Heater problem 1999 trailblazer

You need to fix the coolant leak, then refill and bleed cooling system, then you will get heat and cure overheating problem too

I had a 91 GMC 4.3 do the same thing. Liked to froze to ***** one winter. Found I had a air bubble (air lock) in the engine block and heater. It was a bear to get out. Finally installed back flush kit in the heater hose between engine and heater. Took cap off flush kit, pour coolant in through outlet till coolant ran through back to radiator. Replaced cap and started engine. Repeated again. Heater works as good as new. It's using the same principle as priming a gas powered water pump.

Yes . Your heater is actually not an Electrical Heater or a Separate Heating system . Actually if uses the same Radiator Line . When Radiator is heated up it is cooled down with radiator Fan and coolant . But when you use the heater function , this hot fluid is passed through a heating coil tube and blower fan is used to take that heat inside the car .
So I think now you understand , why you are not heated up in a short run . You really need to make the Engine + Radiator Warmer ( that is natuarlly happened when you run more) to get Warm air inside.
Please rate my answer fairly and Support me With Regards and Season's Greetings Deepak Richard

Leave rad cap off, and start engine from cold, top up coolant and make sure your heater is on, and run engine until operating temperature reached, and make sure you allow it to run for a while after reaching operating temperature. Rev engine slightly for 5 seconds or so, every once in a while to help bleed air out. Check heater as your doing this and you should feel warm air. Once heater is working good, top up coolant in rad & bottle if you have access to both, then put all caps back on, and you should be o.k.
Just be sure to check coolant level in bottle from time to time, and top up if necessary.

faulty thermostat will cause overcooling by staying open at all times.to check this.crank up vechicle until it warm up.becareful watch out for drivebbelt and cooling fan.feel all hoses.plus heater core hoses all hoses should be hot when engine warm up.if engine run until it get hot all hoses feel luke warm thermostat is bad. if all hoses get hot.check your fuses to HVAC mode switch.if all is good.you have a faulty heater air door or it could be stuck.your heater cores could be restricted stopped up.if you have climate control this is a dealership fix.

First you have to identify what causes the problem.
To do this you have to:
1) When engine is cold check the level of coolant. If it is low fill it up.
2) Start engine and check if engine reaches regular temperature. If your engine is not heated enough it means your thermostat does not work properly and should be replaced.
3) If engine temperature is normal, then check the temperature of pipes that go to/from heater core. These pipes are located up the fire wall (easier access is from the passenger side). If pipe/pipes are cold (even engine is worm) it means you heater core is plugged and should be flushed. Keep in mind, flushing the whole cooling system is different from flushing heater core.
4) If pipe/pipes are hot/warm, turn off the fan, switch controller to the hot position, wait for 3-4 minutes and turn on the fan. Note if the cold air blows first or warm air blows first but it cools down in 5-10 seconds. If the air blows cold than it is most probably the problem with blend door or blend door actuator.
5) If air blows warm first and then cools down, then while running engine and remaining the transmission in PARKING position, press on the gas pedal until your engine reaches 3,000 RPM. Check if your heater blows warm/hot air. If it does, then your water pump does not work properly and should be replaced. If the air still cold, I would suggest to flush the heater core.

It is most probably the problem with blend door or blend door actuator. But just in case I would suggest to follow the entire procedure:

First you have to identify what causes the problem.
To do this you have to:
1) When engine is cold check the level of coolant. If it is low fill it up.
2) Start engine and check if engine reaches regular temperature. If your engine is not heated enough it means your thermostat does not work properly and should be replaced
3) If engine temperature is normal, then check the temperature of pipes that go to/from heater core. These pipes are located up the fire wall (easier access is from the passenger side). If pipe/pipes are cold (even engine is warm) it means you heater core is plugged and should be flushed. Keep in mind, flushing the whole cooling system is different from flushing heater core.
4) If pipe/pipes are hot/warm, turn off the fan, switch controller to the hot position, wait for 3-4 minutes and turn on the fan. Note if the cold air blows first or warm air blows first but it cools down in 5-10 seconds. If the air blows cold than it is most probably the problem with blend door or blend door actuator.
5) If air blows warm first and then cools down, then while running engine and remaining the transmission in PARKING position, press on the gas pedal until your engine reaches 3,000 RPM. Check if your heater blows warm/hot air. If it does, then your water pump does not work properly and should be replaced. If the air still cold, I would suggest flushing the heater core.

First you have to identify what causes the problem.
To do this you have to:
1) When engine is cold check the level of coolant. If it is low fill it up.
2) Start engine and check if engine reaches regular temperature. If your engine is not heated enough it means your thermostat does not work properly and should be replaced.
3) If engine temperature is normal, then check the temperature of pipes that go to/from heater core. These pipes are located up the fire wall (easier access is from the passenger side). If pipe/pipes are cold (even engine is warm) it means you heater core is plugged and should be flushed. Keep in mind, flushing the whole cooling system is different from flushing heater core.
4) If pipe/pipes are hot/warm, turn off the fan, switch controller to the hot position, wait for 3-4 minutes and turn on the fan. Note if the cold air blows first or warm air blows first but it cools down in 5-10 seconds. If the air blows cold than it is most probably the problem with blend door or blend door actuator.
5) If air blows warm first and then cools down, then while running engine and remaining the transmission in PARKING position, press on the gas pedal until your engine reaches 3,000 RPM. Check if your heater blows warm/hot air. If it does, then your water pump does not work properly and should be replaced. If the air still cold, I would suggest flushing the heater core.

Check for a vacuum leak. WHERE is the magic question.

Try looking at the theromstat housing and check it very close for a hair line crack either when the engine is cold or hot. They have had problems with the housing getting a small hair line crack and leaking and getting air in the system. Also keep a close eye on the coolant and make sure its clean and full if there is any air in the system it wont heat great at idle when you are going down the road or idle it higher you have more coolant flow thus making it hotter.