Ugh, well I hate for this to be my first post on here. But I've been unable to find threads that have the solution to my problem, even though I have found some with same symptoms.

94 F150 5.8 XL automatic

HIgh Idle only in Park and Neutral, when in gear it idles fine.
Turning on A/C drops the idle, but not to an acceptable level ~1100 - 1300, if I take out the sensor to set the timing it drops to 980 with the A/C on, but if A/C is off its idling around 1500 in park.
The cold start idling stuff seems to work fine, on cold start up it idles very high at least 2000, but drops down in about 15-20 sec
ECT resistance is good
Put new TPS and IAC on, when I unplug IAC idle DOES NOT drop.
Getting around 20 in of vacuum pressure
I plugged the wholes on the butterflies and the helped a tiny bit
No check engine light on, I checked for codes and got none - only 111

Little back story, took it on about a 500 mile road trip, everything ran fine. Although, I have been getting miswiring alert on my aftermarket radio (I didn't install), if I hit reset button on radio it will work again for a while but always goes back to saying miswiring after some time or other. After the road trip I was getting a whining sound, sounded like it was coming from middle of dash, maybe from engine compartment but couldn't tell. After a couple days it just went away. Parked it for a week, then wife drove and said idle was high, it was parked at my work where I know mice and rats can run around, so I figured after a week they chewed through something, but vacuum seems fine. I cleaned my ground connections, but I'm thinking this still might be an issue...

I made the mistake of cleaning the throttle body (about a thousand miles ago i was getting some surging so cleaned out the IAC, installed new IAC and cleaned out TB when this problem arose). Pictured below is amount of air getting passed butterflies, is this enough to cause the high idle? And wouldn't it still idle high in gear if this was the problem?

Not sure where to go from here. Maybe new TB. Is there a sensor or switch that would only cause it to idle high in neutral and park. I haven't been able to figure out what idle not dropping when i unplug the IAC means... maybe I got a bad one since I bought a cheapo from autozone, presuming this was not my problem in the first place...

Thanks in advance, and thanks for everyone on here that has helped me with all the past issues with this truck. I got it for $500 with a blown head gasket, rebuilt the top end and this has been a champ for me. Still runs and drives great except for this high idle in Park...

 

Typically a high idle indicates a vacuum leak.
I would start by proving you have no vacuum leaks.

 

Hey thanks for the reply! I think I've read another high idle thread where you said the exact same thing! I'm not being indignant, I do appreciate your response; my question is is putting a vacuum gauge on a bunch of the different vacuum hoses good enough? Obviously not all of them will be pulling vacuum, egr for example. I guess what I'm trying to say is: I'm a noob, and all I did was put my vacuum gauge into a bunch of different hoses and got 20 in of pressure. Is there something else to do? I didn't feel safe spraying brake cleaner or doing the propane torch thing...

 

No worries, it’s just a start for internet troubleshooting.
I haven’t tried troubleshooting vacuum lines that way.

Me personally, I replace all vacuum lines on old vehicles.
Smoke test seems to work for some.
Spraying something is popular, never heard of a problem with that method.
I use starter fluid for maximum effect.

Some folks remove the hoses one at a time from the Vacuum Trees.

I like to cap the vacuum ports at the manifold, and run a temporary new hose to the FPR & MAP if equipped.
This eliminates all existing hoses, vacuum solenoids & Vacuum Motors in the HVAC System possibilities.

You will need to check all your intake gaskets for leaks, spraying works well for this.
Don’t reuse old gaskets.

Throttle Bodies usually don’t fail overnight, air leaks occur over time with wear.
Check the shaft the Valves are attached to for play, that’s a good spray point as well.

A vacuum leak the size of a toothpick hole, can equate to a 100-300 rpm increase. (an old trick to Lean out the Emissions test)

Some folks never solve their high idle issue, sometimes that’s why they are out here. Most of us feel if the rig were in our garage, we would fix all of them eventually.

You are our eyes and ears over the internet. You are here because you haven't found the problem yet. Don't discount the suggestions you get from this crowd, they have helped me over the years.

You have a CA vehicle, they are in a class of their own……

 

Solid advice above. ^^^

Base timing set at 10* with spout out, then re-connected after verification?

I'd say that you're above the level ECU can adjust idle if no change with IAC unplugged. Is new one aftermarket or Motorcraft?

Still have the old one?

Since you cleaned the TB blades & changed TPS, mizewell try adjusting the throttle stops. That looks like a lot of space, although can't compare top & bottom at same time per that image.

Sounds like you scored a deal.

 

It's been a long time and not I don't remember exactly what I did.

I blocked the air passages through the IAC with "like a gasket". Whatever you have could work, part of a drink can maybe. I think it was just where the IAC bolts down, block it. If you get consistent low idle, it's the IAC or the signal to the IAC.

Really sketchy here. There is something funny about the signal to the IAC. I think it's a stepper motor, it's pulsed to move it? Maybe test it with the frequency counter on a multimeter? Somewhere in the operating cycle it is closed and you can unplug it when it is closed for a low idle. Don't remember if it is during starting or testing for codes?

They don't last forever and if you can convince yourself that the IAC is getting a signal but not reacting, replace it.

 

One really crude and somewhat dangerous test for a large vacuum leak is to block the intake and see if the engine still tries to run. Don't use a body part, it can seriously hurt you. Don't use something it can "suck up", it can hurt itself.

 

I don't think this tells you anything. IIRC, it needs a signal to close.

Unplug it high, stays high. Unplug it low, stays low.

 

The IAC is not a stepper motor. It's a simple solenoid. A spring keeps the plunger closed with no power applied. The computer pulses the IAC on/off to control the idle RPM.

Removing power from the IAC allows it to stay closed and keeps computer control out of the equation.

For reference:


IAC/IAB

 

Thanks for all the replies. definitely have some things to try.

I did try plugging the throttle body, dropped the idle to an acceptable level, but didn't kill motor.
Timing is set at 10 degrees with spout out
Put old IAC back in and no change, I did buy a cheapo thinking that this wasn't my problem in the first place, but... maybe I'll get a motorcraft
I put a meter on the electrical connector going to the IAC, getting 14V, but not sure correct way to test
Sprayed cleaner around and nothing
Cleaned out PVC and made sure I was getting good vacuum back there

I remember seeing a link on a thread for some silicone vacuum hoses, any one know where to buy those? I think they were like $30. I might just replace them all.

When I put the reman heads on, I'm quite sure I got new upper intake gaskets. Maybe i'll take off the upper, make sure bolts are good and put new gasket on.

Thanks for all the help

 

Simple test to narrow down where the vacuum leak is to unplug all the vacuum lines and cap the ports (this includes capping the PCV valve itself) except for two. Run a dedicated line to the MAP sensor. Run a second dedicated line to the fuel pressure regulator.

Unplug the IAC, then start the truck. If the idle RPM is still high the issue is engine related. If the idle RPM drops to normal or the engine stalls your vacuum leak is external to the engine.

At this point there is zero reason to waste money on a new IAC. I would seriously consider replacing all the vacuum lines regardless of what the testing reveals as root cause.

 

1) Unplug the radio circuit. Confirm it is not tied into any live wire in a harness and has it's own separate power supply (not fused off any other circuit).

2) The method you describe is akin to the days of carburetors. If you suspected a lean condition, you partially block the carb intake with a shop rag and if idle improves, you knew it was running lean (vacuum leak somewhere - or carb mis-adjustment).

Try to partially block air intake right before MAF and see if idle improves. If it does, you have a vacuum leak.

How does vacuum gauge read on just manifold vacuum?

 

Thanks for the link. After all these years I still forget what a great resource that site is.

http://www.fuelinjectedford.com/page39.html

This is exactly what I was trying to remember. Even though it is not written very well and doesn't make too much sense.

I still can't get past the stepper motor thing though. I wonder if a Jeep or a Saab had one like that. I had a high idle problem on an old XJ also.

 

This makes me think you have a big vacuum leak.

 

I think you are thinking of stepper motor (several designs) as was used on a carb or CFI throttle body