One of the first questions that should always be asked - and answered - when diagnosing a fuel-related complaint on a fuel injected engine is, "What is the fuel pressure?"

All too often, technicians assume fuel pressure is "good" without actually measuring it with a gauge. If the engine runs, they assume the injectors are getting adequate fuel pressure. If the engine cranks but won't start, and they depress the service valve on the fuel rail and some fuel squirts out, they assume the injectors have pressure. They do, but the question remains, "How much pressure?"

For the engine to start and run smoothly with no stalling, hesitation or misfiring, the injectors have to deliver the proper amount of fuel with every squirt. This is especially important on late-model engines with sequential fuel injection. One bad injector will cause a noticeable misfire and usually set a P030X misfire code (where X represents the cylinder that is misfiring). On older engines where the injectors are all fired simultaneously, the good injectors can often compensate for one or two bad injectors. Even so, for the engine to run right, fuel pressure to the injectors is critical as is the volume of fuel delivered by each injector when it fires.

The fuel calibration curves in the powertrain control module (PCM) are based on OEM dyno testing using a new engine. Fuel pressure is within a specified range for that engine, and the injectors a...