Engine Help with cold starts and cold idle

Jack1424

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Sep 15, 2020
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Greetings,
I’m new to this forum and it’s great. This is my first post as I’ve tried to search as much as I can but can’t seem to find anything on my exact issue. I’ve got a 1990 5.0 that’s almost all original minus a BBK cold air intake and have been having issues maintaining idle upon cold startups. When the motor is warm, no issues with idle. But when i start it up cold, unless I give some gas, the motor will sputter and die. Could this be a MAF issue? Bad IAC? I can’t seem to find any vacuum leaks and like I said, the car idles great when it’s warm. Any help would be greatly appreciated.
 
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7991LXnSHO

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@jrichker can you please post for us the testing specs for the ECT and IAT thermistors and any other ideas for this issue?
@Jack1424 until J gets back to us on that, or you find the charts he has posted before, I’d be using the surging idle checklist to make sure there are no codes beside 11’s, the IAC is working right, and everything else is up running right.
 

General karthief

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Just do the checklist, that's what jrichker will tell you anyway, it will show you the proper procedures to test stuff, run codes and other stuff. Do it step by step, don't skip around.
 

Jack1424

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I was able diagnose my TPS and MAF were bad and have since replaced. Ran the codes again today and I’m showing zero codes besides 11. However car is still experiencing the same symptoms. Cold starts, idle surges between 500-1000 RPM, it’s able to catch itself as idle drops and prevents a stall. As the motor warms, idle still surges between 600-800RPM. When I unplug the IAC, no change in idle. I’m wondering if it’s a bad IAC or I have my idle screw set too high? I’ve also pulled the IAC and cleaned it in accordance with the directions on this forum. Car runs great in all RPM ranges above idle. Any ideas? Bad IAC, vacuum leak? Appreciate any help, thanks.
 

7991LXnSHO

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I went through the whole list cArefully a couple of times working out bugs from the previous owners. When you get tha far, the idle set screw, TPS setting and functioning IAC are vital for a good warm idle.. Have you seen the thermostat test values for the IAT and ECF sensors? Those function a lot like a choke coil, but with better accuracy if they are right.
 
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Jack1424

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I have not seen the thermostat test values for the IAT and ECF. I thought if they were bad it would throw a code? I ordered a new IAC since I’m suspecting mine is bad. Thanks for the reply I appreciate all the help.
 

jrichker

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I have not seen the thermostat test values for the IAT and ECF. I thought if they were bad it would throw a code? I ordered a new IAC since I’m suspecting mine is bad. Thanks for the reply I appreciate all the help.
Read on...

Code 24 - Intake Air Temperature (ACT) sensor out of range.
Bad sensor, bad wiring. The ACT for Mustangs built before 95 is in the
#5 intake runner. It measures the air temperature in the intake to help
computer the proper air/fuel ratio.

Note that that if the outside air temp is below 50 degrees F that the test for the ACT can be in error. Warm the engine up to operating temperature and retest.

ACT & ECT test data:

The ACT & ECT have the same thermistor, so the table values are the same

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

Pin 25 on the computer - ACT signal in. at 50 degrees F it should be 3.5 volts.
It is a good number if the ACT is mounted in the inlet airbox. If it is mounted in
the lower intake manifold, the voltage readings will be lower because of the heat transfer.
Here's the table :

50 degrees F = 3.52 v
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
212 degrees F = .47 v
230 degrees F = .36 v
248 degrees F = .28 v

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
212 degrees F = 2.07 K ohms
230 degrees F = 1.55 K ohms
248 degrees F = 1.18 k ohms

IAC Troubleshooting
IAC doesn't work: look for +12 volts at the IAC red wire. Then check for continuity between the white/lt blue wire and pin 21 on the computer. The IAC connector contacts will sometimes corrode and make the IAC not work. The red wire on the IAC is always hot with the engine in run mode. The computer provides a ground for the current for the IAC. It switches the ground on and off, making a square wave with a varying duty cycle. A normal square wave would be on for 50% of the time and off for 50% of the time. When the idle speed is low, the duty cycle increases more than 50% to open the IAC more. When the engine speed is high, it decreases the duty cycle to less than 50% to close the IAC. An old-fashioned dwell meter can be used to check the change: I haven’t tried it personally, but it should work. In theory, it should read ½ scale of whatever range you set it on with a 50% duty cycle. An Oscilloscope is even better if you can find someone who has one and will help.




Automobile computers use current sink technology. They do not source power to any relay, solenoid 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 positive to negative is what provides the energy 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.

We are going to supply an artificial ground path to the IAC instead of letting the computer supply the ground.

Start the engine and let it warm up.

Take one of the cheap inline fuse holders with a 5 amp fuse in it. Use it to bypass the blue/white wire to ground. You'll have to get creative probing the back side of the IAC wiring with safety pins or paper clips. Since the computer doesn't supply any voltage, but supplies a ground, that can't hurt the computer. The 5 amp fuse protects you and the wiring if there is an internal short in the IAC coil.

The engine should speed up when the fuse holder wire is grounded and slow down or stall when the fuse holder wire is disconnected from ground.



Recommended procedure for cleaning the IAC/IAB:
Conventional cleaning methods like throttle body cleaner aren’t very effective. The best method is a soak type cleaner used for carburetors. If you are into fixing motorcycles, jet skis, snowmobiles or anything else with a small carburetor, you probably have used the one gallon soak cleaners like Gunk or Berryman. One of the two should be available at your local auto parts store for $22-$29. Take the solenoid off the body and set it aside: the carb cleaner will damage some types of plastic parts. Soak the metal body in the carb cleaner overnight. There is a basket to set the parts in while they are soaking. When you finish soaking overnight, twist the stem of the IAB/IAC that sticks out while the blocker valve is seated. This removes any leftover deposits from the blocker valve seat. Rinse the part off with water and blow it dry with compressed air. The IAC/IAB should seal up nicely now. Once it has dried, try blowing through the bottom hole and it should block the air flow. If it doesn't block the airflow, there is still something that is gumming up the works. Reassemble and reinstall to check it out.

Gunk Dip type carb & parts soaker:


Setting the base idle speed:
First of all, the idle needs to be adjusted to where the speed is at or below 600 RPM with the IAC disconnected. If you have a wild cam, you may have to raise this figure 100-150 RPM or so. Then the electrical signal through the IAC can vary the airflow through it under computer control. Remember that the IAC can only add air to increase the base idle speed set by the mechanical adjustment. The 600 RPM base idle speed is what you have after the mechanical adjustment. The IAC increases that speed by supplying more air under computer control to raise the RPM’s to 650-725 RPM’s. This figure will increase if you have a wild cam, and may end up between 800-950 RPM

Remember that changing the mechanical idle speed adjustment changes the TPS setting too.

This isn't the method Ford uses, but it does work. Do not attempt to set the idle speed until you have fixed all the codes and are sure that there are no vacuum leaks.

Disconnect the battery negative terminal and turn the headlights on. Leave the battery negative terminal disconnected for 5 minutes or so. Then turn the headlights off and reconnect the battery. This erases the computer settings that may affect idle performance.

Warm the engine up to operating temperature, place the transmission in neutral, and set the parking brake. Turn off lights, A/C, all unnecessary electrical loads. Disconnect the IAC electrical connector. Remove the SPOUT plug. This will lock the ignition timing so that the computer won't change the spark advance, which changes the idle speed. Note the engine RPM: use the mechanical adjustment screw under the throttle body to raise or lower the RPM until you get the 600 RPM mark +/- 25 RPM. A wild cam may make it necessary to increase the 600 RPM figure to 700 RPM or possibly a little more to get a stable idle speed.
Changing the mechanical adjustment changes the TPS, so you will need to set it. Anything between.6 and 1.0 volt is good. There is no advantage to setting it to .99; that is a BOZO Internet myth, complete with red nose and big floppy shoes.

When you are satisfied with the results, turn off the engine, and re-install the SPOUT and reconnect the IAC. The engine should idle with the range of 650-750 RPM without the A/C on or extra electrical loads. A wild cam may make this figure somewhat higher.