Trent B

Feb 6, 2017
Hey what's up guys, Im new to the forum here.

So here's my dilemma. I have a 1986 GT 5.0 That keeps shutting off. You start the car up, it runs fine, sometimes for a few minutes, sometimes for a few seconds. Sometimes it will shut off on its own, sometimes it will happen if you press the throttle. RPMs will drop and the engine will shut off, just as if you had pulled the key out. Whats weird though is AFTER it does this every time, I hear the fuel pump prime/ go through its cycle- it makes the whining noise.

THEN if i try to start the car back up, I have to pull the key out, re-prime the fuel pump, and then it will start. If I dont, it will just crank and not fire up. I have 40psi fuel pressure, so it's definitely getting fuel to the engine, why it dies just confuses me though.

ALSO this is another goofy thing.. If there's no key in the ignition, and i turn the parking lights on, this will cause the fuel pump to prime. WITH THE PARKING LIGHTS. Then the fuel pump usually floods the engine after this.. So the fuel pumps gotta be shorting on something right? Or maybe i have a loose/bad ground somewhere? This whole thing confuses me, so any help/advice is appreciated... Just wanna hear if anyones had the same/similar issue..

it has new:
battery/ ground cables
spark plugs /wires
cap and rotor
coil pack
tfi module
throttle body
IAC Valve
Throttle position sensor
o2 Sensors

The problem started before this stuff was replaced though.

Thanks in advance -Trent
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General karthief

wonder how much it would cost to ship you a pair
Mod Dude
Aug 25, 2016
List any modifications you have done and other work even if it seems unrelated.
And have you checked for any codes, there is a how to pull codes thread in the tech/how to section


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In Remembrance. Thank you for your contributions
Mar 10, 2000
See electrical & vacuum troubleshooting you can download the whole set of electrical troubleshooting and diagrams for an 86 Mustang courtesy of Jeremy, AKA V8only.
It is a big 61 MB download that you get to unzip and save on your computer. It may take some time to download it, but the results are worth your time.

Another vote for replacing the ignition switch....

Recall on Ford Ignition switches:

Revised 7-June-2014 to add Torx bit picture and source. Also added replacement ignition switch wiring pigtail picture, part numbers and sources.

Some of the symptoms of ignition switch problems are things that don’t work or are intermittent like radio, turn signals, wipers or heater.

There was a FREE recall on Ford ignition switches. They overheat and sometimes catch fire. That burns up the steering column and sometimes the car interior. Since this is very old information, you may not be able to get the switch replaced for free anymore. The auto parts stores sell the switches for $13-$15.




While you are working on the switch, check the wiring and connector closely. A replacement connector with new wiring pigtails is available from most auto parts stores

AutoZone and Advanced Auto Parts have the same Part Number: 434 - may have to order, not always in stock

Advanced Auto Parts alternate part number: PT5534


Saleen0679 was nice enough to dig this up for us awhile back: Replace a 1979-1993 Ignition Switch Assembly

Torx bit set from Advance Auto Parts


Autocraft torx bit set – have small hole in tip for tamper proof screws.
Part No. AC571/W1386 Cost approx. $12 +tax

Diagrams courtesy of Tmoss & Stang&2birds

Ignition switch wiring

Fuel, alternator, A/C and ignition wiring

See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host) for help on 88-95 wiring Everyone should bookmark this site.

Complete computer, actuator & sensor wiring diagram for 91-93 Mass Air Mustangs

Complete computer, actuator & sensor wiring diagram for 88-91 Mass Air Mustangs

Ignition switch wiring

Fuel, alternator, A/C and ignition wiring

O2 sensor wiring harness

Vacuum diagram 89-93 Mustangs

HVAC vacuum diagram

TFI module differences & pin out

Fuse box layout

87-92 power window wiring PowerWindowWiring.gif

93 power window wiring

Visual comparison of the Ford Fuel Injectors, picture by TMoss: