No heat either! (more details)

[Webslinger60]

The heater in my 89 S10 Blazer, in the 9 yrs I owned it, never blew really hot, even though the engine is at operating temp (even the air cleaner gets too hot to touch). This year it’s even worse, only lukewarm at best. Here’s the details
.a) Standard 4.3 TBI engine (no modifications)
.b) Engine is 2yr old Jasper reman. 20k miles
.c) New radiator replaced with engine. (no blockages)
.d) Antifreeze is full & mix good to –10 degrees.
.e) No air pockets (that I know of), parked uphill ovenight with the cap off, it was still full by morning.
. f) Not sure of the T-stat rating, but Im sure it’s not the 160 (you know, the cool one that tricks the computer
for a few extra HP)
.g) I suspected the inner air door maybe wasn’t opening/closing all way because the A/C wasn’t super cold either. But the airflow out the bottom is decent strength, just not very warm.

Is it possible the heater core has an air pocket? Is there a hot water valve like other cars? Is there a way for me to see if the inner door is operating without removing the wholedash?

Id like to head from anyone who’s had these same symptoms, and what they did to fix it.