1993 ford e-150 high idle problem

so i’ve been slowly fixing up an old conversion van i recently got (1993 ford E150 5.0L)… and i’m having problems with this thing idling at about 3000 rpms… i’ve changed the upper intake gasket cause it was leaking, valve cover gaskets,TB gasket, IAC gasket, park plugs, wires, distributor cap and coil… i’ve checked vacuum lines, vaccum leaks, still no change… when i unplug the IAC it idles down to where it’s supposed to be… so i thought it was faulty, and bought a new one…still no change… idles around 3grand unplug it and it idles down… the IAC is getting 11.44 volts to it which is in the good from what the haynes book says… before i changed all the gaskets it was idling around 2grand or so, and it got worse after i changed everything…i’m stumped, if anyone has any ideas i would apreciate it… thanks

Did you install the IAC backwards?

“the IAC is getting 11.44 volts”

That’s actually quite low

How are you determining that you have no more vacuum leaks?

the haynes manual said it should read between 10.5-12.5 volts, i was checking for leaks with a garden hose… kinda primitive, but it’s all i have right now… and the iac is not on backwards


Regardless of what the Hanes manual says, 10.5 volts is low, even with the engine off

If the engine’s idling, it should be about 14V

So the garden hose, how are you using that to check for vacuum leaks?

Letting the engine idle and spray the engine with water . . . if no change, then no vacuum leak?

Absent a smoke machine, I would actually use carb cleaner, wd40, or something along those lines. If the idle changed, you found a vacuum leak

here’s a wild thought . . . are you 100% certain the parts store gave you the correct iac valve? Here’s what I’m getting at . . . just because a certain iac valve was on the van earlier, doesn’t mean it was the right one, and if the parts store gave you the same one you showed them, they just repeated the mistake

Yes, I have seen incorrect iac valves bolt up to Fords. The gasket even matched, but they didn’t work correctly

Another thought, and this is something I’ve also seen . . . the harness to the iac may be brittle

sorry it was 11.44 volts with the motor off…i’ll check the voltage with it on too… i guess i should double check the part that came off… but it was an original ford part so i guess i didn’t think about it… i’ll try spraying wd40 to see if that helps with finding vacuum leaks… i was actually using the hose to listen for leaks… i’ll check the harness also

The high idle might be caused by a faulty coolant temp sensor for the computer.

If the sensor is telling the computer that the coolant temp is at -30 degrees, the engine will go to high idle and stay there.


@BWEB Thanks for the info

Yeah, I’ve actually use a small vacuum hose to listen for wind leaks from window and door seals

It worked fairly well, as a matter of fact

Of course, I wasn’t the one driving . . . !

I like Tester’s idea also. If you have a multimeter, it shouldn’t be too hard to test the sensor. Of course, I don’t have the specs, but generally resistance will be sky high with a stone cold coolant, and low with coolant warmed up

i found something that i think is weird… i thought i was doing something wrong, but testing the IAC again with my multimeter… when the motor is off, and the IAC is plugged in it’s putting out .17 volts… with the motor running it’s putting out 5.7 volts… unplugged it’s running 11.7 from the wire… the old IAC read .17 also while plugged in… but i didn’t get a reading with it running cause i have the other IAC installed… doesn’t seem right to me

Here’s how to test the IAC valve.

  1. Ignition key off.

  2. Disconnect the electrical connector from the IAC valve.

3.Because of the diode within the IAC valve, the positive lead of the meter must be connected to the VPWR pin on the IAC valve, and the negative lead to the ISC pin on the IAC valve.

  1. If the resistance reading of the IAC valve isn’t between 7-13 ohms, replace the IAC valve.


I had a high idle problem on my Corolla one time, and it turned out to be a defective IAC. But in investigating the problem, with the aid of the experts here, I learned some ideas on how to diagnose this problem.

So here’s a couple of other ideas besides the good ones already posted …

  • Along the lines of Tester’s coolant temp sensor idea, the sensor could be fine but the coolant could be too cold for a warmed up engine. If there’s evidence the engine coolant isn’t getting up to proper temperature, a new thermostat would usually fix that. And an engine running colder than it should will be programmed by the PCM to idle faster.

  • There’s several things that are supposed to bump the idle speed. They vary car to car. On my 90’s Corolla they are: Engine coolant temp low, power steering pump pressure high (due to turning the steering wheel), the headlights on, or the rear window defroster is on. In other cars the radiator fan turning on, or turning on the AC or defrost will also bump the idle speed. And there may be automatic transmission related events that will bump the idle speed in cars with automatics. You have to go through all the things that are supposed to bump the idle speed and make sure they are only doing that when they are supposed to be.

  • It’s a good idea to check that the brake booster diaphragm isn’t leaking vacuum. That can be cause of high idle rpm.

  • Also, check that the idle ignition timing isn’t overly advanced from what it should be.

My daughter’s old Taurus had a high idle.
after all the attempts to repair…for some odd reason …I got to looking in and around the cold air intakes just to see.
What I saw was the throttle body butterfly was worn on just one side ( the side where the spring tension pulls it close to that side ) leaving a crescent space that would never close all the way.
Not being a servicable part number without buying a whole throttle body…I cut , filed, and drilled a piece of sheet metal to be the new butterfly plate… ( this was a $500.00 car to start with )… and that corrected the high idle.

"I cut , filed, and drilled a piece of sheet metal to be the new butterfly plate.. "

Ken, you are my new hero.


Resourceful . . . !

A McGyver solution indeed.

Is that guy retired from acting?

Haven’t seen him on TV or movies in years

You know, I don’t ever recall seeing that show on tv. But someone here used the term McGyver as a verb – to McGyver something – so that’s how I learned of that term and that show.

You know, I don’t ever recall seeing that show on tv.

Wha?? Some of us learned much of our mechanical ability and problem solving from watching MacGyver, A-Team, and Gilligan’s Island.

Richard Dean Anderson has acting credits as new as 2013 on wikipedia…though I have not watched any of those after MacGyver…but I DID watch every episode of MacG .
( could you tell ? :slight_smile: )

I have fond memories of the TV show, but I’m not sure if I watched all the episodes