How to prevent voltage until a specific voltage is met?
Connecting a relay to a signal is a bad idea.  Besides not switching when you want to it, the coil is a heavy load to place on a signal wire.  As if that weren't bad enough, the relay could damage something in the throttle sensor or the engine control unit if you ever get the relay to switch at all.
What you'll need is a comparator.  A comparator compares two input voltages and switches its output when one is higher than the other.  It will draw next to no current from the throttle signal, and won't cause other problems for the ECU.
A simple comparator circuit would look something like this:



simulate this circuit &ndash; Schematic created using CircuitLab
The LM393 compares the &quot;Throttle&quot; voltage to the voltage set by R1.  If &quot;Throttle&quot; is higher than the set level, then the output goes high, switching current through the base of Q1.  The current through the collector of Q1 activates the relay.  D1 is there to protect Q1 from the high voltage that is generate when the relay coil is turned off later.
You'll need to make a small circuit board with a 5V regulator and the LM393 (and the other parts.)

I'm not sure if Ron Beyer was joking, but you could replace the LM393 with a properly programmed Arduino (or other microprocessor of your choice.)
I think that's kind of overkill just to translate a voltage into an on/off signal.

This is one of those projects where you need to ask yourself &quot;Do I really need to do this?&quot;
A short circuit on the wire from the throttle sensor to your circuit could cause your vehicle to do strange things - most likely the ECU would decide to shut down, but that's not a sure  thing.
This has the potential to get you in trouble - either at your next inspection, or in the investigation after a crash.
Think it over, and see if maybe there's another way to accomplish your task before you start attaching home grown circuits to the controls of your engine.