Just because your car's engine starts, this doesn't mean it's necessarily in good repair. You may realize that it needs some attention because it's running, but performing poorly in one way or another. While a mechanic can perform a proper diagnosis and get your car running smoothly again, it's always good to know what you might expect by way of repair bills. Note a few diagnostic tips for a car that runs, but not very well.
Starts but then sputters and even stalls
If your car's engine starts but then sputters and even stalls out when on the road, and especially if this happens when you're idling so the RPMs of the car are low, this can mean your car is either not getting enough fuel or not getting enough oxygen. Your car's engine creates combustion with a mixture of fuel and oxygen so when it's not getting enough of either of these, the engine won't create enough combustion to keep running. If you've recently filled up with low-quality fuel, that fuel may be somewhat watery; you might top off the tank with a high-quality, name brand fuel and run the car for several more days, and then top off the tank with high-quality fuel again. You can also add an engine cleaning solution to the fuel tank, as an excess of gunk and debris in the fuel lines will also cut off the engine's supply of needed fuel.
If the fuel is not the problem, your vehicle may need a new oxygen sensor. This tells the engine how much oxygen to bring in and how much to vent away in order to maintain combustion. Usually the "check engine" light is on when your car needs a new oxygen sensor.
Sometimes hesitates when accelerating
If your car won't immediately move when you hit the accelerator pedal, note a few details about the hesitation. If it seems that one tire, or the front or back tires together, are stuck somehow, this could be that the brake pads are worn down and are not releasing as they should when you let off the brake. If the vehicle hesitates and then seems to "slam" forward quickly, this often means that you have a problem with the car's transmission, and the vehicle is stuck in neutral for those few second. Often a leak in the pan that holds transmission fluid is the culprit, and the pan may simply need to be changed.
For assistance, talk to a mechanic.