GMC Troubleshooting Questions? Ask a Mechanic for Answers
On these vans, the most common cause for the airflow problem you're having is the vacuum supply from the engine. The vacuum hose runs from the right side under the hood, across the intake manifold and connects to a fitting on the manifold. This hard plastic line usually gets brittle and breaks in two right at the manifold. You'll have to move the air filter housing out of the way, but you'll likely find the solution to your problem on top of the engine.