The reason why the car starts but engine sputters while starting.
(1) Bad valve.
(2) Problem with fuel injection system.
Car troubleshooting
If the engine also sputters when the engine is idling but not while driving, in other words, it stops sputtering when you start driving or rev up the engine, then the problem is a bad valve. As indicated that the car sputters both when starting and at idle. But, when you start driving, the sputtering goes away, it smoothes out. This is a telltale sign that the valves in the engine are not functioning properly.
What to do?
You need a valve job. Unfortunately, this is usually an expensive procedure. If this vehicle is not under warranty, you'll have to shop around for the best price. Make sure you only have the job done at a shop that specializes in this type of work.
However, if the engine does not only sputter when the engine is idling but also while driving, in other words, it does not stops sputtering when you start driving or rev up the engine and the engine sputter at most speeds, in other words, the engine sputters all the time, when you start it, when you're sitting at a light, and when your driving. Pull off one of the spark plug wires and carefully lay it close to the ground (any clean, grease-free, metal surface will work).
Have a friend crank the engine for a second by turning the ignition key. As the engine is cranking, look for a spark that should jump from the end of the spark plug wire to the ground. Do this same procedure for all of the spark plug wires but make sure you put the last wire you tested back onto the spark plug before you go to the next one.
If you see a spark jump to the ground for all the sparkplug wires tested, when the engine was cranking, a spark jumped from the end of the spark plug wire to whatever ground you laid it near, then a problem with the fuel injection system is possible, problem could be that your fuel injection system is not operating normally.
What to do?
Fuel injection systems are complex and hard to diagnose. If this vehicle is not under warranty, take it to a mechanic that has expertise in this particular make/model. AutoTech highly recommends you get a second opinion if the first mechanic suggests that you need a new fuel injection system. Very often, the problem is not the fuel injection system itself but rather one of its sensors. A skilled mechanic will be able to determine if all the sensors are working properly.
