Rick's Free Auto Repair Advice

Engine hesitates and stalls, transmission slipping – GM SUV

Engine hesitation and stalling, slipping transmission – GM SUV

GM has issued a service bulletin to address an engine hesitation and stalling, slipping transmission problem on 2002 Chevy TrailBlazer, GMC Envoy, or Olds Bravada with the 4.2L (VIN S) engine. The issues are engine hesitation on acceleration, or a stumble or stall on throttle tip-in after being restarted after a prolonged soak. Also, some 2003 models of these same SUVs may encounter a harsh or slipping 1-2 shift or a hard
bump as 2nd gear kicks in.

GM has determined that all of these symptoms could be due to the Evaporative Emissions Self Test. During the EVAP self test, the PCM opens the purge valve for a few seconds to get rid of accumulated fuel vapors in the charcoal canister. Then it closes the purge valve and pulls a vacuum to determine the integrity of the fuel system. However, if the charcoal canister is saturated with fuel, the PCM senses the rich condition as the fuel vapors are pulled into the engine. It will attempt to compensate for the rich condition by driving the fuel system lean. The lean fuel condition and the closing purge valve and fuel system vacuum causes the problems. The problem may be worse in hotter weather, when the ambient temperature is higher than 70°F and on vehicles that experience short trips. Those vehicle generally don’t complete the full drive cycle that would fully purge the fuel vapors from the charcoal canister.

The fix is new software. Take your vehicle to any GM dealer or Independent repair shop and have them update the software. Expect to pay about $150 for the update. That’s a lot cheaper than throwing parts at the problem—parts that won’t fix the software issue.

©, 2013 Rick Muscoplat

Posted on by Rick Muscoplat


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