What causes an engine to race on its own..?


Answers:
bad temp sensor or oxygen sensor. how long has the check engine lite been on?
its called pre-ignition
is it a white beetle with the number 53 on the side?

or a black corvette, with red flashing lights on the front?
not sure but the same happens to my car Ive found that if you disconnect the battery for a while it seems to go back to normal for several months give it a try it cant hurt
It could be the throttle wire sticking in the cable, caused by water penetrating and going rusty. It happened on my golf Mk 11, had to buy a new cable.
have you checked the accelerator cable sometimes it tightens on its own.
think its the choke - but then i'm a girl. Thats what it was on my Ford Fiesta
NO DRIVER
Could be sticking throttle cable but if its fairly new is the idle air control valve sticking.
Overheating could be the cause.
If it's a new car, this happened to ours anyway.
Check the coolant.
depends what car you have.

assuming infernal combustion, then it's caused by the fuel-air mix/quantity changing.
this could be computer on injector system, or mechanically the choke system.

on the Prius hybrid system then the engine speed is not related to the road speed. as the car figures out how much petrol or electric to use at any time.

Best/most efficient motive engines are electric and these don't race unless you take them off load.
Hence the Tesla, made by lotus in Norfolk UK,will out accelerate a ferrari. http://www.teslamotors.com/index.php?js_. based on the ac-propultion tZero. (but don't expect any mention by the fossil fuel heads in the UK media)
Short Answer
First check your choke,it a nob or lever on you dashboard.it is used to make rich mixture of gass for combustion.
second check your throtal or accelator cable, Is it stucking.
third there is small metal clips om the carbrator,under the air filter,where accelator cable is connected.these are called butterflies.chek them if they are pushing the cable back.
throttle cable is sticking.
it would help if u said the modal and year cuz if its a newish car it wont have a thottle cable or choke or carbarettor its all electric! if it is, its more likely to be a sticky idle air control valve or a air leak anywere after the mass air flow meter which is just after ther air filter with a plug on! check the big hose that gos into the top of the engine 4 leaks!
I had this problem simply because the ball joint at the carburetor end of the throttle cable was sticking.
A drop of oils solved the problem.
RoyS
Old engine? Newly Rebuilt Top end? Sounds like shes sucking air at the carb(if it is not FI) or at the intake