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Mar 10, 2000
Dublin GA
Updated 05 July 2022 to add rough idle when cold and smooth idle when up to normal operating temperature. See the EGR tech information, section #15 for more help.

Many of you are familiar with the "Cranks OK, but No Start Checklist for Fuel Injected Mustangs" checklist and the No Crank Checklist. They are very effective in finding and eliminating the problems by use of a systematic process that checks and eliminates possible causes. This idle problem checklist will use the same troubleshooting strategy and methodology.

By the term surging idle, I mean an idle that wanders up and down more than 50 RPM, or an idle that starts low and goes high and stays there.

Engine idles OK cold but not when warm.

Before the engine is warmed up, the engine control computer is running in open loop with predefined settings. It only uses a minimal set of sensor data. After it gets warmed up, it switches modes to closed loop and uses all the sensor data to mange the engine and idle. At that point if you have a bad sensor or sensors or faulty sensor wiring, the computer gets bad sensor data and can't manage the engine or idle properly.

The obvious first step is to dump the codes and see what the computer says is wrong…Codes may be present in the computer even if the Check Engine light isn’t on.

Here's the link to dump the computer codes with only a jumper wire or paper clip and the check engine light, or test light or voltmeter. I’ve used it for years, and it works great. You watch the flashing test lamp or Check Engine Light and count the flashes.

Be sure to turn off the A/C, and put the transmission in neutral when dumping the codes. On a manual transmission car, be sure to press the clutch to the floor.
Fail to do this and you will generate a code 67 and not be able to dump the Engine Running codes.



If your car is an 86-88 stang, you'll have to use the test lamp or voltmeter method. There is no functional check engine light on the 86-88's except possibly the Cali Mass Air cars.


The STI has a gray connector shell and a white/red wire. It comes from the same bundle of wires as the self test connector.

89 through 95 cars have a working Check Engine light. Watch it instead of using a test lamp.


The STI has a gray connector shell and a white/red wire. It comes from the same bundle of wires as the self test connector.

WARNING!!! There is a single dark brown connector with a black/orange wire. It is the 12 volt power to the under the hood light. Do not jumper it to the computer test connector. If you do, you will damage the computer.

What to expect:
You should get a code 11 (two single flashes in succession). This says that the computer's internal workings are OK, and that the wiring to put the computer into diagnostic mode is good. No code 11 and you have some wiring problems. This is crucial: the same wire that provides the ground to dump the codes provides signal ground for the TPS, EGR, ACT and Map/Baro sensors. If it fails, you will have poor performance, economy and driveablity problems

Some codes have different answers if the engine is running from the answers that it has when the engine isn't running. It helps a lot to know if you had the engine running when you ran the test.

Dumping the Engine Running codes: The procedure is the same, you start the engine with the test jumper in place. Be sure the A/C is off, and clutch (if present) is pressed to the floor, and the transmission is in neutral. You'll get an 11, then a 4 and the engine will speed up to do the EGR test. After the engine speed decreases back to idle, it will dump the engine running codes.

Trouble codes are either 2 digit or 3 digit, there are no cars that use both 2 digit codes and 3 digit codes.

Alternate methods:
For those who are intimidated by all the wires & connections, see Actron® for what a typical hand scanner looks like. Normal retail price is about $30 or so at AutoZone or Wal-Mart.

Or for a nicer scanner see EQUUS DIGITAL FORD CODE READER (3145) – It has a 3 digit LCD display so that you don’t have to count flashes or beeps.. Cost is $30.​

Find and fix the coded items and that will clear many problems

Decision tree: If you don't get any codes but an 11, this will help you figure out if you have a mechanical problem or sensor related problem.
Disconnect the IAC? Does the engine keep on spittering and sputtering or does it smooth out and settle down? You may have to press on the accelerator to keep it running, but what happens?
Still sputters: disconnect the MAF if the engine is equipped with one.
Still sputters: remove the SPOUT
Still sputters: you have now disabled most of the computer related possibilities. That means you probably have a mechanical problem like a vacuum leak, sticking valve or something broken inside the engine.

So far I have the common items that cause most problems:
1.) Dirty or defective IAB (or IAC) - clean or replace IAB. Some TB's are coated and are marked "Do not clean". If they have no markings, spray them down & use a toothbrush to do the tough places that refuse to come clean. Spray some more and wipe up the mess with a paper towel. The rest will go through the engine with no problems. The stalling when an engine is first cranked up or cold is a clue to a non-functional IAB. See the Recommended procedure for cleaning the IAC/IAB in the next post in this series.

IAC, IAB or Idle Air Bypass. The IAB is an actuator similar to the Mustang door lock actuator. It moves a valve that opens or closes to allow air to bypass the throttle butterfly. This will speed up the idle as the valve opens and slow down the idle as it closes. It is spring loaded to the closed position. The closed position will make the engine idle at whatever speed the mechanical idle speed screw on the throttle linkage is set.

The IAB draws 12 volt power from the same red wire that powers the injectors. It is powered on anytime the ignition is in the run position. The computer provides a switched ground that completes the circuit and makes the valve open and speed up the idle. It switches the white/lt blue wire to ground many times a second to provide enough air to make the engine idle at a preset speed determined by the computer.

Using a voltmeter, look for 12 volts on the red wire for the IAB, measure between the red wire and the engine block. No 12 volts and the engine runs, then you have bad wiring or bad connections. A safety pin to probe the connections from the back side of the connector will be helpful.

Next measure between the red and white/lt blue wires: you will get a reading less than 12 volts but more than .5 volts. This is because the computer switches this wire to ground many times a second, creating a voltage that varies in average value. If you had access to an oscilloscope, you would see a square waveform that varied in duty cycle. It would switch from 12 volts down to 1 volt or less and then back to 12 volts.

If you have a small 12 volt test lamp, probe the red & white/lt blue wires. With the engine idling, the lamp will glow at less than maximum brightness. Use the red wire to power the test lamp and ground the other test lamp lead on the engine block. You will see just how bright the lamp glows. If the lamp did not glow when you probed the red & white/lt blue wires, then there is an open connection (broken wire or bad connection) between the computer and the IAB.

If there is a short to ground in the white/lt blue wire, the voltage will not go above the 1 volt point. There may be a bare wire rubbing against metal or the IAB may have an internal short between the case and the wiring inside it. The resistance of an IAB would typically run between 10-14 ohms across the two connections and open or greater than 1 Meg ohm between the case and either one of the connectors.

The IAC depends on the computer to provide a ground to make the IAC work. Bad wiring, dirty connections or a failed IAC circuit inside the computer can keep the IAC from adjusting the idle like it should.​

2.) Defective TPS - replace TPS. An idle that sticks high and won't come down unless you turn the key off & restart the engine is a sign of a bad TPS or dirty TPS electrical connections. The Idle will often start up at normal RPMs and then slowly increase in RPMs as you drive.

Disconnect and clean the TPS electrical connector using a jeweler's screwdriver to scrape the outside of the male pins and inside of the female pins. Use some WD40 to wash things down after you finish scraping. Check and clean the white 10 pin electrical connector pins (see item #6). Clean pins 1,2, & 5 since they are all TPS related.


Setting the TPS: you'll need a good Digital Voltmeter (DVM) to do the job. Set the TPS voltage at .5- 1.1 range. Because of the variables involved with the tolerances of both computer and DVM, I would shoot for somewhere between .6 and 1.0 volts. Unless you have a Fluke or other high grade DVM, the second digit past the decimal point on cheap DVM’s is probably fantasy.

Since the computer zeros out the TPS voltage every time it powers up, playing with the settings isn't an effective aid to performance or drivability. The main purpose of checking the TPS is to make sure it isn't way out of range and causing problems.

The Orange/White wire is the VREF 5 volts from the computer. You use the Dark Green/Lt green wire (TPS signal) and the Black/White wire (TPS ground) to set the TPS. Use a pair of safety pins to probe the TPS connector from the rear of the connector. You may find it a little difficult to make a good connection, but keep trying. Put the safety pins in the Dark Green/Lt green wire and Black/White wire. Make sure the ignition switch is in the Run position but the engine isn't running.

Here’s a TPS tip I got from NoGo50

When you installed the sensor make sure you place it on the peg right and then tighten it down properly. Loosen the back screw a tiny bit so the sensor can pivot and loosen the front screw enough so you can move it just a little in very small increments. I wouldn’t try to adjust it using marks.

A.) Always adjust the TPS and Idle with the engine at operating temp. Dive it around for a bit if you can and get it nice and warm.

B.) When you probe the leads of the TPS, do not use an engine ground, put the ground probe into the lead of the TPS. You should be connecting both meter probes to the TPS and not one to the TPS and the other to ground.

C.) Always reset the computer whenever you adjust the TPS or clean/change any sensors. I just pull the battery lead for 10 minutes.

D.) The key is to adjust the TPS voltage and reset the computer whenever the idle screw is changed.

The TPS is a variable resistor, must like the volume control knob on a cheap radio. We have all heard them crackle and pop when the volume is adjusted. The TPS sensor has the same problem: wear on the resistor element makes places that create electrical noise. This electrical noise confuses the computer, because it expects to see a smooth increase or decrease as the throttle is opened or closed.

TPS testing: most of the time a failed TPS will set code 23 or 63, but not always. Use either an analog meter or a DVM with an analog bar graph and connect the leads as instructed above. Turn the ignition switch to the Run position, but do not start the engine. Note the voltage with the throttle closed. Slowly open the throttle and watch the voltage increase smoothly, slowly close the throttle and watch the voltage decrease smoothly. If the voltage jumps around and isn’t smooth, the TPS has some worn places in the resistor element. When the throttle is closed, make sure that the voltage is the same as what it was when you started. If it varies more than 10%, the TPS is suspect of being worn in the idle range of its travel.​

3.) Vacuum leaks - locate & replace leaky gaskets & hoses. Spend $8 or so at the auto parts store for enough various sizes and lengths of vacuum hose to replace all the vacuum lines. The hard plastic lines get brittle over time and will crack and leak. See http://www.veryuseful.com/mustang/tech/engine/images/mustangFoxFordVacuumDiagram.jpg and http://www.veryuseful.com/mustang/tech/engine/images/88Stang5.0Vacuum.gif for vacuum diagrams. The carbon canister plumbing is often damaged or missing, causing vacuum leaks. The big hose coming from the bottom of the upper manifold and going to the front of the engine is for the carbon canister.

To find vacuum leaks around bolted joints, use motor oil in a squirt can. When you find a leak, the oil will be sucked and the engine speed will change. The oil is messy, but works great and will not pose a flash fire hazard. Avoid using flammable fluids like carb cleaner or propane gas – flash fires are not pretty and are very hazardous to your health.​

4.) Bad O2 sensors or bad or missing O2 sensor ground - replace O2 sensors and check the ground wire. The ground comes out of the fuel injection wiring harness & is a orange wire with a ring terminal on it. The O2 sensor ground bolts to the back of the cylinder head, or one of the intake manifold bolts. An alternate ground spot is the engine to body ground behind the intake manifold. After 60 K miles of usage, the O2 sensors are generally on the downhill side of things and you should consider replacing them.

Because the oxygen sensor generates its own voltage, never apply voltage and never measure resistance of the sensors. To measure voltage signals, use an analog voltmeter with high input impedance, at least 10 megohms. Remember, a digital voltmeter will average a changing voltage Here's a tip: the newer the sensor, the more the voltage changes, swinging from as low as 0.1 volt to as much as 0.9 volt. As an oxygen sensor ages, the voltage changes get smaller and slower - the voltage change lags behind the change in exhaust gas oxygen.

Measuring the O2 sensor voltage at the computer will give you a good idea of how well they are working. You'll have to pull the passenger side kick panel off to gain access to the computer connector. Remove the plastic wiring cover to get to the back side of the wiring. Use a safety pin or paper clip to probe the connections from the rear. The computer pins are 29 (LH O2 with a dark green/pink wire) and 43 (RH O2 with a dark blue/pink wire). Use the ground next to the computer to ground the voltmeter. You can expect to see the voltage switch from .2 volt to .6 volt on the average O2 sensor. More voltage swing is good, less voltage swing is bad.​

5.) Insufficient voltage at idle - reduce electrical load, replace or upgrade alternator. Use a good Digital Voltmeter (DVM) to measure the voltage. At 1000 RPM you should see 13.8 – 14.2 volts on a warm engine. Keep in mind that at 650-725 RPM, the output will be less, and may be below the 13.2 volts required to keep the battery charged. This is not good and can cause problems: underdrive pulleys may aggravate the situation.

6.) Dirty 10 pin wiring connectors or damaged wiring going to/from the 10 pin salt & pepper shaker wiring connectors.

Diagram courtesy of Tmoss & Stang&2birds

10 pin connector location

See the graphic for the 10 pin connector circuit layout.

The injector power pin is the VPWR pin in the black 10 pin connector. [/b]

Clean the 10 pin connectors with electronic parts cleaner or non-inflammable brake parts cleaner (same stuff in a bigger can and cheaper too). See http://oldfuelinjection.com/?p=85 for more help.​

7.) Dirty or defective MAF element: Clean or replace the MAF element. Disconnect the MAF and start the car. If the idle smooths out, then proceed from here. Keep in mind that this does not work on every car.

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 and ground.

See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host)

Ignition switch wiring

Fuel pump, alternator, ignition & A/C wiring

Computer, actuator & sensor wiring

8.) MAF meter on CAI system that needs clocking or protection from engine compartment air turbulence. A cone type filter located inside the engine compartment is almost sure to have surge problems due to the hot, turbulent airflow around it.

Don't do this: it is a big NO NO.


Try cleaning the MAF element & then "clock" the MAF by rotating the entire MAF housing to see if changing its position helps.

9.) Clogged fuel filter, damaged fuel lines or dirty fuel pump sock. Poor fuel delivery will cause severe problems.

10.) Bad grounds in a computer controlled engine will make all sorts of strange problems.

Any car that has a 3G or high output current alternator needs a 4 gauge ground wire running from the block to the chassis ground where the battery pigtail ground connects. The 3G has a 130 amp capacity, so you wire the power side with 4 gauge wire. It stands to reason that the ground side handles just as much current, so it needs to be 4 gauge too.

The picture shows the common ground point for the battery , computer, & extra 3G alternator ground wire as described above in paragraph 2. A screwdriver points to the bolt that is the common ground point.

The battery common ground is a 10 gauge pigtail with the computer ground attached to it.

Picture courtesy timewarped1972

Correct negative battery ground cable.

A.) The computer has its own dedicated power ground that comes off the ground pigtail on the battery ground wire. Due to its proximity to the battery, it may become corroded by acid fumes from the battery.
In 86-90 model cars, it is a black cylinder about 2 1/2" long by 1" diameter with a black/lt green wire.
In 91-95 model cars it is a black cylinder about 2 1/2" long by 1" diameter with a black/white wire.
You'll find it up next to the starter solenoid where the wire goes into the wiring harness.

See http://assets.fluke.com/appnotes/automotive/beatbook.pdf for help for help troubleshooting voltage drops across connections and components. Be sure to have the maximum load on a circuit when testing voltage drops across connections. As current across a defective or weak connection, increases so does the voltage drop. A circuit or connection may check out good with no load or minimal load, but show up bad under maximum load conditions. .

Voltage drops should not exceed the following:
200 mV Wire or cable
300 mV Switch
100 mV Ground
0 mV to <50 mV Sensor Connections
0.0V bolt together connections


11.) Dirty or defective ECT and ACT sensors. Look for codes 21, 24, 51, and 54 when you dump the codes. The ACT sensor will get coated with gunk over time and may need to be cleaned.

ECT sensor = Engine Coolant Sensor. It is normally located in the heater feed tubing on the passenger side of the engine. Some cars without heater feed tubing may install this sensor directly into the lower intake manifold in place of the heater feed tubing. The computer uses this sensor to find out how hot the engine is and adjust ignition timing and air/fuel ratio.

ACT = Air Charge Temperature. It is normally located in #5 intake runner on the driver's side of the car. It has two wires that connect it to the wiring harness. The temp gauge sender is nearby, but only has one wire and it is a push on connector. The ACT provides information to the computer about what temperature the incoming air is and uses that information to adjust ignition timing and air/fuel ratio. Sometimes the ACT may be relocated to the intake filter airbox because the manifold isn't drilled for it.

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 :

Voltages are measured across the two connector pins of the sensor with the sensor connected. Some safety pins used to probe the connector from the rear will be helpful
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 v
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

12.) Defective PCV. The PCV is almost impossible to see unless you have the engine out of the car, have the intake manifold off, or you are a snail & have an eyeball on a stalk. :D

The PCV fits in a rubber grommet that plugs in the engine block at the rear end of the lower intake manifold. The rubber grommet is notorious for not fitting tight or sealing like it should. It connects to the upper manifold by a 3/8" vacuum hose coming from the rear of the upper intake manifold. The easiest way to find it is to follow the hose with your hand and pull it out of the rubber grommet. Check the screen below the PCV while you have it & the grommet out if you can get to it.​

13.) Mismatched aftermarket MAF (Pro-M, Mass Air Systems, C&L, etc) and injector size. The aftermarket MAF must have a matching flow rating for the injectors. If you have 24 lb in injectors, you need a 24 LB aftermarket MAF.
Ford OEM MAF's are matched to the computer and the injector size calibation is done in the computer's program. The Cobra computer is an example of this rule for Ford OEM MAF's. It uses a standard MAF and changes the internal tables to modify the fuel curve for 24 lb injectors.
Some aftermarket devices can modify a mismatch between MAF, computer and injector size, but they are not very common.

14.) VSS sensor defective or wiring damaged. The MAF and Speed Density cruse control equipped cars will have a vehicle speed sensor on the speedo cable pickup. The purpose of the VSS is to increase the idle speed as the car slows to a stop. If the sensor is defective, you can experience stalling as you slow to a stop.

15.) EGR leaking or partially open. An EGR that doesn’t seal or has vacuum applied when the engine is at idle can cause rough idle.
I forgot about this one and spent $$$ and Feb, March and May of 2022 off and on chasing it. It had a bad EGR vacuum regulator that put vacuum on the EGR at idle. When the engine warmed up and went into closed loop mode, the idle smoothed out and it idled perfectly.
The culprit was a failed EGR vacuum regulator that put vacuum om the EGR valve at cold idle. Replacing it fixed the problem.
Look for EGR code 34 in this case. Disconnect the EGR vacuum from the EGR sensor and cap the line and observe if the idle improves.

Carbon between EGR the pintle valve and seat can hold 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).

16.) Over advanced ignition timing can cause the engine to not idle and stall on cold starts. Check the timing with the SPOUT out and it should be in the range of 8-14 degrees BTC. Be sure to put the SPOUT back in when you finish.

17.) Engine seem to load up on fuel and may have black smoke at the tailpipe. Fuel pressure regulator failed. Remove the vacuum line from the regulator and inspect for fuel escaping while the pump is running. If fuel is coming out the vacuum port, the regulator has failed. Check the regulator vacuum line for fuel too. Disconnect it from the engine and blow air through it. If you find gas, the regulator has failed.
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OK, now you have gone through the list and eliminated most of the possible problems. Now your mechanical and electronic problems are fixed, you can set the base idle speed.

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.

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.

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.

An engine that whose idle speed cannot be set at 600 RPM with the IAC disconnected has mechanical problems. Vacuum leaks are the #1 suspect in this case. A vacuum gauge will help pinpoint both vacuum leaks and improperly adjusted valves. A sticking valve or one adjusted too tight will cause low vacuum and a 5"-8" sweep every time the bad cylinder comes up on compression stroke. An extreme cam can make the 600 RPM set point difficult to set. Contact your cam supplier or manufacturer to get information on idle speed and quality [/INDENT]

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 damages 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. Reassemble and reinstall to check it out.

Gunk Dip type carb & parts soaker:

Thanks to Tmoss & Stang&2birds (Mustang FAQ - Wiring & Engine Info) and Ford Fuel Injection) for diagrams
Now for some fixes courtesy of those who have made suggestions that worked for them.There were a lot of good ideas, but I only have a limited amount of space. These are some of the highlights...

Idle bypass plates - they work for some, don't for many. The idle bypass plate fits between the IAC and the throttle body and allows a screwdriver adjustment of the idle air. A side benefit is that it adjusts the idle air without changing the mechanical settings. This keeps you from having to re-set the TPS voltage settings every time you make an adjustment. Here is a link to mustangs unlimited. "Idle Adjustment Plate". http://www.mustangsunlimited.com/it...y=&catkey=74-01 or from your local Ford dealer, use #f2pe-9f939-aa as the part number for the idle air adjuster

From dwhiskie and Hissin50: I mounted my aftermarket IAC upside down, no more surge.

Ranchero5.0’s comments on engines with other than stock cams:
A little dragon slaying lore here:

99% of the time on a cammed car opening up the divider between the ports on the IAC with a dremel so the motor idles at 1000rpm with the IAC unhooked, the throttle plate shut and the TPS at .98vdc fixes all surge related problems. Found about to do that on my '93 with a very mild cam and good induction it didn't like idling below 900rpm. The IAC can't react quick enough to a lopey cam induced RPM fluctuation so instead of dampening the surge it increases it. Every E cammed car I've ever worked on needed this to keep a stable idle. Similar to Fords idle bypass plate without the cobbled look. Just dremel out a little at a time till it idles around 1k. In my experience the stock puter doesn't like to idle a cammed car down low.

If that doesn't do it check the 12vdc to the heater on the O2's. One smack of wiring on headers wipes out the fusible link hidden in the wiring on the engine side of the firewall where the main puter harness goes through. This will cause the o2's to slowly go out of tolerance and the puter flips out. check this if the car's running really rich a idle too. Ranchero got nailed when first installed and the '93's done it too. I actually ended up soldering on a stereo inline fusible link and installing a 20a fuse to make the repair quicker.

For an elaboration on the o2's. The two white wires on the o2 sensor are for a the o2 heater. Without them working ,especially on long tubes the o2's cool off at idle and slow cruise and stop working correctly and the puter flips out. Use a paper clip or two and check for 12dvc between the two wires. No voltage, no heaters. Ford actually made a change and put the fusible link on the outside of the harness in the early 90's

More to come as I get time...


Blackfiveoh's approach to the same problem was solved by trimming the gasket between the two ports inside the IAC. Same effect and a easy way to test the results to see if it works for your car.

From Snake1
If you have R12 in your A/C you might want to check the charge on your A/C.
Low Charge = hunting idle Only possible if the hunting idle only happens when the A/C is on.
unplug your compressor and see if it makes a difference. It did with my 88 GT and it went to a steady 650 RPM.

jrichker's notes: This is one that will get you if you aren't careful, and it will do it with R134 too. Watch the A/C clutch while the car is in neutral and idling. If it cycles on and off, frequently, you are either low on refrigerant or have a misadjusted low pressure cut out switch. The cutout switch is supposed to shut off the compressor electrical power at pressures below 18 PSI. If it is set too high, then the compressor will cycle on and off continually, causing an idle surge.

From ArtMan with a supercharged 5.0

One other cause of a surging idle that may have already been addressed (I didn't read every post of the past 5 pages) is one that afflicts supercharger owners.

After installing your Vortech you may find that the engine won't idle correctly. If you are able to rule out the other causes noted above, you should verify the crankcase is vented properly. This doesn't effect all sc owners the same, but be assured its an annoying problem.

The nipple at the oil filler can suck in air at idle. If this happens air is ingested into the combustion chamber that is not metered by the MAF. An erratic or sticky high idle results. When Vortech first sold their kits they had you run a hose to the filter "area". Now they include bosses so you may run a hose from that nipple to the intake elbow that fastens to the compressor. Since I own an older kit I had to order the updated elbow special.

But it cured my surging idle instantaneously and the car runs like silk (even with my cam)

From garystocker:
I had an idle of 1500rpm. And an occasional surge. Stayed that way until I disconnected the throttle cable! Idled right down!!

I pulled the cable backwards out throught he maze of vacuum lines and rerouted it in a nice sweeping way...Perfect idle. No surge. No 1500rpm. Bliss!
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Great idea Joe!

i dont have anything decent to add (like i ever do after you have posted your excellent and thorough ideas), but i will toss out a couple of things (which you likely thought of and omitted for a very good reason). I have not had many idle issues, so I dont have a lot of experience with 'fixes' other than helping friends out.

the idle bypass plates - they work for some, don't for many. might be another option for folks.

a note about some aftermarket IAC's installing upside down might be good - some folks dont read the little piece of paper that comes with them, which note this.

i dont have nearly the resources that you do in terms of known cool internet links, but here is one article which is fairly decent (i dont agree wtih a couple things), but it shows the general idea and has pics (Pics are good. LOL).


There is another thing or two i generally think of, but they are escaping me right now.

anyhow, like i said, you will likely not want to include anything in my post but i wanted to at least toss the idea(s) out.

great idea on the list!
I cleaned the throttle body, helped a lot, last I remember it would rarely surge after that. But it is coming back. Dunno if it helped it cleaning it a few times or from putting some miles on it (the car sat for 3 or 4 years covered in a garage.)
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jrichker said:
4.) Bad O2 sensors or bad or missing O2 sensor ground - replace o2 sensors and check ground. The ground comes out of the fuel injection wiring harness & is a orange wire with a ring terminal on it
Sometimes you can clean the O2 sensors instead of replacing them. I was getting a check engine light in my Jeep, so I was testing it w/ a voltmeter and a torch, but I guess I ended up burning the junk off and fixing the problem :D

Anyway, just an idea. However, since the O2's aren't used until open loop, they would only cause idle issues on a warm start, sitting at lights, etc right?
crunchie12268 said:
what are the "salt/pepper shakers" for?
They are the 10 pin connectors for the fuel injection harness. See the following picture for all the circuits...

Notice that the IAB is in the "black shaker" along with all the fuel injector wiring.