94 gt idle issues..

dcm0123

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Mar 11, 2017
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Thank you- found a plate and ordered it with gaskets.
When hot and you unplug the IAC your idle should drop to about 800 plus or minus depending how someone set the throttle plate stop screw. I presume you have an OEM Ford throttle body with no other adjustments.

If it does not drop, you may have a vacuum leak. Check the hose to the PCV and make sure the valve is in the pocket where it belongs. Also make sure the valve works as it should- blocks air in one direction and lets it flow in the other.

If it does drop but jumps back up when you plug the IAC back in, you likely do not have a vacuum leak. Something is telling the computer to raise the RPM by opening the IAC.

If you adjust the settings for the throttle stop to set the idle speed with the IAC unplugged, after you are done you should unplug your battery and the spout connector for about 30 minutes to activate a learn cycle. The throttle plate should never close completely otherwise it can stick.

I solved idle hanging by reducing the idle speed with the IAC disconnected.

My current problem is surging when you let up lightly on the accelerator.
 
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General karthief

wonder how much it would cost to ship you a pair
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I was having idle issues with the cobra I had, the 94-5 mustangs have the charcoal canister hidden behind the inner fender cover at the back of the front fender, check there, the vacuum tank is back there too, my lines were bad back there.
 
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dcm0123

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Regarding TPS voltage- reading other posts it says on 94 and 95 the computer will re-calibrate itself if you change the position of the throttle stop. The eralier models do not do this so the TPS voltage is dritical.
 

General karthief

wonder how much it would cost to ship you a pair
Mod Dude
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Ahhh, no, the same applies to 94-5 GT/Cobras I believe.
This will tell you how to set your TPS, go to #2.
Keep in mind that changes to the throttle body idle screw can change the TPS reading, computer controlled cars rely on sensor input to operate so there are procedures when making adjustments, they must be followed to give the computer correct info.
If you are having idle issue I would do the checklist from the beginning, don't skip around.
 

7991LXnSHO

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The checklict is worth a shot because it gets people thinking
You realize however, there is no idle reset procedure in the Ford manuals
You will not see me, ever never ever, play with the screw on the throttle plate
It is simply not acceptable and will get you fired? at the dealer
If you think your plate is maladjusted, you need a new one
Back in 1985 and 1986 there was a procedure
Long since gone with the coated plates
If yours hunts for idle or runs crappy it is something else
Instead, Ford made an adjustable gizmo to let in more air another way. The IAC spacer has two screws to adjust with, not just one.
If the parameters of the parts have been changed from stock, the air requirements should also change.
 

Smurfstang88

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Ahhh, no, the same applies to 94-5 GT/Cobras I believe.
This will tell you how to set your TPS, go to #2.
Keep in mind that changes to the throttle body idle screw can change the TPS reading, computer controlled cars rely on sensor input to operate so there are procedures when making adjustments, they must be followed to give the computer correct info.
If you are having idle issue I would do the checklist from the beginning, don't skip around.
Exactly this, which is why i tried to follow this post as best i could when diagnosing mine, which i still cant be sure its 100% right yet but i digress. Also the tpi seems to not be near as adjustable as the fox’s on these sn95s. But if you adjust the throttle stop screw because some other jackleg in the past messed with it as long as the tpi stays in range it doesnt seems to cause any issues. If its adjusted too much i know it will throw the engine light on..
 

Smurfstang88

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Instead, Ford made an adjustable gizmo to let in more air another way. The IAC spacer has two screws to adjust with, not just one.
If the parameters of the parts have been changed from stock, the air requirements should also change.
Iac spacer?? What? There was no spacer from factory.. the iac itself has a screw inset i showed a pic in a previous post showing it.. thats the only screw these had to adjust as far as im aware..

Edit: unless you are referring to an aftermarket part they offered..
 

7991LXnSHO

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Iac spacer?? What? There was no spacer from factory.. the iac itself has a screw inset i showed a pic in a previous post showing it.. thats the only screw these had to adjust as far as im aware..

Edit: unless you are referring to an aftermarket part they offered..
Ford had a repair part for a TSB on Windsors that would not idle correctly. Doing a base Idle reset and following the checklist would have fixed most. But the part was a legit Ford fix. It is listed for up to 93, and although the design changed, it looks like the same gasket pattern.
My point is that the info another member posted is not entirely accurate and might prevent you from an easy fix by following well tested, but not Ford manual, steps.
I understand what he’s trying to get at, but if the screw was never needed, it would not have been included, and what are the chances of it not having been adjusted in the last 28 years?

 
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dcm0123

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Thank you for attaching the link above. It has a lot of good info for checking out the EEC system including the idle.


In case you need to check them, I used to check the TPS and the EGR sensor with an analog ohm meter (I am old). The EEC diagram below shows they are still using potentiometers as sensors so you should be able to check the same way. Disconnect the connector and attach the meter direct to the TPS or EGR sensor. There are 3 terminals. Across 2 the resistance should start high and drop when you slowly push the plunger or rotate the shaft. Across another pair it will do start low and go high. Looking at the diagram below I would guess the one highlighted in yellow is the common lead.

EGR sensors had the most problems because the EGR valve gets hot which shortens the sensor life. An intermittent sensor will cause problems for the computer. Only buy OEM for sensors.

If the meter jumps around as you are moving there is a problem. It should travel smoothly as you rotate the shaft or push the plunger. If someone has a Ford manual they used to state the high and low resistance readings.


1663807941029.png
 
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Smurfstang88

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I finally got the IAC restrictor plate, and i could not be happier! Seriously its the best 10$ i ever spent (could have easily made it myself but nonetheless). It does just what it’s advertised to do, the throttle no longer hangs up at 1600rpm and the car feels just as good to drive as with the iac unplugged, WITH the perk of being able to idle with the A/c on. No surging, and no trying to die on me with the A/c on and turning the wheel. Its seriously wonderful, if you are having these problems i promise you this is the fix you have been searching for as long as there are no extenuating circumstances like bad vacuum leak, or motor problems etc..

Also i want to update about the conclusion I’ve come to about why my original problem of high idle occurred, and hopefully this thread becomes useful to someone in the future.
Im 99% sure my high idle problem started because i cleaned the throttle body. Carbon most likely built up causing the the car to barely run so some bozo adjusted the idle up with that throttle stop stud that you shouldn't touch. So when i cleaned the TB it removed all that carbon around the blade and then in turn was sucking in more air around it while it was in the closed position. So then i had to turn that stop stud back to where it should be and Voilà no more high idle.

Also i want to speak my mind on something else of course its just my opinion and i might catch flak, but im sure people will also agree with me that have worked on these things. Anyways, this would be what i would call myths about why your car isnt running right. And that would be the tps “adjustment” and also the common mysterious base idle reset. Ive been aware of both these things for quite a while and have spent a ton of time trying them on foxbodys and this car, just trying to work out why cars are just not running and idling right. Well i have after many times messing with that stuff have never seen any of it make a noticeable difference. While the tps voltage is important, it should not be necessary in my opinion to have to mess with it unless you have seriously screwed something up and its WAY out of adjustment. No tiny little movements and adjustments will make any difference it just needs to be in range, if you are having serious problems getting your crap to run right, base idle reset and adjusting your tps all day arent gonna fix your problems. My most recent proof is this car, i tried all that stuff multiple times and it never solved my issues. If your car is surging all to hell and dying on you at idle randomly a base idle reset sure as heck isnt gonna fix your car. I just slapped that plate on plugged the iac back in and the car runs perfect. Ive adjusted the idle on this car and foxbodys without the super specific procedure of the reset, that so many people have made up over the years and is always different depending on which person said it lol. And it had never caused the car to just run like crap and not idle, unless i adjusted it was too far. Basically if your car is running piss-poorly and surging extremely and dying on you, you have a ACTUAL issue going on and no fairy dust or .0002v of tps adjustment or a base idle reset will fix your junk, mark my words. So find the actual problem and dont waste hours on end with this mythical fixes. Now im not saying they will never do anything, but think about it do you seriously think that if you need to change your battery that your car will run like crap until you base idle reset it? Do you guys with the battery disconnects that leave your cars in storage for a while base idle reset your car to get it to run right every time? I dont think so. And how about when these cars were newer do you think that people were scrambling to the dealer to have their car fixed and it just needed a base idle reset? Also what about all the other cars in the world, i have never seen anyone in a camaro forum rambling on about a base idle reset or a corvette or any other car.. just these cars. Nobody will ever convince me that a base idle reset is ever necessary on a car that doesn’t actually have something wrong with it..
 
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dcm0123

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Update- Tried the new Motorcraft IAC and no help.

Bought 2 plates- one with the .25" diameter hole and the other with the .375" hole. The .25" diameter hole reduced the airflow through the IAC to much. Could not idle on a cold start without keeping my foot on the accelerator. Had to open the throttle plate to the point where the idle would not drop down enough when hot. I could have drilled it out but chose to try the other.

The one with the .375 diameter hole ran the best. Hole was installed closest to the end of the IAC with the motor (solenoid). It did help reduce the idle hunting before it settles down after you let your foot off the accelerator (foot on the clutch).

The high speed idle after letting your foot off the accelerator was previously fixed by reducing the throttle opening by adjusting the stop. As Smurfstang88 noted in a previous post, when you let in to much air the IAC can not compensate for it properly.

I also reduced the fuel pressure. Was running 45PSI with the hose disconnected, cut back to 40PSI. This helped reduce the surging to the point I feel better about driving it. Was concerned the severe surging/bucking on deceleration would damage the drive train.

This is a built motor 5.8 with a blower, custom cam, Tweecer etc.
 
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jozsefsz

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Regarding TPS voltage- reading other posts it says on 94 and 95 the computer will re-calibrate itself if you change the position of the throttle stop. The eralier models do not do this so the TPS voltage is dritical.
For posterity, this is actually correct @General karthief. The 94-95 computer accepts whatever voltage (up to ~1.2 I believe) the TPS puts out at startup as the baseline and doesn't assume it's exactly 0.98V (if I remember) that the Fox requires. You can actually mess with how the pedal functions by holding down the throttle a little bit when you start the car. If you hold it down too much but not WOT (flood-clearing) it throws a code (if you're above ~1.2V). No harm in hogging out the TPS holes to set it back to 0.98V, but it doesn't do anything on the 94-95 unless you're way out (>1.2V).
 

General karthief

wonder how much it would cost to ship you a pair
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For posterity, this is actually correct @General karthief. The 94-95 computer accepts whatever voltage (up to ~1.2 I believe) the TPS puts out at startup as the baseline and
doesn't assume it's exactly 0.98V (if I remember) that the Fox requires.

You can actually mess with how the pedal functions by holding down the throttle a little bit when you start the car. If you hold it down too much but not WOT (flood-clearing) it throws a code (if you're above ~1.2V). No harm in hogging out the TPS holes to set it back to 0.98V, but it doesn't do anything on the 94-95 unless you're way out (>1.2V).
I'm a bit confused here, you're say'n the 87 through 93 computer does not read the TPS voltage at start up?
I agree the tps voltage does not have to be a certain setting (.99 or what ever) and moving the peddle before turning the key on will change the idle parameters at start up.
My understanding is the 87-95 computers read the TPS voltage when the key is turned on and does not 'require ' the TPS voltage to be set at a certain reading as you stated, the computer does not 'assume ' anything.
 

manicmechanic007

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They discuss strategy in Screwel
Generally. it starts using tables of predetermined values for each sensor (open loop they call it)
It runs for approximately 2 minutes then switches to closed loop and uses all of the sensors
The TP sensor input does get used in both loops
To me, it seems both the new and old computers work the same
The new ones are just smarter
They told us a 84 Tempo had more computing power than the space shuttle
TP voltage .8 to 1.3v will not turn on the light, and is the spec for idle
 

dcm0123

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My understanding is the same as jozsefsz, I read the 94-95 is the only years which can accept the wider voltage setting and self compensate for it. By self compensate I mean when you turn the key on it assumes the throttle plate is fully closed and uses that voltage (as long as within the allowed range) for the fully closed setting.

The settings for the earlier models is more critical because the computer can not self compensate. This being said I had many Ford 5.0 fuel injected "family" vehicles dating from the early 80's and never had to set the TPS voltage.
 
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Smurfstang88

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small update-
My car has been running flawlessly after i put that plate on. No issues starting in the cold or anything. Also I measured my mpg for 4 tanks of gas before i installed the plate, and now after ive gone through another 2 fill ups and my mpg increased after the plate install. It was 14-15mpg before and now its is 17mpg. This is driving on all the same roads and not taking it easy on the car either, keep in mind my car has 3:55s and most of this driving is probably 55-75mph