Q: I've been searching the Internet to diagnose an issue I have with my 2003 Chevrolet Express van with the 6.0-liter engine. I was driving on the expressway, and suddenly the truck started to behave as if the engine had no power. I shifted to Park and the engine ran smoothly, but once I shifted to Drive or Reverse, it was missing power, started to rattle, and suddenly it turned off. It got to the point where it wouldn't even start in Park. I called an auto mechanic and he bypassed some fuses under the hood and managed to make the truck start, but as soon I shift into gear it has the same problem.
A: I have no idea what bypassing fuses was all about, but your general description of the problem points to a torque converter clutch problem. Almost all modern automatic transmissions use a lock-up torque converter. The torque converter itself is a hydraulic link between the engine's crankshaft and the transmission's input shaft. One function of the converter is to allow the vehicle to come to a stop without stalling the engine.
A lock-up torque converter uses an internal clutch. When driving at highway speeds under light-load conditions, the lock-up clutch is applied inducing a solid mechanical link between the engine and transmission. This provides a significant advantage in fuel economy. Let's compare that with a manual transmission. Say you're cruising on the freeway at 60 mph in fifth gear, and you let off the gas without changing gears or depressing the clutch. As speed decreases to about 30 mph, you'll note the lack of power when attempting to accelerate (of course, 30 mph in fifth gear). Then, as you slow down even more, you'll begin to feel the bucking and jerking. And when you come to a complete stop, the engine will abruptly stall. Same thing when the solenoid that applies the lock-up torque converter gets stuck in the applied position. Once stopped, and Park or Neutral is selected, the engine starts and runs great, but stalls when engaged in any gear, because the engine is actually locked to the rear wheels. It's like releasing the clutch pedal on a manual transmission with your foot on the brake.
A couple items in your question didn't add up, but have the PCM checked for DTCs and evaluate the lock-up converter operation. The most common cause is a failed (stuck) torque converter clutch solenoid, which is not expensive to replace.
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