A car can hesitate or stall while going uphill for a number of reasons, but typically the problem occurs when the engine's valves and rings are worn. Because driving uphill puts the engine under additional stress, less compression is dispensed, which weakens the engine over time. Stalling can also occur when the engine's sensor no longer works.
Fuel intake deposits can also make it hard to start a car or may cause the vehicle to hesitate and stall. Adding a fuel system cleaner usually alleviates the problem. If the car smokes after acceleration from a stop, it is typically an indicator that the piston rings or valve guides need to be replaced. Fuel injector or carburetor deposits are often the reason for reduced gas mileage or performance. If the engine no longer idles smoothly, it may be due to a deposit build-up in a car's fuel intake system, specifically on the throttle plate.
To determine any kind of problems with hesitation or stalling, it's best to have a mechanic run a diagnostics test to exact a cause. By running the test, a driver can avoid paying for a part he does not need or spending more than necessary to fix his car.