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Had to look at what year you have. yes some of your system has vac. operated doors so I would guess it comes off the intake so any loss can cause issues. I'm sure someone who knows the L98 better than I will post up.
Vacuum leak will cause hvac to default to defrost. Typically leak is a source in engine compartment. Hissing under dash need to follow sound. There maybe a situation wear air comes out wrong ducts.
to answer your question on the vacuum leak - depends. you could have a break in the HVAC vacuum circuit that would not affect the engine. it would depend on which side of a check valve a break occurs. check this first -- at the intake plenum there are two ports (right side/rear) - one comes off to a two way check valve. that check valve splits and goes two places - the cruise control servo and to a plastic (nylon/ABS/???) 5/32" tube into a harness, thru the firewall, and to your HVAC panel. check all connections at the plenum first. side question - does your cruise control work? a common issue - the check valves are plastic and break from old age. if you need a replacement - any auto box store - about $7. dorman p/n 47150, (or GM service replacement, p/n 15733271). it's not unusual for the plastic tubing to fracture due to engine heat and old age. if that's the case, just bridge with a section of rubber vacuum hose (5/32"). if everything under the hood looks OK, you may have to start digging into the connections at the HVAC panel.
check this first -- at the intake plenum there are two ports (right side/rear) - one comes off to a two way check valve. that check valve splits and goes two places - the cruise control servo and to a plastic (nylon/ABS/???) 5/32" tube into a harness, thru the firewall, and to your HVAC panel. check all connections at the plenum first. side question - does your cruise control work? a common issue - the check valves are plastic and break from old age. if you need a replacement - any auto box store - about $7. dorman p/n 47150, (or GM service replacement, p/n 15733271). it's not unusual for the plastic tubing to fracture due to engine heat and old age. if that's the case, just bridge with a section of rubber vacuum hose (5/32"). if everything under the hood looks OK, you may have to start digging into the connections at the HVAC panel.
Hmmm...That check valve is not even there LOL -- and no, the cruise control does not work which may explain it. Where does the hose connect toward the front of the engine if you don't mind me asking?
Hmmm...That check valve is not even there LOL -- and no, the cruise control does not work which may explain it. Where does the hose connect toward the front of the engine if you don't mind me asking?
Thanks!
aft /right side of plenum, two ports, you want the aft port. the forward of the two ports goes to the fuel pressure regulator.
