If your car has throttle-body fuel injection under the air cleaner, it may suffer from hesitation and stumble if the fuel injector leaks or, on cars with two injectors, if one injector fails.
Remove the air-cleaner cover and unplug the wiring connector or connectors. Have a helper crank the engine. If fuel drips from an injector tip, have the injector replaced.
If the throttle body has two injectors, crank the engine with the wiring attached as well as detached. Observe whether both injectors spray a cone shaped spray. If one does not, reverse the wiring connectors and repeat the test.
The other injector now fails to spray? The wiring is at fault. The no spray injector still doesn’t work? It is defective and should be replaced.