I have a 1992 gmc safari van when i turn the heater on no air is blowing through the vents could you please tell me what might be wrong

open the hood and check for leak on any plastic vacuum lines.

the heater core on a Safari/Astro van is higher then the radiator, to solve my probem, I installed a reverse flush kit form Prestone, then I was able to fill the entire cooling system.

Your selector might be stuck. Is it blowing on the windshield?

Try stopping on a steep incline--does the heater stay warm? If yes, what you have is an air bubble that hangs at the heater hose until pressure from the water pump blows thru it as the engine speeds up. Park on an incline, open the radiator cap, run the engine until the thermostat is open, and fill the radiator to the neck. Wait until the level stops dropping and make sure bubbles don't continue to come up. If they do, you may have a blown head gasket that will keep forming these bubbles until you change the gasket. Please let me know the outcome, as we are getting a lot of these cases and are trying to improve the answers.

This is a common problem on all Astro/Safari vans. The problem is a leak in the vaccum lines that control the vaccum switch responsible for changing air direction from the defrosters, dash vents, floor vents..etc.
Many of the problems that plaque the Astro/Sarfari vans and many other "Mini-Vans" is the build up of excess heat in the engine compartment. It\'s a tight place to put a V6-4.3 Vortec engine and the heat generated bounces back and cooks everything on the engine (coils, modules, sensors, wiring, and of course plastic vaccum hoses). No need to mess with your switches or anything behind the dash -- replace those thin plastic vaccum hoses to operate the vaccum switch (located just below the A/C canisters, to the right of the heater/ac blower housing) and your dash switches will magically send heated or cooled air where you want it.

1993 GMC Safari Van M
4.3 liter V-6

check vac hoses by firewall iin engine compartment. If disconnected reconnect.

replace the fuel pressure regulator and the fuel filter, the pressure is low.

Is the headlight switch turned all the way to illuminate the dashboard lights? If it is, check the fuse for instrument panel, also marked as instrument cluster.

Check under the hood for deteriorated small vacuum hose coming off the intake over to the heater box area.
Repair the vacuum hose.