Engine I'm gonna fix this thing if it kills me!

Help me create the "Surging Idle Checklist"

• Thread starterjrichker
• Start dateApr 10, 2005
• Tags1986 1993 86-93 check engine light code 67 cranks ok dump codes eec-iv mustang no start

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jrichker
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SN Certified Technician
Mar 10, 200026,8912,55722473Dublin GA
Apr 10, 2005

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Updated 1 Apr 2017 to add closed and open loop surge problem description and correct spelling and formatting.

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.

Theory:
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.

Troubleshooting:
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.
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.
Skip down to #7 where the maf is discussed, it shows how to test the maf and it's wires to the computer.
this is from the checklist I mentioned earlier.