91 Gt Starts Dies Dumped Codes

Code 18 - SPOUT out or wiring fault - look for short to ground in SPOUT wiring going
back to the computer. Possible bad TFI or defective 22 K resistor in the IDM wiring

Use a timing light to check the timing: remove the SPOUT and observe that the timing retards at least 4 degrees. Put the SPOUT back in place and observe that the spark advances at least 4 degrees.
This code can disable spark advance and reduce power and fuel economy.

Remove the passenger side kick panel and disconnect the computer connector.
There is a 10 MM bolt that holds it in place.
Disconnect the TFI module connector from the TFI and the measure the resistance between the yellow/lt green wire and ground.
You should see greater than 100 K (100000) ohms.
Check the resistance from Pin 4 on the computer connector (dark green/yellow) and the dark green/yellow wire on the TFI connector. You should see 20-24 K Ohms (20,000-24,0000 ohms).


Resistor location: A big thanks to liljoe07 for this information:

liljoe07 said:

Check over by the brake booster. Its not in the harness on the TFI, its on the main part of the harness over by the plugs that connect to the dash harness. About 6" or so from that, going back toward the EEC.

If I remember right, the resistor is covered in a shrink tubing that is sealed to the wires. So, you wont be able see any markings. The shrink tubing is labeled though. It's a 22kohm 1/2 watt resistor.

Here is the location.

Click to expand...

Next measure the resistance between the yellow/lt green wire on the TFI module connector and Pin 36 on the computer connector. With the SPOUT plug in place, you should see less than 2 ohms.

The following is a view from the computer side of the computer connector.


This diagram is the wire side of the computer connector.


Diagram courtesy of Tmoss & Stang&2birds





Code 34 Or 334 - EGR voltage above closed limit –

Revised 26-Sep-2011 to add EGR cleaning and movement test for pintle when vacuum is applied to diaphragm

Failed sensor, carbon between EGR pintle valve and seat holding 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 clean or, replace the EGR valve ($85-$95).

Recommended procedure for cleaning the EGR:
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. There is a basket to set the parts in while they are soaking. Soak the metal body in the carb cleaner overnight. Don’t immerse the diaphragm side, since the carb cleaner may damage the diaphragm. If you get any of the carb cleaner on the diaphragm, rinse it off with water immediately. Rinse the part off with water and blow it dry with compressed air. Once it has dried, try blowing through the either hole and it should block the air flow. Do not put parts with water on them or in them in the carb cleaner. If you do, it will weaken the carb cleaner and it won’t clean as effectively.

Gunk Dip type carb & parts soaker:



If you have a handy vacuum source, apply it to the diaphragm and watch to see if the pintle moves freely. Try blowing air through either side and make sure it flows when the pintle retracts and blocks when the pintle is seated. If it does not, replace the EGR.


If the blow by test passes, and you have replaced the sensor, then you have electrical ground problems. Check the resistance between the black/white wire on the MAP/BARO sensor and then the black/white wire on the EGR and the same wire on the TPS. It should be less than 1.5 ohm. Next check the resistance between the black/white wire and the negative battery post. It should be less than 1.5 ohm.

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.

Let’s put on our Inspector Gadget propeller head beanies and think about how this works:
The EGR sensor is a variable resistor with ground on one leg and Vref (5 volts) on the other. Its’ resistance ranges from 4000 to 5500 Ohms measured between Vref & ground, depending on the sensor. The center connection of the variable resistor is the slider that moves in response to the amount of vacuum applied. The slider has some minimum value of resistance greater than 100 ohms so that the computer always sees a voltage present at its’ input. If the value was 0 ohms, there would be no voltage output. Then the computer would not be able to distinguish between a properly functioning sensor and one that had a broken wire or bad connection. The EGR I have in hand reads 700 Ohms between the slider (EPV) and ground (SIG RTN) at rest with no vacuum applied. The EGR valve or sensor may cause the voltage to be above closed limits due to the manufacturing tolerances that cause the EGR sensor to rest at a higher position than it should.

The following sensors are connected to the white 10 pin connector (salt & pepper engine harness connectors)


This will affect idle quality by diluting the intake air charge



Code 96 causes & tests 91-93 models. – KOEO- Fuel pump monitor circuit shows no power - Fuel pump relay or battery power feed was open - Power / Fuel Pump Circuits. The fuel pump circuit lost power at one time or another.

Revised 07-apr-2013 to add check for corrosion and damage in fuel pump relay socket

Clear the codes by disconnecting the battery and turning on the headlights for about 5 minutes before reconnecting the battery. This will clear any remaining codes. Drive the car for several days and dump the codes again. In many cases, this clears the 96 code.

Look for a failing fuel pump relay, bad connections or broken wiring. The fuel pump relay is located under the Mass Air Meter on Fox bodied stangs built after 91. On earlier model cars is under the passenger seat. On Mass Air Conversions, the signal lead that tells the computer that the fuel pump has power may not have been wired correctly. See Mustang Mass Air Conversion | StangNet

Diagram of the fuel pump wiring for 91-93 cars.


Look for power at the fuel pump - the fuel pump has a connector at the rear of the car with a pink/black wire and a black wire that goes to the fuel pump. The pink/black wire should be hot when the test connector is jumpered to the test position. To trick the fuel pump into running, find the ECC test connector and jump the connector in the lower RH corner to ground. No voltage when jumpered, check the fuel pump relay and fuse links.




Power feed: Look for 12 volts at the pink/black wire (power source for fuel pump relay). No voltage or low voltage, bad fuse link, bad wiring, or connections. Remember that on 92 or later models the fuel pump relay is located under the Mass Air meter. Watch out for the WOT A/C control relay on these cars, as it is located in the same place and can easily be mistaken for the fuel pump relay.

Relay: Turn on the key and jumper the ECC test connector as previously described. Look for 12 volts at the dark green\yellow wire (relay controlled power for the fuel pump). No voltage there means that the relay has failed, or there is a broken wire in the relay control circuit. Be sure to closely check the condition of the relay, wiring & socket for corrosion and damage.



91-93 Models:
Using the diagram, check the dark green/yellow wire from the fuel pump relay: you should see 12 volts or so. If not the relay has failed or is intermittent. Check the inertia switch: on a hatch it is on the drivers side by the taillight. Look for a black rubber plug that pops out: if you don't find it, then loosen up the plastic trim. Check for voltage on both sides of the switch. If there is voltage on both sides, then check the Pink/black wire on the fuel pump relay: it is the power feed to the fuel pump. Good voltage there, then the fuel pump is the likely culprit since it is getting power. No voltage there, check the Pink/black wire, it is the power feed to the fuel pump relay & has a fuse link in it. Good voltage there & at the dark green/yellow wire, swap the relay.

All testing is done with the ignition switch in the Run position. Do not forget this crucial step.

The pink/black wire s should have the same voltage as the battery positive terminal +/- 0.25 volt. If not, then the fuse link for the fuel pump has opened up.

With the test jumper in place the green/yellow wire should be the same voltage as the pink/black wire +/- 0.25 volt.

If not, look at the red wire: should have the same voltage as the battery positive terminal +/- 0.25 volt.
If not, then check the yellow wire on the EEC relay located on top of the computer. This one is hard to get to. It should have the same voltage as the battery positive terminal +/- 0.25 volt. If not, then the fuse link for the computer has opened up.

If the red wire does not have the same voltage as the battery positive terminal +/- 0.25 volt and the yellow wire on the EEC relay does, then check the red/green wire on the EEC relay. It should have the same voltage as the battery positive terminal +/- 0.25 volt. If not, then the ignition switch is defective or the fuse link in the ignition wiring harness has opened up, or the EEC relay is defective.

All testing is done with the ignition switch in the Run position. Do not forget this crucial step.

The pink/black wire s should have the same voltage as the battery positive terminal +/- 0.25 volt. If not, then the fuse link for the fuel pump has opened up.

With the test jumper in place the green/yellow wire should be the same voltage as the pink/black wire +/- 0.25 volt.

If not, look at the red wire: should have the same voltage as the battery positive terminal +/- 0.25 volt.
If not, then check the yellow wire on the EEC relay located on top of the computer. This one is hard to get to. It should have the same voltage as the battery positive terminal +/- 0.25 volt. If not, then the fuse link for the computer has opened up.

If the red wire does not have the same voltage as the battery positive terminal +/- 0.25 volt and the yellow wire on the EEC relay does, then check the red/green wire on the EEC relay. It should have the same voltage as the battery positive terminal +/- 0.25 volt. If not, then the ignition switch is defective or the fuse link in the ignition wiring harness has opened up, or the EEC relay is defective.

Diagram courtesy of Tmoss & Stang&2birds

Holy cow, I'm worn out just scanning through all that. Why not figure out if your problem is fuel or spark. 2 simple tests. Remove boot from throttle body. Spray some starter fluid into intake. If fuel car will start, then die 1-2 seconds later. If not pull #1 or #5 spark plug boot. Get a loose spark plug or spark plug tester, I like the adjustable type. Lay against block crank motor. Spark no spark. Trouble shoot from there. It's been my experience that thick film ignition goes out most. Due to most boneheads not putting dielectric grease on proper or doing silly things with battery connected. Just my 2 cents, I've owned 3 of these stangs. The old Moose!


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I stand corrected, I use dielectric on all other connectors. But you are right sir! Heat sink paste applied proper. It usually comes with new tfi module. If you buy new one, make sure someone didn't rob little pack of it.


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Or go to radio shack and get some arctic silver paste... and only use a Ford TFI.. Aftermarket and parts store TFI's are garbage. Rockauto.com sells the correct part.

Moosee1955 said:

Holy cow, I'm worn out just scanning through all that. Why not figure out if your problem is fuel or spark. 2 simple tests. Remove boot from throttle body. Spray some starter fluid into intake. If fuel car will start, then die 1-2 seconds later. If not pull #1 or #5 spark plug boot. Get a loose spark plug or spark plug tester, I like the adjustable type. Lay against block crank motor. Spark no spark. Trouble shoot from there. It's been my experience that thick film ignition goes out most. Due to most boneheads not putting dielectric grease on proper or doing silly things with battery connected. Just my 2 cents, I've owned 3 of these stangs. The old Moose!


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Click to expand...

The idea is to provide technically correct information that covers all the possibilities. Since I am not present and cannot sort the wheat from the chaff for them, it comes as one big data dump. The feet on the ground observer can then sort through it and use the information that is most helpful. One pass and he has all of the technical information needed to find and fix the problem.

So is was the thick film igniter acting up? Good deal . Usually they either work or don't, but not always. Glad to hear you got it running tight and right ! Don't worry about module, I've got an oreillys right around the corner from me. Their great on warranty replacements. Just make sure you put grease on tfi good.


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This goes out to j richker, I know you know ten times what I know about electronics and mechanical tech on these mustangs and probably all cars/trucks. I was just throwing the reader some short cuts I learned years back and through my own personal experience. I've been wrenching since 1970. I'm learning something new everyday. I also participate on first/second generation ford expedition forums. Your way is the proper way, as I was taught some 40 years back. I was lucky enough to have a friend or two show me some shortcuts, that I like to pass on. Hope you understand. Between the two of us he fixed his problem. Cheers, Moose


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I'm a little confused, you said you had power/ ground to all injector wires. How did just 1 cylinder in a tfi module fail. Usually they either lose spark completely, or car runs, then stalls. After cool down 10 minutes or so, starts again. But 1 cylinder? That's a new one for the books.


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Martman said:

Got it resolved. After taking the detail from the crank no start list of @jrichker and @Moosee1955 quick idea of what path to focus on it was the TFI. I
@jrichker you obviously have a level of electrical knowledge of these systems that most can only dream of, or you are very good at googling and compiling information.

I still have code 34 so I am going to take the egr valve off and clean it tomorrow. Do you all have any suggestions on the best product and approach to getting the carbon deposits off of an egr valve?

Click to expand...

The checklists are a result of 16 years spent here on Stangnet and the background I have in electronics and computers. I have seen everything that can happen multiple times and have spent a lot of time digesting how the electronics in the computer, electrical system and ignition system actually work.

Almost every checklist procedure I create has a blue text revision information and date with it. It helps keep up with what is current and what is old. The checklists pop up in sites and places I have never posted before and I am surprised when I find one while browsing the web.

So for code 18, if the wires shoot good per jrichker's test, we are left with a bad TFI by default, correct?

I am chasing this gremlin right now and am not a fan of throwing parts at problems for troubleshooting purposes.

We were able to get it running by cranking the idle screw up a bit and the code 18 was pulled from KOER. Cylinder balance test was good. May be dealing with two different issues as I feel a cylinder balance test would catch an injector firing with no spark. Frustrating.