Let me tell you more than I know. On older Chrysler engines, there is a crankshaft position sensor and a camshaft position sensor. Both have to send signals to the Engine Computer for it to turn on the fuel pump relay. That means the engine has to be rotating, (cranking or running). If either signal is lost, the engine will stall and not restart. On newer models the same thing happens, however, if the fuel pump relay is bypassed, like you're doing, the engine will run on just one sensor. That sensor is providing the timing signal for the injectors and the ignition coil(s), but the computer isn't turning on the fuel pump relay. Typically the crankshaft position sensor provides the timing information for the ignition coils and the camshaft position sensor provides timing pulses to synchronize the injectors, but when the camshaft position sensor signal is lost, the crankshaft signal is used as a backup strategy. The injectors will fire in the right order, just not at precisely the right time.
On older GM engines, if the camshaft position sensor fails on a V-6 engine while you're driving, you'll never notice it until you stop the engine and try to restart it. Then, there is a 33 percent chance the computer will fire the correct ignition coil at the right time. If it does, it will just continue to fire all three in order each time it gets the right timing pulses from the crankshaft position sensor. There's a 66 percent chance it will guess wrong and the engine won't start. It will continue firing the coils in the wrong order until you turn the ignition switch all the way off, then back on and try to restart it, then it will guess again and have another 33 percent chance of being right. For this reason, you can have multiple failures to start, then unexpectedly it will start and run.
You likely have some similar situation. Be sure to check your fuel pump relay. Two terminals must have 12 volts with the ignition switch on. You don't have to be cranking the engine for those to appear. If those are there but the computer isn't turning that relay on during cranking, I would suspect the camshaft position sensor or the crankshaft position sensor. You'll need a scanner or code reader to see if the computer detected a missing signal from one of them. If no diagnostic fault code was set, you'll need the scanner to view live data to see if those sensors are listed as "yes" or "no" while cranking the engine.
Tuesday, February 24th, 2015 AT 8:22 PM