Help with heat

The other day I changed my t-stat because the heat was blowing cold now it's still blowing cold on higher fan speeds and on lower speeds 1 or 2 I can get temps of about 103 to 104, but higher speeds it's only about 94 to 95. Any Ideas? Is my heater core plugged? HELP! Please! I have to take the truck on the annual Christmas trip four hours north next weekend.

Take the return hose off the core, run the engine and see what comes out, for starters.

i had the same thing last month. i took off both lines to the heater core and hooked a garden hose up to one and blew it out. heat works great now. i think there was crap in there from when i did my intake gasket.

Thanks for the replies I did that this morning, actually took off both inlet and outlet hoses{ consequently broke off the plastic fitting off the inlet hose}, and put the sprayer to it. Hooked them both back up and now it's worse?! Now it won't even touch 90 on low speeds. Maybe it's still plugged, I'll try just doing the outlet.Also noticed a few air bubbles just trying to massage some air in case too much got into the system, but just small ones not big burps?! Thanks Again.

I've used the "feel" method. With the engine up to temp and running "feel" both hoses going into the core. If one is noticably cooler than the other you've got blockage. If they both feel about the same you don't. Another problem could be the temp control door or motor on the plenum. You didn't mention year of the truck??

Sorry guys it's a 99 w/a 4.8 manual tranny.I might try some co2 today maybe the sprayer wasn't enough juice to unclog it! Thanks!