Totally stumped - Help is GREATLY appreciated

Any codes?

Has the coil been swapped too?

Step 1.) Dump the codes and fix any code related problems.

Dumping the computer diagnostic codes on 86-95 Mustangs

Revised 19-May-2009 to update drawing for dumping the codes on 86-88 Mustangs with no check engine light.

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 way 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. Fail to do this and you will generate a code 67 and not be able to dump the Engine Running codes.

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 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.

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.

See Troublcodes.net Trouble Codes OBD & OBD2 Trouble Codes and Technical info & Tool Store. By BAT Auto Technical





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.

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.

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 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.



Step 2.) No significant codes or all codes are related to removed/disconnected smog gear:

The TFI module mounted on the distributor is usually the culprit for a high speed miss on a warm engine. If the problem does not occur when the engine is cold, the TFI module is definitely suspect. You may need a special socket to remove the TFI module, but most auto parts stores will have one for $5-$7.

Be sure to use plenty of the heat sink grease on the new TFI and clean the old grease off the distributor.



See Automotive Tools Specialty | Auto Mechanic & Technician Diagnostic, Testing Equipment | Thexton

ch47600 said:

Thanks for the advice guys. I pulled the codes and got 44, 94 and 21. From what I've read it sounds like 44/94 may be smog pump related? Is 21 just replacing the engine coolant sensor or is it indicating that the operating temperature is too high?

I replaced the coil wire but not the coil itself. Would you recommend that I replace the coil as well?

Thanks!

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Was the engine up to temp when you pulled codes? If not, you can generate a 21. You can test the ECT sensor to check its calibration if you desire.

The TFI and coil are two items that can cause a 4K-ish break up. You noted having a new dizzy (and I assumed a new TFI. Perhaps that was a bad assumption) so that left the coil. I would not buy a new coil but if you had a spare (half the Fords of that era use the same coil), I might try it to see if the symptoms change.

Any arching with your plugs or wires?

Check the voltage ouput of the LH O2 sensor. The code 91 may be due to the ignition misfire.

Code 41 or 91 Three digit code 172 or 176 - O2 sensor indicates system lean. Look for a vacuum leak or failing O2 sensor.

Revised 24-Sep-2009 to correct computer pin description for 94-95 5.0 Mustangs

Code 41 is a RH side sensor,
Code 91 is the LH side sensor.

Code 172 is the RH side sensor
Code 176 is the LH side sensor

The computer sees a lean mixture signal coming from the O2 sensors and tries to compensate by adding more fuel. Many times the end result is an engine that runs pig rich and stinks of unburned fuel.

The following is a Quote from Charles O. Probst, Ford fuel Injection & Electronic Engine control:
"When the mixture is lean, the exhaust gas has oxygen, about the same amount as the ambient air. So the sensor will generate less than 400 Millivolts. Remember lean = less voltage.

When the mixture is rich, there's less oxygen in the exhaust than in the ambient air , so voltage is generated between the two sides of the tip. The voltage is greater than 600 millivolts. Remember rich = more 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.

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 a high input impedance, at least 10 megohms. Remember, a digital voltmeter will average a changing voltage." End Quote

Testing the O2 sensors 87-93 5.0 Mustangs
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. The O2 sensor voltage should switch between .2-.9 volt at idle.


Testing the O2 sensors 94-95 5.0 Mustangs
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 43 (LH O2 with a red/black wire) and 44 (RH O2 with a gray/lt blue wire). Use the metal next to the computer to ground the voltmeter. The O2 sensor voltage should switch between .2-.9 volt at idle.


Note that all resistance tests must be done with power off. Measuring resistance with a circuit powered on will give false readings and possibly damage the meter. Do not attempt to measure the resistance of the O2 sensors, it may damage them.

Testing the O2 sensor wiring harness
Most of the common multimeters have a resistance scale. Be sure the O2 sensors are disconnected and measure the resistance from the O2 sensor body harness to the pins on the computer.

The O2 sensor ground (orange wire with a ring terminal on it) is in the wiring harness for the fuel injection wiring. I grounded mine to one of the intake manifold bolts

Make sure you have the proper 3 wire O2 sensors. Only the 4 cylinder cars used a 4 wire sensor, which is not compatible with the V8 wiring harness.

Replace the O2 sensors in pairs if replacement is indicated. If one is weak or bad, the other one probably isn't far behind.

If you get only code 41or 91 and have changed the sensor, look for vacuum leaks. This is especially true if you are having idle problems. The small plastic tubing is very brittle after many years of the heating it receives. Replace the tubing and check the PVC and the hoses connected to it.
A secondary problem with only a code 41 or 91 is for cars with an intact smog pump and cats. If the tube on the back of the heads clogs up, all the air from the smog pump gets dumped into one side. This excess air upsets the O2 sensor calibration and can set a false code 41 or 91. The cure is to remove the crossover tube and thoroughly clean the insides so that there is no carbon blocking the free flow of air to both heads.

ch47600 said:

Man this thing is frusterating. I've replaced the coil, TPI, spark plug wires, spark plugs, rotor, rotor cap and smog pump and this thing still runs like crap. G0od news is that I cleared the old codes but now I get 91 with both koeo and koer tests. The engine misses at almost any RPM range. It's drivable but absolutely no power and it sounds bad. Any other ideas where to continue the mission? Thanks!

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take a second look at your plugs and wires, i had a problem like this once and replaced plugs, and wires. it seemed to be a little better at idle, but any time i pushed down on the gas it just fell in its face! turned out that i had a bad plug, and a bad plug wire, and not even on the same cylinder! good luck, and keep us posted!

I had something like that happen to me also, turns out it was the hall-effect sensor inside the distributor. Long story short and 3 distributors later, my coil was going bad and "shorted" out the HES (my theory...lol). Well at least it stopped happening after the coil went toally south and I repalced it. Havent had a problem since.