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post #1 of 15 Old 09-19-2006, 03:44 PM Thread Starter
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surging idle

im guessing that this is a common problem. when the car is in neutral (AOD) it surges from about 600-1200 rpm over and over again. i think ive read just about every post on idle issues and nothings worked. i checked the IAC, cleaned the MAF, set the TPS to .96 vdc and im 99.9% sure that there arent any vaccum leaks. if i manualy adjust the idle screw to about 1200 rpm's it finally steadies out. i created a vaccum leak on the itake and that fixed the problem so i thought that it would be the IAC. if anyone has any suggestions i would greatly appreciate it.
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post #2 of 15 Old 09-20-2006, 07:57 AM
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I don't want to have you just throw parts at it, but have to tried changing the IAC because many times even cleaning them doesn't help.

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post #3 of 15 Old 09-20-2006, 02:33 PM Thread Starter
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yeah, i tried a new iac and it still did the same thing. i really dont understand because when i create a vaccum leak in the manifold, i problem goes away and the car runs fine. it really seams like it would be the iac. i dont know.
thanks anyway
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post #4 of 15 Old 09-20-2006, 02:38 PM
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I know you said you tried manually changing the idle but have you tried resetting the base idle?

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post #5 of 15 Old 09-20-2006, 03:12 PM
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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 that 100 RPM, or an idle that starts low and goes high and stays there.

The obvious first step is to dump the codes and see what the computer says is wrong…

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.


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 88 model Cali Mass Air cars.

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.

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

For those who are intimidated by all the wires & connections, see 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 – It has a 3 digit LCD display so that you don’t have to count flashes or beeps.. Cost is $35.

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

First of all, the idle needs to be adjusted to where the speed is at or below 600 RPM with the IAC disconnected. 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 RPMs to 650-725 RPMs

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. When you are done, reconnect the IAC electrical connector. Changing the mechanical adjustment changes the TPS, so you will need to set it.

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. 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.

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.

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 ˝ 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.
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.

Try this... Currently there is some dispute about the effectiveness of setting it at .99 volts, but anything less is probably OK. All you need is less than 1.0 volt at idle and more than 4.25 at Wide Open Throttle (WOT). You'll need a good Digital Voltmeter (DVM) to do the job.

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. Set it at .97v-.99v, the closer to .99v the better.

jrichker's notes: the .99 volt value may not be the best setting for you engine. Play with the adjustment until you get the best idle quality, just be sure that it is less than 1 volt when you are done. Increasing the TPS voltage may result in an increase in RPM with the IAC plugged in. If you set the TPS at .99 with the IAC unplugged, don't be suprised to see the idle speed increase when you re-connect the IAC.

(copied from MustangMax, Glendale AZ)

1.) 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.

2. 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.

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

4. The key is to adjust the TPS voltage and reset the computer whenever the idle screw is changed.
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 and 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. After 60 K miles of usage, the O2 sensors are generally on the downhill side of things.

Because the oxygen sensor generates its own voltage, never apply voltage and never measure resistance of the sensor circuit. 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. See for the 10 pin connector locations. See / for the wiring inside the 10 pin connectors. Clean the 10 pin connectors with electronic parts cleaner or non-inflammable brake parts cleaner (same stuff in a bigger can and cheaper too).

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.

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)
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 turbulent airflow around it. Try cleaning the MAF element & then "clock" the MAF by rotating the entire MAF housing to see if changing its position helps any.

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.

The secondary power ground is between the back of the intake manifold and the driver's side firewall. It is often missing or loose. It supplies ground for the alternator, A/C compressor clutch and other electrical accessories such as the gauges. Any car that has a 3G alternator needs a 4 gauge ground wire running from the block to the chassis ground where the battery pigtail ground connects.

The computer has its own dedicated power ground that comes off the ground pigtail on the battery ground wire. Due to it's proximity to the battery, it may become corroded by acid fumes from the battery. It is a black cylinder about 2 1/2" long by 1" diameter with a black/lt green wire.

All the sensors have a common separate ground. This includes the TPS, ACT, EGE, BAP, & VSS. This ground terminates inside the computer, but still uses the computer power ground as its source.
See for help troubleshooting voltage drops across grounds

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.
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 :

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

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
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.

The PCV fits in a rubber grommet that plugs in the block at the rear 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 MAF and injector size. The MAF must have a matching flow rating for the injectors. If you have 24 lb in injectors, you need a 24 LB MAF. The Cobra computer is the exception to this rule, since it uses a 19 LB MAF & internal tables to modify the fuel curve. Some aftermarket devices can modify the mismatch, 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. Look for EGR code 34 in this case. Disconnect the EGR vacuum 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).

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". 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...

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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post #6 of 15 Old 09-20-2006, 04:40 PM Thread Starter
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yes i did try the base idle rest. the only thing is that it tells you to disconnect the iac connector, do you disconnect it at the same time as you disconnect the battery or right before you hook it up. and also, it tells you to set the base idle with the iac disconnected, how am i supposed to set the base idle if the idle keeps surging between 600-1200 rpms?
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post #7 of 15 Old 09-21-2006, 09:49 AM
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Well if the base idle is still surging with the IAC disconnected then i'd say the problem is elsewhere. The problem is that it could technically be many things. I once had a bad MAF connection that made the car idle like crazy, but i'd guess that even something like dirty fuel injectors could do things like this also.

Sorry, wish i could be of more help.

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post #8 of 15 Old 09-22-2006, 12:52 PM Thread Starter
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when messuring the output of the MAF on pions C & D, do you messure the pins together or pin C to ground and pin D to ground?
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post #9 of 15 Old 09-22-2006, 01:29 PM
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The MAF? or TPS? I've never tested the MAF myself so I can't help out there.

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post #10 of 15 Old 09-22-2006, 03:33 PM
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there are 2 big plastic wiring harnesses (a black and a beige/white) clipped on to the back on your plenum spacer (it might be clipped to the intake mani, i cant remember). Take those appart, clean them out with electrical cleaner stuff. in the female part of the harness' you can pry the metal connector so that they have a tighter fit and better contact when the harness' are put back together.

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