Question about 1999 GMC Sonoma
Sounds like more than one issue, not getting as hot as before, and slow to change from vent to defrost, The first I suspect you have a thermostat failing, not a big job or expensive either, remove the thermostat putt it in an old pan full of water bring it up to a boil, with a thermometer (I use a meat thermometer from the dollar store as you won't want to use it for food again) watch at what temp the thermostat pops if it doesn't pop at around 180 then replace it.
second is the delay in switching from vent to defrost, again is not to expensive but is a bit troublesome as it sounds like you have a weak vacuum the louvers are moved by little dash pots the dash pot that moves the louver behind the dash could have a bad diaphragm or a crack vacuum line. not expensive just a pain to get at. a can of wd40 with the straw in place can help diagnose the problem if it changes properly when sprayed at time of activation (the line first then the dash pot) you will have found the bad boy Good luck
Posted on Apr 14, 2017
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Posted on Jan 02, 2017
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
Posted on Oct 21, 2009
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"
This is what I did to solve the exact same problem.I got a pump that you can hook to a drill that has garden hose connections on it.I made 2 hoses to pump into my heater core and hook to the pump. I bought some radiator cleaner at Auto Zone and put it in a 5 gallon pail with about 2 gallons of hot water.I pumped the cleaner from the pail into the OUTLET side of the heater core.(By going in the outlet side you are back flushing the core.) I put an old cloth over the top of the bucket and let the outcoming cleaner run onto this cloth acting as a filter for the dirt coming out of the heater core and let the pump run about 10 minutes. Believe me,A LOT!!!!!!!!!!! of dirt,rust etc. came out of my heater core. The heat now works excellent! Good luck.
Posted on Jan 29, 2010
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