hvac problem 99 gmc envoy
#12
The vacuum system must be leak free to operate correctly. If the black vacuum line does not hold vacuum, it means there is a leak between the mode switch and the source, (line, reservoir, check valve). If there is a leak anywhere in the system, including actuators & lines in the dash, the air distribution system will have issues. A hand vacuum pump with a vacuum gauge is an invaluable tool for this job. It makes detecting & locating leaks much faster & easier.
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