I have a 2002 Chevy Blazer Xtreme, 4.3L V6. I stop the other day waiting on a train to pass. While I was setting in park I notice my heater started blowing cold air out the vents. When the train passed, and I started driving, the heater started blowing warm then hot air again.

Thanks!

 

Sounds like your heater core may be partially clogged.

 

Going to tomorrow to get a flush and fill! Thanks!

 

You can do it yourself --> Heater Core Flush

Also, you can confirm that the core is partially clogged by feeling the temperature of the hoses leading into and out of the core. If things are working properly, they both should be hot (around the same temperature).

 

Yea I notice that earlier when I was doing a search here on the forum. It's too COLD to go outside and do it myself right now, but I might do it later when it warms up. The temp is not getting out of the 30s for the next three days! We just went thou a Snow/ice Strom this past weekend. Still have some snow and ice on the ground!

 

UPDATE: I went today to get my Blazer checkout at the Express Oil Change. Come to find out I have a a leaky water pump. The coolant level was low causing my heater not to work right. Thought at first it was the thermostat, but found out there was a leak coming from the water pump. Getting it fixed tomorrow.