I've RIPPED the dash apart and it sounds like it's comming from behind the heater core. when I take the condinsation hose out of the floor and put my ear to it it's the loudest. At first I thought it could be some air flow regulator but so far all the doors iv'e seen are open and closed electronically.
SOURCE: When turned to defrost, no
I had the same thing happen on my 2000 Sonoma, last week. I quickly found it to be a broken vacuum line. Look between the brake master cylinder and the drivers side fender- up top. Easy to find, there are 3 vacuum lines connected to a tee. One goes towards/ and into the fender itself. On my truck this line was cut due to rubbing on the fender opening. Withthe engine running it was drawing a good deal of air. I put a screw (plug) in the open line and my vent controls now work. Unfortunately I cannot find a vacuum diagram to tell me where the line into the fender ends up.
SOURCE: 97 gmc heater core diagram
the hoses end at the firewall to the best of my knowledge. They have to be serviced from the engine compartment. The heater is another matter. Many of the old GM Truck products could have the heater replaced without pulling the dashboard. I would go to the local parts store & buy a repair manual on that truck for about $25 then check your hoses, then check the heater core. you can always by-pass the heater core by splicing the 2 heater hoses together with a $3 plastic hose repair kit & a couple of hose clamps. as long as the hoses are long and supple enough & not short or stiff & cracking. you can actually hear old hoses crackle when you pinch them between thumb and forefinger gently. replace as needed
SOURCE: At idle, Heater Core Return Line is flowing only
two things--first replace your cabin air filter.second reverse flush your heater core with a garden hose and that should remove the build up of material in your heater core and restore the flow,hope this helps
Testimonial: "Hello; I backflushed heater core, cooling system flush. Heater floor vent temp. went from 96 degrees to 140; Thanks for your excellent advice. Robert"
SOURCE: Poor coolant flow through heater core. Heater core
on some GMC models there is a valve that opens when you turn on the heat inside. It is called a heater control / actuator valve, it works off engine vacuum thru hoses connected under the dash, and going to the top of the valve. Now junk and crud and old rust will get in the valve and restrict it's flow. You will have to pull the valve off and clean it, but it is best to replace it, seeing it is hard to get all the junk out of it.
It looks like a L shape pipe with a hose on top & bottom, black plastic,and a smaller line on top mounted close to the wheel well about even with the exhaust manifold on the pass side. Drain the coolant, remove the valve and see if this is the cause, lots of gunk in the one hose. It is not real hard not real easy to replace this valve. Now if your truck does not have this valve check the pipe that the hose hooks to on top of the engine, remove the hose and see if a small screw driver will go down inside or if it is stopped up. .
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