Heating problem

I have a 1993 Ranger 3.0 6 auto 2wd. I was getting no heat/low heat after the engine was warmed up. I took it to the repair shop and they said I needed to get a new thermostat, and a flush and fill of radiator fluid. I installed the new thermostat and got the flush and fill at a oil change joint. I still get periods of no heat, and low heat, but never good normal heat after driving for a while. Should I just replace the heater core at this point? Or is there something else I should be looking for?

Second question which is a general thing. If I buy a new car and replace some components with aftermarket parts that are used for enhanced performance and street racing and whatnot, am I generally getting a far superior product than stock? I actually do no race at all, but if I got a performance clutch for instance and drove normal then the chances that I ever have to replace that cluch is low right, same with brakes and other things.

Ranger's have a blend door problem do a search as a link was posted with pictures.
To check the heater core when the truck is up to operating temperature feel both hoses feel the top and bottom hose they should be hot if your getting good flow if real cold may be some change.

It could be 2 things, clogged heater core or blend door problem.
That model had vacuum operated blend doors...make sure that you don't have a bad vacuum hose in the engine compartment.

i'll check those two issue out, thanks

Problem solved. I have heat. It wasn't a vacuum problem because the little motor/piston thingee goes up and down when you change from cool to heat and the book says that is controlled by vacuum.

Changed the heater core and BAM!!! Heat!!

To anyone even thinking about having a mechanic change your heater core, DON'T LET THEM DO IT (unless you just got money to burn and no time I suppose), the job is just that easy to do.

I've had multiple cars that heaters stopped working. So far, I got them all to work by: putting in a back flush kit, and then flushing with a mild flushing agent. the problem with the super strong flush is that it can expose leaks in your cooling system.

Good luck.

Stan