Question2004 Hyundai Sonata, 2.4 4cyl, It starts sometimes and doesn't start others. When it is running (driving it) there are no power loss or sputters. When it doesn't start it cranks fine, it seems not to fire. It happens randomly it won't start when cold or hot. I am not sure what causing it. Fuel pump, crank sensor, coil pack(plugs), Relays? What is the best step to check to see what it is?
AnswerI'm going to presume the engine cranks normally in all circumstances. If it cranks slowly, you should immediately suspect the battery, starter, and battery cables.
You'll first want to see what the nature of the problem is. While in most cases, this won't give you the specific cause, it'll help you eliminate some potential causes.
Start by checking fuel pressure and spark when the problem is occurring.
If you have no fuel pressure, check the fuel pump and the circuit that controls it.
If your fuel pressure takes a long time to build up, suspect a faulty fuel pressure regulator.
If you have no spark, suspect a faulty crank sensor or a problem preventing the engine control module from operating.
You can probably eliminate plugs and coils as potential causes as these would most likely enable the engine to start but run very poorly.
If all the above are working properly, your best next step is to take it to a reputable and competent shop which has a scan tool capable of reading the data stream. This will help them determine if there is some sort of problem causing the ECM to deliver an improper fuel mixture.
