i recently did a motor swap on my inline 6 wrangler, and used my old components from my previous motor that were completely fine when on the old motor. I rebuilt the motor and when i started it up, it began to accelerate off the bat, as if the gas pedal were floored, so i turned it off immediately. I then tried again and the same. i read online that a poorly seated intake or exhaust manifold could do this, so i bought a new gasket and bolted them both down securely. I know i have a severely clogged heater core, but would this cause such a high acceleration as soon as the car is turned on?