Okay so i have a 86 Ford Mustang LX 3.8L v6 so i have a brand new starter, starter solenoid and ignition lock cylinder installed all not even a half a week old so my issue is when i go to start my car it cranks over just fine and fires up when i release the key to just let it run it instantly dies it will run if i just fine if i hold it in place, i have only one idea what it could be and thats ignition switch in steering column but idk for sure what do yall think
Use a test light on the hot side of the ignition coil. There should be power at that terminal when the key is in the RUN position. I vaguely remember the wire being red with a green tracer.
If there is no power then odds are the ignition switch is bad.
Sounds to me like it’s the classic symptoms of a bad ignition switch.
The TFI module might be the cause. It’s a somewhat cheap and simple part to swap.
This is a classic example of what happens when the Starter Solenoid goes bad…sound strange eh? I encountered just this issue ALL OVER the Ford world…lots of Ford vehicles will do this when the solenoid fails internally… The solenoid…when something fails doesnt allow ignition power to be supplied when the key is on and engine running…or should be running. It supplies ignition power under CRANK…but in the run position…it cuts ignition feed…why? How? I dunno. But this is what they do when they fail.
I DO know how silly this Starter solenoid theory sounds…but the running vehicles and money in my pocket is proof enough for me that I’m not off base. In fact I always carry one of these solenoids in my bag for roadside repairs on broken down motorists scratching their heads… LOL
If its not the starter solenoid its the ignition switch…but the column switch is EZ to test…very easy. SO test the ignition output wire on the column…I believe its a yellow wire? I forget…it doesnt matter tho… a simple test light is all you need…or better yet a volt meter.
If the ignition wire is being supplied power with key on…then it is the Starter Solenoid…Which is EXTERNAL to the starter and very easily replaced.
SO…test your ignition sw wire for key on power…and if its good. Buy and install a new Starter solenoid… Its about a 20 min job. Trust me on this one…
Blackbird
This DOES sound weird, but I know Honda to be a very experienced and knowledgeable mechanic.
Now I’m doomed to strain my brain trying to figure it out. 
I dont think its the starter solenoid i just replaced it yesterday when i replaced the a tual starter
The simple test for this is to start the engine. Instead or releasing the key and letting the internal spring turn the key to “Run”, you only release the key part way to the “Run” position. Just until the starter is disengaged.
The explanation is that as you turn the key to start, you first close the contacts to power the ignition, then a bit farther you engage the starter contact (along with the ignition contact).
So backing the key off slightly you open the contacts to the starter, yet you are still keeping the ignition contact closed. If it continues to run this way, it is a faulty ignition switch.
Yosemite
Honda Blackbird: I think I addressed in another thread carrying a new starter solenoid/relay switch plus basic tools in all my Fords glove box. I could change one in 10 minutes. OP states the solenoid was replaced. I had one “new in box” that was as bad as the failed one…
I agree with @OK4450 advice about checking for ignition power at the coil. A very simple and fast test to prove the ignition switch connection. Power at the dash fuse panel could also be checked but you would have to know which fuses to check for ignition power to them.
I don’t know if it applies to your 86 Mustang, but I had that exact symptom on my early 70’s Ford truck 302 one time. It turned out to be related to the ignition resistor bypass wire. During normal running the coil is powered via a resistance wire. I forget now what they call that thing, but it is a wire made of some kind of metal that has some inherent resistance, like 2 ohms or something. You know, the kind of wire in your toaster ni-chrome wire has some inherent resistance, otherwise it wouldn’t heat up.
During starting the Ford team wanted maximum spark so they installed a wire that bypasses that resistance wire, but only during cranking. If the resistance wire fails for some reason, it will crank and seem to start, but as soon as you return the key to “run” the spark goes away and the engine dies.
@GeorgeSanJose That’s called a ballast resistor and this car has one. I agree it’s a good possibility.
@GeorgeSanJose
It doesn’t apply.
The resistor wire/block was replaced with the ignition control module.
https://www.rockauto.com/en/moreinfo.php?pk=4587906&cc=1133862&jsn=426
Tester
Oh yeah, that’s exactly what it is called. Thanks for the reminder. It’s not a name that pops into a person’s mind, I wonder why they called it that? Ballast makes me think of boats, not cars.
Tester is right if this Mustang has electronic ignition, newer cars tend not to use separate ballast resistors powering the coil. But I still think they electronically up the spark voltage during cranking, and this symptom may be related to that function.
Water getting into the spark system can be a cause too. My dad , this was years ago, he decided to install what’s called a “capacitive discharge (CDI) ignition system” onto his Ford truck. The idea was so the spark would be hotter and make the truck run better. Now there wasn’t anything wrong with the way the truck ran before, but as far as I could tell that was beside thepoint … this truck was going to have a CDI function come hell or high water … lol …
Anyway, after that the truck still ran fine, the way it did before. Except when it rained, then it wouldn’t run at all. Fortuneatly he installed a bypass switch around the CDI so he could still use the truck when it rained.
It does have a ballast resistor!
https://www.rockauto.com/en/moreinfo.php?pk=2414986&cc=1133862&jsn=447
Tester
Well then it just could be that ballast resistor has failed.
GSJ gave the best answer…If it has a single coil, run a jumper wire from the + battery post to the + connection on the coil. It should start and run…Replace the ballast resistor or resistance wire…Don’t run it for long with the jumper in place, the coil will overheat…
I don’t believe the resistor wire in the Rock Auto catalog is correct for this application. A resistor is not shown in the wiring diagram nor is a full voltage bypass during cranking. That function is performed within the ignition module, which has likely failed.
I got it to work it was the ignition switch