If the idle goes to the proper rpm such as 500-800 rpm when you plug the line with your finger or other object then the booster is bad. It should skyrocket if nothing plugs it (Open line). And look for as many connectors as possible and clean them. And spray your throttle body shaft bushing with starting fluid the plastic gets old with age then doesn't seal as well as it could and let air in. And there should be a way to adjust idle as a really really bad bandaid on those throttle bodies I think I've seen people do that. That's the hard part about old fuel injections you sometimes have to fire the parts cannon and hope... But your doing good your slowly finding your way there. If you ever give up and you still want fuel injection you could go to a sniper EFI setup. Or run a good ol' carburator.

 

I could see why the loop could be an issue. I wonder if the IAC is adjusted correctly same with the TPS. The computer is confused somewhere or is acting up and faulty hard to tell.

 

Alright fellas here's todays post as I'm warming up a little and getting some lunch in me.
Drove the truck to my woman's place last night with the IAC disconnected; starts up and sits very precariously at 4-500 rpms, then manages to raise itself to about 8-900 rpm as it warms (Proper idle with a functioning IAC). This morning I got back home in it and with it warm I poked and prodded a couple of the sensors with my multimeter. Something peculiar I noticed is that VREF again. 4.96V Key On Engine Off. 4.05-3.96V Key On Engine Running. (I find this suspect.)
I tracked down a beautiful diagram last night that I will post below. EEC pin #26 VREF should deliver 5V to the TPS, MAP, and EVP. My limited knowledge of voltage drop would lead me to believe that one of these three sensors could potentially be shorting internally and eating up some of that 5V VREF? Or the harness could be experiencing an increased resistance somewhere restricting voltage to these sensors? Though if that was the case would VREF not also be fubar when engine is not running? Curious as to how low VREF to the TPS would affect the EEC's interpretation of throttle position, is that why its blasting the IAC open? (Or the EEC is bad even though she's brand-spanking new and "tested good")
Heres something else fun. I did pick up on a slight whistling noise from the passenger side of the engine at idle this morning with the IAC unplugged. Vacuum leak, right? Blasted that whole area with another can of starting fluid and heard no change in Idle RPMs.
To rule out the last guy to mess with this truck messing with the throttle plate adjustment screw, (not an idle adjustment screw), I followed fords adjustment procedure. (I marked it before adj. and it was in the same position after.) So that can be ruled out.
Voltage while idling at the VPWR to the IAC, the red wire at the connector is battery voltage, in my case, idling with zero electrical accessory load on the system, is 14.4v. Is this voltage supposed to be regulated by the EEC to be 12V flat or is whatever the charging voltage for the battery appropriate for the IAC?

In summation, I'm now headed back into the cold to attempt to trace down a running engines VREF voltage drop across 3 sensors/the harness, and to triple triple check for a vacuum leak. As todays Hail Mary, I've decided I am going to disconnect the pcv from the intake and plug the port into the intake to see if that brings the idle down some. Perhaps the new PCV is crappy and is letting it breathe too much. The PCV can vent atmospheric, EPA be damned. 8-900 rpm idle with the EGR blocked and IAC unplugged means it is STILL getting air somewhere. I am tempted to make another tin plate for the IAC. Maybe even with it unplugged it is still opening and letting air in?

Project5.0 / Dentside, next time you're in my neck of the woods come get a beer or 3 on me. Thanks for being a sounding board here.

 

 

PCV really helps with crank pressure, I wouldn't recommend deleting it and I wouldn't be surprised if one the sensors are shorted inside, my brothers 01 dodge TPS shorted inside making it run like garbage, fresh new replacement too. New parts are junk nowadays. I don't think rockauto sensors would last very long. And have you checked your fuses? Could be a blown fuse causing an increase in resistance somewhere in the systems because I could be wrong but isn't the entire system feeding off one major power supply the fuse box? So in theory something in the system could be in theory blown or drawing to cause his IAC to not function. And like I said before check your throttle shaft bushings there plastic and age and leak air in to the system.

 

Alright, I'm indoors for the evening. I really hope you find this post as much of a "head-scratcher" as I am currently finding it. It makes absolutely zero sense, but I cannot deny the facts that are infront of me.

After my last post, I went outside and re-connected the battery, started up the truck and let it idle. Everything connected. IAC, TPS, EGR, Every vacuum line. the whole 9 yards. Followed dentside's advice, reluctantly, and did not touch the PCV.

Let the truck idle for about 5 mins while I recorded the dashboard and talked to my phone about all the sensors and grounds and vacuum lines I've checked and the problems the truck was exhibiting. While warming up, I took my multimeter and tested the TPS, EGRV, MAP's Orange wire, the VREF that was previously displaying less than 5V at running idle, all of these sensors are now displaying a perfect 5V VREF. Not the 3.9-4.05v I measured previously. That's weird. IAC at this point was measuring 14.4v red and 10.5v white. When the temp gauge displayed the needle at the line that precedes the NORMAL marking I tapped the throttle a few times and sure enough the Idle surged up to 1500 rpms, I tapped the throttle again, and it rose to 2k. I got out of the cab and went and tested the VREF at the TPS, EGRV and MAP's orange wire and got a perfect 5v. TPS at 2k rpm was reading .97.. IAC 14.4v red, 8.5v White... WTF? No check engine light on dashboard, nothing.

This is where **** gets weird and makes less than zero sense.
In a previous post, I mentioned that I followed the ford procedure for setting base idle with that magic "do not touch screw". Which is really (from the numerous pages of ford corp-speak and forum know-it-alls) supposed to just keep the throttle butterflies from binding in the throttle body. In some serious summarizing, the procedure is effectively, turn on a bunch of electrical and engine load ; wipers, headlights, A/C, run the truck with the IAC unplugged, and dial the screw in till the point it stalls out. Then your supposed to do a bunch of hokey **** like starting the truck 5 times under different loads for the EEC to realize what's going on. I'm really simplifying here. It was a massive pain in the ***.

Well, when the piece of **** had it's idle on the moon, I figured for ****s and gigs I'd try to push the throttle butterflies closed via the arm that connects to the throttle cable, right? If my massive vacuum leak raising idle isn't from any of my lines, not from the EGR, not from the IAC, then maybe the throttle itself. I'll be damned, it idled down a hair. I took a 5/16 wrench and kept backing that throttle butterfly stop screw out until it was way off the throttle stop screw plate. It idles PERFECT. Losing my bananas. I went and drove the truck down the highway, perfect, sat in traffic, perfect. An hour and a half I drove it around aimlessly with zero issues. Came back home, turned it off and waited an hour till it was stone cold. Steve Austin. Started it up and as soon as that idle dropped drove it off to run some errands with absolutely ZERO issues. Please keep in mind I had already removed the throttle body and cleaned it, lubricated the pivot points, literally everything to ensure it was as it should be.

I drove the truck back to the mechanic I bought it from on my way home, grilled him, "When the truck wouldn't run right with the IAC plugged in did you **** with the Idle set screw so it would run okay with the IAC unplugged???" "Nope, what are you talking about..." Walked him through all this bull****, he was dumbstruck, said it didn't make any sense.. I know!!!

I cannot comprehend how it was giving me less than 5V VREF when "acting up" before, but now delivers 5V VREF when acting up now. I do not understand how I can deviate entirely from the "procedure" for setting Idle stop screw, back that sumbitch entirely off the plate and have it run SO PERFECTLY. It makes zero sense. The EEC-IV, for all intents and purposes is a goddamn brick. My Texas instruments graphing calculator is a more complicated "computer" than it is, I cannot fathom a world in which it is somehow at fault for this mess. I am at a loss for words. After all the testing and digging and searching and diagnostic I have performed, the answer was just sending the Idle stop screw to the basement?? Against the advice of ford, and every mechanic under the sun? This is not my first project, nor basket-case.. It doesn't jive with any of my previously learned knowledge.

I am pulling my hair out, I have spent days total in some cold *** weather in the rain and snow testing wires galore for this.

Some thoughts, last guy, mechanic, owned the truck for 10+ years, drove it weekly, short trips, never had an issue. It was only 5-6 months before I inquired about the truck that he had the high Idle issue, he swapped the IAC, same issue. Left it unplugged to drive it <5 miles home. Ran like poop for him with IAC unplugged. Ran like poop for me with IAC unplugged. Ran like poop for me with a new computer with a new IAC unplugged. He never touched the idle stop screw, I had already given him cash for the truck months ago, what reason is there for another grown man to lie to me after finance has already been extruded?? All of the sensors and senders are still testing at exactly the voltages and readings that they are supposed to. Nothing electrical has changed, nothing vacuum has changed, and yet its no longer an issue! I am thrilled, and livid at the same time.

Something about looking a gift horse in the mouth? I am drinking heavily and watching John Carpenters "Christine" with the better half tonight.
I will update this thread if this issue presents itself again. In the meantime, I am now an expert on EEC-IV's, Bricknose ford's 5.0 engine wiring and vacuum setups, and idle diagnostics.
Godspeed, Gabe.

 

Was it a gift horse in the mouth?? Just kidding.

Now for the truck. If you have the butterflies closed all the way (stop screw) not touching and the IAC is unplugged while idling 800-900 rpm’s, you have a BAD vacuum leak to find! Making the plate for the IAC will help you diagnose if the leak is from the IAC or elsewhere. Butterflies closed all the way should never start. The procedure I found and used was unplug IAC, turn truck on and adjust stop screw until your rpm is at 550. Shut truck off, verify tps signal and adjust accordingly. Reset computer, plug IAC back in and drive.

Do you have up and down play with the throttle blade shaft? Grab the linkage where the cable attaches and see if you can move it up and down or side to side. It’s not the most common thing, but the butterfly shaft and get worn out.

You could try unplugging each sensor (one at a time) and see if your 5vref returns. Check the computers ground on driver side by hood hinge as well.

Hissing on passenger side of engineside can be vacuum canister or tubing on wheel well, intake manifold, EGR tube (If original, very possible), Pcv or intake gasket (upper or lower). Map sensor and tubing is on that side also.


If you’re butterflies are completely closed, squirt starting fluid in the air inlet hose by the air filter housing, that will tell you if your leak could be coming from IAC/throttle body.

 

If the butterflies are shut completely then you may still have a vacuum leak somewhere in the system. I'd do what 90project5.0 said.

 

A few ideas and suggestions.
Is the plastic throttle cable sleeve that goes through the firewall oriented with the bend toward the passenger side? If not, the ferrule will hang up on it




Is this E Clip present on top of the throttle body?

(In this picture the E clip is missing)

How's your evap system and charcoal canister? There should be a rubber line carrying gasoline vapors to the throttle body from the charcoal canister. Is there a solenoid in the line?

Do these two grounds have a clean, tight connection.to the battery negative terminal? They're the power grounds for the ECM and the ECM relay.



Check the ECM relay socket and blades for corrosion.
It's the relay with the brown base laying on the driver's side inner fender or in a black plastic enclosure.

Did you try disconnecting the vacuum line that goes to your vacuum reservoir and the one that goes to the EGR solenoid and temporarily blocking the vacuum tree ports where they connect? That would take the EGR and air injection solenoids out of play.

 

My 90 doesn’t have the purge solenoid. Side note to what you said @Soup bean do you have the write up on the relay conversion to newer style? I couldn’t find it anywhere.

 

My '88 doesn't have the solenoid either.
I don't have the relay conversion write up.

 

Alright, I jumped the gun a little. In my defense, it was behaving well, and I was hoping this headache would go away on its own. Guilty of laziness and excessive optimism for sure.
Spoiler, it did not go away on its own.
But, I have found the gremlin. Finally. Looking back, half of this process has been learning the truck and its system, and half has been bringing an old, neglected "brick" up-to-date.

The very night I made my last post in full confidence that the issue was resolved via my throttle stop screw adjustment, the truck "acted up" again. My lady and I had just finished watching "Christine", I drove her home, the truck revved to the moon and there I was trying to explain It was not possessed and was not going to harm her or her friends. Not a great time. I have had my tail between my legs for the last 48 hrs. I was so confident it would behave properly.

Fortunately. I have figured it out.
The key was in the voltage drop in that VREF reading. 5v normal to 3.9-4.05v when misbehaving. 90project5.0 had advised I create an aluminum plate to block off the EGR. I did so, and in an effort to follow "procedure" I disconnected the battery. My perfect drive was during the "learn" period for the EEC when the battery was reconnected and the EEC was learning protocol. That evening, with the EGRV connected but the EGR still blocked off, the IAC opened wide and rpms surged to 3k.

That surge was due to an intermittent short in the EGRV, which shares the same VREF circuit with the MAP and TPS. When the EGRV ate up some voltage from the VREF line, the EEC received incorrect readings from the MAP and TPS, causing that TPS and EGR check engine light I mentioned earlier, sending a signal to the IAC to open up wide to compensate, raising the rpms to 3k, effectively a massive vac leak.

Sure enough, upon replacing the EGRV and putting 300 miles on the truck, I have no problems.

The EGRV was nowhere near the top of my checklist for issues. Anywhere you read will tell you, for a high idle issue, TPS/IAC failure first, Vac leak second, some form of throttle cable kink or bind or issue third, improperly adj. butterflies fourth, wiring or ground issue fifth, EEC failure at final.

Dudes. I followed that checklist to a T. Initially, upon my purchase of the truck, the IAC read bad resistance, the tps read way out of specification, my throttle cable's plastic housing was cracked and shot, I rectified these issues and still had problems. I chased all vac lines, I tidied up old crappy wiring, cleaned grounds, checked connectors. Still had problems. Found melted capacitors and replaced my EEC and still had issues! Blocked off the EGR and still had an issue. I really do not think I would have ever found this issue without consulting this forumn and finding that lovely EEC pinout wiring diagram.

Soupbean, thanks for posting pics to guide me. I appreciate that, I had already made certain those "simple" causes were not the primary issue, on an old truck, they are bound to need attention, and I had ensured I did not make a mistake like mounting the throttle cable backwards in the firewall, or missing that clip on my linkage for the butterflies. I did have an issue where that ferrule on the cable bound on the rubber floor mat, and the small wire grounds to the battery were attached poorly upon purchase of the truck, but those were rectified immediately, and the primary issue persisted.

This issue caused me to consume 2 bottles of diagnostic vacuum leak finding starting fluid, a throttle cable, 2 IAC's, 3 TPS, an EEC, a battery and atleast 2 weeks combined of days off work. My parts cannon was smoking. What an effin headache. I challenge you. Find me one article, service bulletin or professional who will tell you to check for an intermittent short at the EGRV for a high idle issue.

I have since re-adjusted the butterflies correctly so they do not bind, and the truck runs like its brand new, (not for nothing, but with the 5 rock auto magnets I collected from this nightmare it better!!!)

Like I had said earlier in this thread, 90% of this process I figured out from forumn posts from decades ago. If you see this thread and still have an issue, reply to it, it will reach my email, and I can get in touch with you to try to help.

Fellas, thank you so, so much for the help and advice. I'll be posting on here sooner or later with all the other things I want to get sorted, like headers, fuel selection valves, front end work, interior upgrading, the whole 9.

All the best.

 

Good deal!! Hopefully it stays driving smooth and you don’t have to deal with that irritation/embarrassment when it happens! You readjusted the stop screw, you have the IAC plugged back in? Did you check tps closed voltage?

 

Yup! Everything is reconnected as it should be, stop screw adjusted as you've recommended, TPS voltage is 5V VREF .97v SIG REF and a good clean ground.

 

Bingo! Drive her like you stole her! Hopefully it’s fixed, you’ll know soon enough!