Why I get lukewarm heat air flows ok, and the temp gauge does not move?

Hi Susan, My name is Si, I am a journeyman mechanic. I had a 1987 S15 that had the same sort of issue. I changed the thermostat to a hotter one ( yes you can get them that allow the engine to get warmer) for those of us that live in Canada, we need that. The heater works off a small radiator very much like the large radiator the engine uses. Its just smaller. You can flush this radiator easily. what you do is disconnect the hoses from the engine end of the hose. This will give you some hose to work with. Once you have them disconnected blow air in through one, then through the other. All you are doing at this point is getting as much coolant out of it as possible. Then pour in a bottle or two of CLR or Radiator Flush (available at your auto parts store) and let it sit for half an hour. then hook a garden hose up to either hose. To do the hose connection... cut the end off an old garden hose where the nozzle attaches. See if you can squeeze the garden hose into one of the heater hoses. If you cannot... get a piece of connector pipe for garden hoses and make the connection. Use electrical tape to tape up the connection point to keep it from leaking. Run water through the radiator at full flow. You should get flow out the other hose equivalent to about one quarter of the normal garden hose flow. You want to force the gunk out of the radiator. Finally blow the water out of the radiator (same as before) then run pure antifreeze through it as soon as possible. (you don't want the water to freeze). Now reconnect the hoses and top up the system coolant with pure antifreeze. This will flush the heater core (ie. small radiator) With the radiator or coolant tank cap open and off... run the engine and keep watch on the coolant level. Top it up as needed. Now... Run the engine for 10 minutes or so. (maybe go for a drive) warn up the engine. With a new thermostat and a flushed heater core you will likely get heat. If you do not... (As was the case for me) The water pump on the engine was faulty. I put on a new water pump and that was it. The water pump. What happened was the pump bearings wore and the impeller in the pump rubbed the casting and then was not pumping enough to get heat. Note... if you do not get good flow through the heater core you can put a can of Radiator Flush in the main cooling radiator system and run it for a few days. Then flush the entire system to get the chemical out and refill with fresh coolant and distilled water. (not tap water). I think this should get you heat.

Yes i believe your thermostat is partly stuck open. Low temp readings on gauge and warm air. Here is a good test place a throw rug in front of grill to block air flow through radiator start and watch temp gauge if it goes up higher turn heater on and see if air is hot then remove rug if goes cold then replace thermostat. stays the same feel heater hoses if one is hotter than the other the core is plugged or heater valve is restricted. all else is vent door isnot fully seating in heater box cable adjustment.

First check and see if the fluid in the radiator is up where it should be. When was the last time the thermostat was changed? Could be no good.These things cause just warm air If the blower is working as it should you will have air flow but it your antifreeze is not warming up like it should it will be cool. .