start out on the vac tree there is a vac line that runs over to where the heater core is. Then check the under the dash on the driver side there is a vac line that runs one of those vac. solenoids . the vac line may have come off of it. That is what happend on my car. There is one behind the glove box to. The one under the dash is hard to get to, and the line is just long enough to fit.