Depends what year, what system you have. It's got no accel cable right, using a potentiometer? Could be bad Pot, or even the adjustment will throw a code. The stop bolt on floor, if changed can cause a code. The wide open throttle switch out of adjustment, same thing. When it codes out, truck goes into limp mode, crawls.
A bad ignition coil or wiring can cause it to code out. Toyota has too many different systems, and buying a manual from dealer is a crap shoot.
You dont need a scan tool on older models, just ground the right plug and codes flash