Heater problems
 

Ok this is a LONG one..
Had to take a friends S10 blazer on a long trip today 450 miles round trip.
So yesterday we went over the engine to make sure fluids and such were they should be.
His wife said the heater was not blowing hot air And that she recently got an oil change.
We ended up adding 2 quarts of oil to truck. And topped off engine coolant. Started engine.. brought it up to temp.. heater blowing cold air.
Friend suggested heater core clogged up.. - That was last night.
This morning. Checked oil before leaving. Level was where we left it last night.
Heater blew cold air in morning even at temp. Drove 220 miles. Picked up item, stopped to refill gas tank, checked oil. Level normal, no leakage.
Heater starting to blow warm air. Blew warm air, (not hot) for over 150 miles before going cold on us.
Arrived home.. Checked fluids.. everything at level they were when we started the trip.
Is this a sign of a clogged heater core or is there something else? Thanks.

With engine at temperature. Check the heater hose to see if they are both hot. If one is hotter than the other one it is a sign of a clogged heater core. What year is the blazer?

2003 s10 blazer.. So would a clogged core, be hot one time and cold another after its ran at temp for over an hour?? mmm.. guess that does make sense..