are a system pass code to my understanding. The engine now timed correctly is sitting anywhere from 1100-1500 and surges to 3000 for a second after starting. At idle the timing roams from 20 roughly to 27 seeming randomly. Sorry for the formatting of this post, I’m writing this from my phone

 

[Pix3L8]

One other thing, the screw your talking about. Is it located on the throttle body right next to where the throttle linkage clips in? If so I have this all the way out to where it is barely touching the plate on the linkage

 

[RonD]

Yes, engine must be warmed up to do the IAC Valve test

The red wire on the IAC valve will always have battery voltage, not really a test point, the other wire is a Pulsed Ground, computer controls this
So if you want to test IAC Voltage you can pierce this "other" wire with a sewing pin and with engine running, use Volt meters ground probe on the pin with meter's red probe connected to battery positive, should read 7-11volts depending on engine temp
Computer use the Ground to control voltage to IAC Valve

But not your problem
What brands are the IAC Valves?

IAC Valve should close every time you unplug its wires, period, so RPMs should drop everytime
If its not then IAC Valve is sticking or it was already closed all the way, so vacuum leak

You can plug in the IAC Valve when its off the engine
Turn on the key and IAC Valve should open all the way for starting
Then watch the valve while you unplug it, it should close all the way
Then plug it in again and it will open all the way
Repeat as much as needed

It only moves 3/8", so not much, but should move to same place open and closed each time

Yes that could be the anti-diesel screw, it sets minimum throttle plate opening

Make sure throttle plate is not loose

 

[Pix3L8]

Yes, engine must be warmed up to do the IAC Valve test

The red wire on the IAC valve will always have battery voltage, not really a test point, the other wire is a Pulsed Ground, computer controls this
So if you want to test IAC Voltage you can pierce this "other" wire with a sewing pin and with engine running, use Volt meters ground probe on the pin with meter's red probe connected to battery positive, should read 7-11volts depending on engine temp
Computer use the Ground to control voltage to IAC Valve

But not your problem
What brands are the IAC Valves?

IAC Valve should close every time you unplug its wires, period, so RPMs should drop everytime
If its not then IAC Valve is sticking or it was already closed all the way, so vacuum leak

You can plug in the IAC Valve when its off the engine
Turn on the key and IAC Valve should open all the way for starting
Then watch the valve while you unplug it, it should close all the way
Then plug it in again and it will open all the way
Repeat as much as needed

It only moves 3/8", so not much, but should move to same place open and closed each time

Yes that could be the anti-diesel screw, it sets minimum throttle plate opening

Make sure throttle plate is not loose

The original is a motorcraft and the new one is from orileys with no branding on it. It seems to me that on this EFI engine the three things controlling the idle are the MAP, IAC, and o2 sensor, correct me if I’m wrong. I’m thinking about systematically testing each sensor with a multimeter and seeing what’s what. Because if they all work what’s left? Ordering a new emc maybe? I have two ecms but they are both from junkyards and could be faulty, one won’t even connect to my code reader so who knows.

 

[rusty ol ranger]

Check for vacuum leaks.....

Not just at the hoses....upper intake gaskets like to fail too...PCV line, hell could even be a bad brake booster....

 

[RonD]

Computer controls IAC Valve, but yes based on sensor data

Main sensor for idle is ECT sensor, computer uses this to set high idle when engine is cold and then lowers idle as engine warms up
And then TPS(throttle sensor), this tells computer when it should "take over" to set idle RPMs, i.e. throttle is closed all the way


MAP sensor tells computer vacuum level in intake, the voltage at IAC valve would be important, but not part of the control of the IAC Valve, TPS voltage as well, as that along with RPMs would establish Load on the engine for air/fuel mix
O2 sensor wouldn't be part of IAC Valve control either, thats just feedback on if air/fuel mix was correct

Really really good site to read up on the older and newer Ford fuel injection systems: http://www.fuelinjectedford.com/page1.html

They call the IAC Valve the, Idle Air Bypass(IAB)

You are running an EEC-IV which uses Speed Density(SD), so MAP sensor instead of MAF sensor

 

[Pix3L8]

Computer controls IAC Valve, but yes based on sensor data

Main sensor for idle is ECT sensor, computer uses this to set high idle when engine is cold and then lowers idle as engine warms up
And then TPS(throttle sensor), this tells computer when it should "take over" to set idle RPMs, i.e. throttle is closed all the way


MAP sensor tells computer vacuum level in intake, the voltage at IAC valve would be important, but not part of the control of the IAC Valve, TPS voltage as well, as that along with RPMs would establish Load on the engine for air/fuel mix
O2 sensor wouldn't be part of IAC Valve control either, thats just feedback on if air/fuel mix was correct

Really really good site to read up on the older and newer Ford fuel injection systems: http://www.fuelinjectedford.com/page1.html

They call the IAC Valve the, Idle Air Bypass(IAB)

You are running an EEC-IV which uses Speed Density(SD), so MAP sensor instead of MAF sensor

Just read through some of that website, that’s a great new resource! I’ll report back once I do some more testing. I’ll test for vacuum leaks again but if not I’ll start looking at the sensors now that I know the parameters for each one. I apreciate all the feedback from you guys with my never ending 2.9 problems