Idle staying at 2,000 RPM

savegoodautonfg said:

So are you possibly telling me that it is possible that my IAC/IAB is fine but that my ground is'nt good?

Click to expand...

The ground on the engine block has nothing to do with the IAC circuit.

The computer provides the ground for the IAC circuit. If the computer's dedicated power ground was bad, the car would not run, or run very poorly.

Automobile computers use current sink technology. They do not source power to any relay or actuator like the IAC, fuel pump relay, or fuel injectors. Instead the computer provides a ground path for the positive battery voltage to get back to the battery negative terminal. That flow of power from postivie to negative is what provides the engery to make the IAC, fuel pump relay, or fuel injectors work . No ground provided by the computer, then the actuators and relays don't operate.

Check out this thread in the link below.
http://forums.stangnet.com/showthread.php?t=695720
I basically had the same kind of problem.
Your idle screw may be advanced too far. That may be the reason that there is no difference when you unplug the IAC and the idle hangs. I had the same problem.
Read the thread that I linked.
What I did worked for me. It's very simple and worth a try.
What kind of MAF Meter set up are you using?
If it's a C&L and you have a conical filter is in the engine bay, this can cause some problems also. I just installed a stock air box with my C&L MAF Meter and it runs even better.
Good Luck!!
I hope this helps you out.

savegoodautonfg said:

Jritcher,

Basically, your saying to buy a new IAC?

Click to expand...

If you have worked all the way through the "Surging Idle Checklist and it is still broke, then purchase a new IAC. Be sure that you do not skip a step, because that might be the very thing that fixes your problem.

The reason I am so insistent about not skipping any steps is that I cannot be there to see, hear and feel what the engine is doing. The only way I know to compensate for that is to create a laundry list of steps that cover all the possible conditions that I know about.

When the idle hangs high, disconnect the IAC electrical connector. Does the idle speed drop back to the 600 RPM base mechanical idle?

Nick, that restrictor plate is real common with the mod motor or HCI 94-95 guys. I don't like those things to start with, nor do I think it will work for you because your issue is variable. Sometimes it works fine and sometimes it doesnt. A mechanical piece of metal like that (make one out of a soda can if you want) can phase shift the range of issue but cannot address an individual aspect of it (like helping only when your idle soars).

Jrichker's advice to disconnect the IAC and try to deduce things from that is the approach I'd take. I dont have a means of seeing IAC DC on a fox III so I kinda deal with everything else first and if all else fails, consider replacing the IAC.............
But as JR said, you should be able to have a rock solid hot base idle without the IAC being connected. JR's idea to diagnose it as it occurs is the only way you're going to find the issue (without kinda getting lucky and tossing a part at it). Should you have a spare IAC (as you had a spare TPS), I'd throw it on.


Good luck.

Set the base mechanical idle speed using the Surging Idle Checklist directions.

Make sure that you have no mechanical problems like vacuum leaks that affect the idle. A cracked vacuum line might show up only when it is forced to flex a certain way. Check all the vacuum lines, including the PVC line behind the intake manifold.

Have you dumped the codes lately? There may be a signal ground problem that would show up as a code 23.

Dude i dont think you are listening to what hissin and Jrichker are saying! This post is frustrating to read because you keep repeating the same things and jrichker has told you a thousand times that the computer only provides a ground path for the IAC circuit. IF that electrical ground connection wasnt working the car would not be running well, if at all. Obviously the IAC works intermitently because you dont have this problem all the time. You need to go through the surging idle checklist and set the base mech. idle and go from there, step by step through the check list. Print the thing off and bring it outside to the car with you and go through EVERY step...even if you dont think you have that problem.

You really do need to fix the codes before you attempt to do anything about the idle.
It is the first step in the Surging Idle Checklist


Code 34 Or 334 - EGR voltage above closed limit - Failed sensor, carbon between EGR pintle valve
and seat holding the valve off its seat. Remove the EGR valve and clean it with carbon remover. Prior to
re-installing see if you can blow air through the flange side of the EGR by mouth. If it leaks, there is carbon
stuck on the pintle valve seat, replace the EGR valve ($85-$95).

If the blow by test passes, and you have replaced the sensor, then you have electrical ground problems.
Check the resistance between the black/white wire on the MAP/BARO sensor and then the black/white wire
on the EGR and the same wire on the TPS. It should be less than 1.5 ohm. Next check the resistance between
the black/white wire and the negative battery post. It should be less than 1.5 ohm.

Note that all resistance tests must be done with power off. Measuring resistance with a circuit
powered on will give false readings and possibly damage the meter.

Let’s put on our Inspector Gadget propeller head beanies and think about how this works:
The EGR sensor is a variable resistor with ground on one leg and Vref (5 volts) on the other. Its’ resistance
ranges from 4000 to 5500 Ohms measured between Vref & ground, depending on the sensor. The center
connection of the variable resistor is the slider that moves in response to the amount of vacuum applied.
The slider has some minimum value of resistance greater than 100 ohms so that the computer always sees a
voltage present at its’ input. If the value was 0 ohms, there would be no voltage output. Then the computer
would not be able to distinguish between a properly functioning sensor and one that had a broken wire or
bad connection. The EGR I have in hand reads 700 Ohms between the slider (EPV) and ground (SIG RTN)
at rest with no vacuum applied. The EGR valve or sensor may cause the voltage to be above closed limits due
to the manufacturing tolerances that cause the EGR sensor to rest at a higher position than it should.



This will affect idle quality by diluting the intake air charge

Code 51 Engine Coolant Temperature (ECT) sensor signal is/was too high -
Possible bad ECT sensor,
or wiring. Possible missing signal ground –
black/wire wire broken or bad connection. With the power off, measure the
resistance between the
black/white wire and battery ground. You should see
less than 1 ohm. Check
the same black /white wire on the TPS and MAP
sensor. More than 1 ohm there and the wire is probably broken in the harness
between the engine and
the computer. The 10 pin connectors pass the
black/white wire back to the computer, and can cause problems.

Pin 7 on the computer - ECT signal in. at 176 degrees F it should be .80 volts

Voltages may be measured across the ECT by probing the connector from the rear.
Use care in doing it
so that you don't damage the wiring or connector.

Here's the table :

68 degrees F = 3.02 v
86 degrees F = 2.62 v
104 degrees F = 2.16 v
122 degrees F = 1.72 v
140 degrees F = 1.35 v
158 degrees F = 1.04 v
176 degrees F = .80 v
194 degrees F = .61

Note that all resistance tests must be done with power off. Measuring resistance
with a circuit powered on will give false readings and possibly damage the meter.

Ohms measures at the computer with the computer disconnected, or at the sensor with
the sensor disconnected.
50 degrees F = 58.75 K ohms
68 degrees F = 37.30 K ohms
86 degrees F = 27.27 K ohms
104 degrees F = 16.15 K ohms
122 degrees F = 10.97 K ohms
140 degrees F = 7.60 K ohms
158 degrees F = 5.37 K ohms
176 degrees F = 3.84 K ohms
194 degrees F = 2.80 K ohms

Diagram courtesy of Tmoss & Stang&2birds



See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host) for help on 88-95 wiring http://www.veryuseful.com/mustang/tech/engine/

http://www.veryuseful.com/mustang/tech/engine/images/IgnitionSwitchWiring.gif

http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif

http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif


Code 66 MAF below minimum test voltage.
Insufficient or no voltage from MAF. Dirty MAF element, bad MAF, bad MAF wiring, missing power to MAF.
Check for missing +12 volts on this circuit. Check the two links for a wiring diagram to help you
find the red wire for computer power relay switched +12 volts. Check for 12 volts between the red and
black wires on the MAF heater (usually pins A & B). while the connector is plugged into the MAF. This may r
equire the use of a couple of safety pins to probe the MAF connector from the back side of it.

There are three parts in a MAF: the heater, the sensor element and the amplifier. The heater heats the
MAF sensor element causing the resistance to increase. The amplifier buffers the MAF output signal and
has a resistor that is laser trimmed to provide an output range compatible with the computer's load tables.

The MAF element is secured by 2 screws & has 1 wiring connector. To clean the element, remove it
from the MAF housing and spray it down with electronic parts cleaner or non-inflammable brake
parts cleaner (same stuff in a bigger can and cheaper too).

The MAF output varies with RPM which causes the airflow to increase or decease. The increase of air
across the MAF sensor element causes it to cool, allowing more voltage to pass and telling the
computer to increase the fuel flow. A decrease in airflow causes the MAF sensor element to get warmer,
decreasing the voltage and reducing the fuel flow. Measure the MAF output at pins C & D on the MAF
connector (dark blue/orange and tan/light blue) or at pins 50 & 9 on the computer. Be sure
to measure the sensor output by measuring across the pins and not between the pins and ground.

At idle = approximately .6 volt
20 MPH = approximately 1.10 volt
40 MPH = approximately 1.70 volt
60 MPH = approximately 2.10 volt

Check the resistance of the MAF signal wiring. Pin D on the MAF and pin 50 on the computer (dark blue/orange
wire) should be less than 2 ohms. Pin C on the MAF and pin 9 on the computer (tan/light blue wire)
should be less than 2 ohms.

There should be a minimum of 10K ohms between either pin C or D on the MAF wiring connector and
ground. Make your measurement with the MAF disconnected from the wiring harness.


See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host)
for help on 88-95 wiring http://www.veryuseful.com/mustang/tech/engine/

Ignition switch wiring
http://www.veryuseful.com/mustang/tech/engine/images/IgnitionSwitchWiring.gif

Fuel pump, alternator, ignition & A/C wiring
http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif

Computer,. actuator & sensor wiring
http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif

Fuse panel layout
http://www.veryuseful.com/mustang/tech/engine/images/MustangFuseBox.gif

Vacuum routing
http://www.veryuseful.com/mustang/tech/engine/images/mustangFoxFordVacuumDiagram.jpg

Code 96 – KOEO- Fuel pump monitor circuit shows no power - Fuel pump relay or battery power
feed was open - Power / Fuel Pump Circuits. The fuel pump lost power at some time while the ignition
switch was in the run position.

Look for a failing fuel pump relay, bad connections or broken wiring. The fuel pump relay is located under
the Mass Air Meter on Fox bodied stangs built after 91. On earlier model cars is under the passenger seat.
On Mass Air Conversions, the signal lead that tells the computer that the fuel pump has power may not
have been wired correctly. See http://www.stangnet.com/tech/maf/massairconversion.html

To help troubleshoot the 96 code , follow this link for a wiring diagram for 87-90 models
http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif

for 91-93 models see http://www.autozone.com/images/cds/gif/large/0900823d80195960.gif

Look for power at the fuel pump - the fuel pump has a connector at the rear of the car with a pink/black
wire and a black wire that goes to the fuel pump. The pink/black wire should be hot when the test
connector is jumpered to the test position. . To trick the fuel pump into running, find the ECC test connector
and jump the connector in the lower RH corner to ground.




No voltage when jumpered, check the fuel pump relay and fuse links.

86-90 Models:
Using the diagram, check the red/black wire from the fuel pump relay: you should see 12 volts or so.
If not, check the inertia switch: on a hatch it is on the drivers side by the taillight. Look for a black rubber
plug that pops out: if you don't find it, then loosen up the plastic trim. Check for voltage on both sides
of the switch. If there is voltage on both sides, then check the Pink/black wire on the fuel pump relay:
it is the power feed to the fuel pump. Good voltage there, then the fuel pump is the likely culprit since
it is getting power. No voltage there, check the Orange/Lt blue wire, it is the power feed to the
fuel pump relay & has a fuse link in it. Good voltage there & at the Pink/black wire, swap the relay.

91-93 Models:
Using the diagram, check the dark green/yellow wire from the fuel pump relay: you should see 12 volts
or so. If not the relay has failed or is intermittent. Check the inertia switch: on a hatch it is on the drivers
side by the taillight. Look for a black rubber plug that pops out: if you don't find it, then loosen up the plastic trim.
Check for voltage on both sides of the switch. If there is voltage on both sides, then check the Pink/black
wire on the fuel pump relay: it is the power feed to the fuel pump. Good voltage there, then the fuel pump
is the likely culprit since it is getting power. No voltage there, check the Pink/black wire, it is the power feed
to the fuel pump relay & has a fuse link in it. Good voltage there & at the dark green/yellow wire, swap the relay.

Click here http://forums.stangnet.com/showthread.php?t=698148 to find the link to dump the codes.

The ECT is located in the water feed tube for the heater on the passenger side of the car. It is near the front of the engine.