Surging idle checklist?

Discussion in 'Fox 5.0 Mustang Tech' started by chobracobra, Mar 31, 2006.

  1. I know there used to be a very extensive checklist on here but i cant find it. My car always ran perfect until this week. When the motors hot, the idle surges up a little and drops low and usually stalls if i dont play with it. This happens when i throw it in nuetral when i'm stopping or slowing down. When it is in gear with my foot off the gas, it runs fine. its not the cam lope either, i know its sound from a mile away. the parts are in my sig. help!
    I know this is the most common woe with 5.0's. I just need that list.
  2. I just started it up, and its sounds solid.We'll see how she does on the way to class once it gets hot. Does the computer need time to adjust to air temp? here in NJ the temp has been rising almost 10* a day. Its been the warmest this year and thats when the problems started. Any ideas?
  3. PM the moderators and request that they restore it. For some unknown reason, it was removed.

    Here's the summary version that had most of the fixes in it...

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

    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 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. ​

    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

    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.

    (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. Check the procedure for your year, on my 90 I have to turn the idle screw until it just touches the tab, then insert a .010 feeler gauge and give it about one more turn. Then you adjust the TPS voltage to .98v, reset the computer. Start it up, if the idle is to low then turn the screw in until it is just right, then readjust the TPS voltage to .98v and reset the computer and start it up. 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. :D

    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.

    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.
    Here are a couple quick pics of an IAC with the port mod sitting next to a stocker and the air die grinder I do it with.

    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.
    did with my 88 GT and it went to a steady 650 RPM.

    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!
    Enrique Caro likes this.
  4. Which mods should we pm? The ones with the big blue letter names? I'll pm them.

    The surging idle checklist has become like a bible for me.
    Noobz347 likes this.
  5. jrich, thanks man I was looking for that post too,
  6. We lost another sticky?
    Boy am I glad I printed that off the other day!

    Being posted in this forum is like a death sentence to all stickies with useful info.

    :uzi: USEFUL INFO

    That's it I'm going on strike. :)

    Good luck getting anything out of the moderators. I pm'd them a couple of times about the 351w sticky and got nothing back.
    Hope this comes out better.

  7. grate i have the same issue this is going to suck
  8. The sticky has been revived!:nice:

    This is thanks to Moderator fiveofanatic. :hail2:

  9. When checking the IAC valve... Check the voltage(koeo with the plug to the iac disconnected) on both the red power wire AND the white/ light blue wire that goes to the ecu. If you have significant voltage on the white/light blue wire, it could that the diode in the circuit is bad.

    This issue was driving me CRAZY for the last few weeks, turns out 2 dollars at Radio Shack solved the problem!

    Great thread by the way!

  10. Hi all I'm new to the site. I thank you for all this great information. I was able to eliminate all vacuum leaks replaced TPS, cleaned ISC, and the engine will not idle down from about 1000 RPM. What can cause the high idle? Thanks for you assistance.
  11. Have you tried unplugging the IAC, and backing down the throttle screw? After that, reset the computer by unplugging it for 30 mins. Then plug the IAC back in and fire it up

  12. Thanks for the fast response. Yes I backed out the idle screw completely and disconnected the ISC. I didn't leave the battery disconnected that long. On my way to try it out. I'll get back to you. Thanks again
  13. I spent an afternoon trying to sort out the problem and found a large vacuum port under the upper manifold was open, I plugged it and the engine pretty much returned to normal Still have a vacuum leak at the brake booster or booster valve, I have to fix. Thanks again..
    Noobz347 likes this.