GMC Troubleshooting Questions? Ask a Mechanic for Answers
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Try crimping off the vacuum hose to the brake booster with the engine running but not when driving the vehicle and see if the engine then idles down.If it does then this indicates the brake booster itself has a leak and needs replaced in most cases.If it does not idle down then or if this model does not use the vacuum brake booster assembly let me know so we can continue.
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