The advice is free - the tools to do the job may be quite expensive...
If you have no electrical talent and are not good at using a Digital Multimeter, this tech note is a waste of your time.
1.) Buy, beg, borrow or rent an infrared temp tool to read the heat generated by the combustion process. You are going to use it to measure the exhaust header tube temperatures.
$130 at
Extech 42515 Infrared Thermometer with Type K Input - instrumart.com
You need to see if the cause is due to lean mixture or a hot spot in the combustion chamber like a carbon deposit, a sharp edge on the combustion chamber or an exhaust valve. A lean mixture will show as a significant difference between adjacent cylinder’s exhaust header when the engine is running under load. No significant difference between adjacent cylinders exhaust header means that you have a hot spot in the combustion chamber. Make sure you take your measurements the same distance from the cylinder head exhaust header flange. That will reduce any cooling effect from measuring too far away from the heat source.
2.) Significant difference between adjacent cylinders as detected by the infrared temp tool means that either the injector, the power feed to it or the computer provided ground is not functioning as it should.
Swap the injector with one from another cylinder that isn’t showing abnormal temp readings. Run the engine under load and recheck the exhaust header temp readings. If the problem follows the injector, it is defective. If the problem remains with the same cylinder, it is a hot spot in the cylinder.
Some basics about the computer: Remember that the computer does not supply power for any actuator or relay. It provides the ground necessary to complete the circuit. That means one side of the circuit will always be hot, and the other side will go to ground or below 1 volt as the computer switches on that circuit.
You will need access to an oscilloscope to measure the pulse voltage and waveform shape on the injector as the engine is running. Connect the oscilloscope probe to the injector return side wire. The injector will have a red wire that supplied constant voltage to the injector and a brown/red return side wire that connects to pin 12 on the computer. Start the engine and observe the difference in pulse between #3 cylinder and #2 (white wire, computer pin 59) or #4 (brown/lt blue wire, computer pin 13). The pulses should look the same. If they don’t, go the next step and check the injector wiring.
No oscilloscope, no idea of where to find someone who has one and is willing to help. You will need a good DMM (not a $20 from elcheapo supply) with good batteries in it. With the engine running , measure the red wire voltage on #3 injector. It should be the same as the voltage on #2 or #4 injector. If it isn’t the engine fuel injector harness is defective. With the engine off, connect Set the DMM to the ohms range and connect one probe to the brown/red wire on the injector connector and the other to pin 12 on the computer wiring harness. Note your measurement results. Move the probes to #2 cylinder injector connector white wire and computer wiring harness pin 59. The two readings should be the same within 3%.
4.) Using oscilloscope, the pulse on #3 injector looks bad when compared to the pulse on #2 or #4 injector and the voltage and wiring resistance tests are good. Swap the computer.
Diagram courtesy of Tmoss & Stang&2birds
See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host) for help on 88-95 wiring
Mustang FAQ - Wiring & Engine Info Everyone should bookmark this site.
Ignition switch wiring
http://www.veryuseful.com/mustang/tech/engine/images/IgnitionSwitchWiring.gif
Fuel, alternator, A/C and ignition wiring
http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif
Complete computer, actuator & sensor wiring diagram for 88-91 Mass Air Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif
Vacuum diagram 89-93 Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/mustangFoxFordVacuumDiagram.jpg
HVAC vacuum diagram
http://www.veryuseful.com/mustang/tech/engine/images/Mustang_AC_heat_vacuum_controls.gif
TFI module differences & pinout
http://www.veryuseful.com/mustang/tech/engine/images/TFI_5.0_comparison.gif
Fuse box layout
http://www.veryuseful.com/mustang/tech/engine/images/MustangFuseBox.gif
