1991 Camaro RS 5.0 auto. Idiot previous owner tried to fix an intake gasket leak with block sealer. Have replaced the intake gasket, radiator, heater core, heater control valve, water pump, thermostat (195 degree), coolant temp sensor, lower radiator hose, blower motor, vacuum switch thingy on the back of the HVAC that's always broken on older GM cars, and drained/flushed/refilled the coolant at least 10 times. I have verified that the blend door is operating correctly, and the metal pipe at the back of the passenger side of the engine that's impossible to remove without pulling the engine seems to be flowing okay from what I can tell. Heater core is hooked up correctly as well.

Of course the heat gets plenty hot in the summer, but rarely blows warmer than 70 degrees or so in the winter, unless the temp gauge is showing 220+, which it only does for brief periods in bumper to bumper traffic. In normal driving in the winter, my temp gauge rarely reads above 160.

Any ideas? I can't think of anything, and I'm tired of being cold.

Is it an aluminum, aftermarket, heater core?

Heater core is a Spectra 94606 replacement. I'd assume it's aluminum. Original one wasn't leaking, just assumed it had flow problems from the block sealer. Also have a Spectra replacement in my '02, no problems there.

I have the same one and my heat doesn't really blow all that much warmer than the outside air, once the engine is up to temp, and I have a 195 degree thermostat in my 4.8. I have read several times that the new heater cores don't work that well. I have two take outs that I am planning on having re-cored eventually since this doesn't work that well.

I honestly don't think it's the heater core, unless my old one was really badly clogged, and the new ones are complete garbage. It was blowing just as cold with the original copper one. Will keep that in mind though if no one else comes up with anything.

What temp thermostat do you have installed?
Does your car still have the heater core valve that is vacuum driven? If so that may either be not getting enough vacuum to fully open or it could also be clogged up/broken. I would try bypassing it with just some straight fittings and see if that improves your heat. Also you may want to take the AC cover off the top of the ac box (under hood firewall) and make sure it still has all the weather stripping in there.
Verify that the AC isn't running in the heat position (it should run in defrost).
Does it happen to have a under drive pulley on the water pump?

If you have a vacuum pump like you would bleed brakes with I would test the lines under the dash for leaks.

Verify that the source vacuum is hooked up to the engine and not reversed with the selector valve. (under hood)

I don't think it would matter too much but verify the heater core input/output is right and not reversed?

My Factory copper heater core will cook you... I was thinking of swapping it with aluminum simply so I had all the same metal after the ls swap... maybe I should rethink that .

Once the engine is up to operating temp. do the heater hoses get hot?

I'm having this same issue right now. I have my vacuum line to the heater hose switch going to the front of the Holley throttle body, I was wondering about vacuum being an issue too. Unfortunately my blower motor is going in and out too. Probably because it's cold outside!

I had a similar issue. I had to remove the blower motor from the engine bay side, add RTV around it to make a sealed gasket and re tighten the bolts all the way in (didnt help that 3 were missing)

basically it was sucking in engine bay air (if its snowing in america i cant imagine the engine bay air is particularly warm)

Thermostat was a 190, replaced it with another 190 yesterday, no difference.

If you're referring to the heater control valve, I just replaced it a few months ago, vacuum line is still hooked up to it. Will have to get a vacuum pump to test as I don't own one.

Heater core hoses are on correctly, just re-checked all routing yesterday.

Pretty sure I resealed the ac box with 3M strip caulk when I changed the blower motor.

No underdrive pulley, but the air pump has been removed if that makes a difference.

Checked everything with an infrared thermometer this morning, here's what it's currently at:

Outside ambient temperature: 51 degrees
Engine temperature: 160 degrees (this is going by the gauge, not the infrared thermometer)
Heat temperature from vent: 110 degrees
Heater core hose that's closest to the passenger's side: 130 degrees
Heater core hose that's closest to the driver's side: 90 degrees

110 vent temp with 160 degree water temp is about right for it. How is the air flow. Sounds like you have good water flow, it may be leaking ducts, airbox, and the vacuum actuators at each vent that opens them on select, or a vacuum leak.

Especially if your dropping the water temp 40 degrees across the heater core... Though that may indicate the flow is too slow so I would still be tempted to replace the valve with straight fitting and test.

Air comes out from the vents fairly strong. Only weird thing is that occasionally I can hear one of the vacuum actuators behind the dash moving way after I have set the HVAC controls (like 10-15 minutes later), and it sounds like the actuator is corroded and/or not operating smoothly. Usually a rusty metal sound followed by a pop. No change in temperature or airflow from any of the vents when it does this though. Didn't think it was worth removing the entire dashboard assembly to investigate since nothing else changes when I hear the noise, and it's really random and difficult to duplicate.

Your problem is your engine coolant is not getting up to operating temperature. If the engine temp was at 190 degrees, your heater air temperature would also increase another 30 degrees which would put it at 140 degrees. Sounds like you have too much engine coolant bypassing somewhere and not allowing the engine to get up to operating temperature.

have you checked vacuum at the heter control valve....I personally hate those things and toss them on any GM vehical I get that's equipped with one...my 84 never had one and the ac and heat worked fine...I would ditch the valve for some straight tubing just to verify its not the issue or at least check the vacuum going to it

Have any pictures of your engine bay? Maybe the lines are run incorrectly.