Why does my starter continue to turn the engine even after I remove the key?

You do know that the starter solenoid is on the d/s inner fender, not attached to the starter, right? Did you try banging on the solenoid when it was stuck?
Assuming that's what you meant, and it is the ignition switch(which I doubt. I'd bet on the solenoid), you will need a tamperproof torx bit(I believe a t27) to take it off

The starter solenoid is similar to a relay. apply a voltage across it and it will allow power to flow through (from the battery to the starter). This makes it easy since you can have a small amount of current running through your switch while switching something on that requires alot of current. I agree with strtrcr50, if your solenoid sticks on, your key won't be able to turn it off.....

I second that...The solenoid is horrendously notorious for sticking like that.I had one on my 89 get stuck and start smoking the solenoid.The one on my 92 gets stuck the oppositte way.I have to smack the solenoid a couple time to get it to crank.The chance of a solenoid sticking upon start greatly increases if you have a '90 or older stang.Those dont have the high torque mini starters that '91's and up have.The older starters are heavy..bulky and demand a lot more amperage.If the solenoid has already stuck once,I would replace it.Chances are that it may have overheated from the constant cranking.Maybe replace it with a heavy duty solenoid instead.

I doubt it's the ignition switch.The ignition lock has a pin that goes into a small hole on the outside of the switch.That hole is part of a slide plate which has contacts on one side and a ball bearing and a spring on the under side.The contacts on the plate slide against multiple stationary contacts on the lower half of the switch.The problem with these switches is that they have a bad habit of coming apart.Fires start when the slide plate breaks and a hot ignition contact hits a ground on the metal case side.I know,they wern't made all that well.I've had 2 of them break on me.Lucky they just come apart and I couldn't crank the engine.Does the switch seem any looser or does it seem like it gets stuck?

NAPA used to sell these awesome red solenoids. After going through 3 stock replacements from autozone/advanced, I got NAPA's heavy duty and never had another problem.

FastDriver said:

NAPA used to sell these awesome red solenoids. After going through 3 stock replacements from autozone/advanced, I got NAPA's heavy duty and never had another problem.

Click to expand...

Yeah that's the heavy duty solenoid I was talking about.If it's the one that has the terminals on the sides,then yeah,it's a good one.I think they were originally made for boats.Summit Racing jas them.

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Just to be sure, I would disconnect the starter cable at the solenoid (so you can connect the battery and not be grinding your starter forever). Now if you have the key off and the output terminal on the solenoid is still passing 12V, check the thin slide-on wire on the solenoid for 12V. If you do NOT have 12V in this wire but the normally open solenoid terminal is hot, the solenoid is latched.

I would just get a new solenoid and do this test as you're about to change out the solenoid - the chances of the interlock circuit being shorted to constant 12V is slim but PO's do some weird wiring sometimes so it's worth checking IMHO.

It's all good, man. Don't beat yourself up. Did you get the NAPA unit? If not and it happens again, do so.

Chris