The Check Engine light on means there is at least one diagnostic fault code stored in the Engine Computer. That code will lead to the circuit or system with the problem, not necessarily the bad part.
Fuel pressure can be tricky. There are actually three circuits to look at. The Engine Computer turns on the fuel pump relay for the fuel supply system, and it turns on the automatic shutdown relay to send power to the ignition coil pack, alternator field, injectors, oxygen sensor heaters, and on some models, the fuel pump or pump relay instead of having its own circuit.
The third circuit is the trigger circuit that tells the Engine Computer to turn on those relays. THAT's the circuit that makes the most trouble, however, since you have spark, that proves the trigger circuit is working. (It involves the crankshaft position sensor and the camshaft position sensor).
If your mechanic verified fuel pressure by poking the test fitting on the fuel rail, that can be misleading. The fuel pump will run for one second after turning on the ignition switch to be sure pressure is up for starting the engine. After that, the computer will turn it on again when it sees engine rotation, (cranking or running). It knows that by the pulses from the crank and cam sensors. Unlike GM pumps that let you sit on the highway, Chrysler pumps rarely quit while they are running. They DO fail to start up when the brushes in the motor are worn. Sometimes banging on the bottom of the gas tank will get them going. Now that I shared all that, if the pump is running for that one-second burst, (you should be able to hear it), it is probably running while you're cranking the engine too. When the cam or crank senor fails, the pump will still run for that one second, but not during cranking. A lot of people find the low fuel pressure but don't realize they have to check for spark too.
Besides fuel and spark, that just leaves compression. It's not common for the timing chain to jump, but it IS somewhat common for the flex plate to crack around the six bolts that hold it to the crankshaft. There are holes in the outer ring that are detected by the crankshaft position sensor to determine when to fire the spark plugs and injectors. The flex plate could have turned slightly on the crankshaft, but usually that causes backfiring and loss of power.
One stupid thing you don't want to overlook is the fuel. We had two cars come in with this problem at the same time. The very experienced mechanic scratched his head for hours on the first one. Finally, he drew some fuel from the test port, threw it onto the floor and threw a lit match on it. It put the flame out! New tank of gas and off they went into the sunset.
Monday, December 6th, 2010 AT 10:45 PM