Why Does My 2004 Envoy Hesitate Under a Load?
- Your truck's powertrain control module is its brain, so any kind of malfunction here is going to wreak havoc on everything it controls. Odds are good that the PCM is fine, particularly if the hesitation only happens under load and you're not experiencing any other problems, but you may want to check it for loose connections. Loose connections and corrosion aren't uncommon, particularly in humid environments.
- A rear O2 sensor malfunction typically won't cause hesitation under load as it only monitors catalytic converter function, but a code indicating rear O2 malfunction is something to watch for. A clogged converter keeps exhaust trapped in the engine, which can momentarily kill power. Front O2 sensor malfunctions are a different animal altogether. Acceleration and high-load conditions are transitional periods where the engine has to perform at its optimal condition, while things inside change very quickly. A slightly slow or malfunctioning O2 sensor will typically manifest under load before it does at any other time. A bad crankshaft or camshaft position sensor may initially manifest as hesitation, but will almost certainly throw a check engine light with a specific code for that sensor.
- A maladjusted or malfunctioning throttle position sensor, dirty or malfunctioning mass-airflow sensor or bad manifold air pressure sensor will manifest with many of the same symptoms as a bad O2 sensor, and for the same reasons. In the same way that the O2 sensor monitors the gases coming out of your engine, these sensors keep an eye on the air that's going in. If you've recently disconnected the battery and suddenly developed a hesitation problem, you might want to consider cleaning the throttle body. The Envoy uses an electronic throttle body that will adjust to minor obstructions built up over time. Disconnecting the battery will cause your computer to forget that the obstruction is there, which will result in an airflow discrepancy and hesitation.
- Your Envoy uses several different kinds of filters, and all can cause hesitation under load, if badly clogged. It uses a "sock" filter around the fuel pump, another fuel filter in the line and another small screen inside each injector. All of these can cause a momentary drop in fuel pressure resulting in hesitation. A single, slightly clogged injector won't typically cause hesitation, but several dirty screens will. Check the air filter, particularly if it's ever gotten wet. Vacuum leaks typically manifest as a momentary hesitation under load, particularly if they're small enough that the computer can compensate for them after the air/fuel ratio runs lean.
