95 chevy lumina blows cold air only

Could use some advice. Last yr my heater stopped working, so the thermostat was replaced and worked fine. About 3 mos ago, the low coolant lite came on and the coolant was completely empty. This is when we noticed something leaking and making a hissing sound and the hood of the car was smoking. There must be a slow leak as the coolant needs to be added occ. The temperature gauge never rises above 1/4 way thou. Now the heater is no longer working again and blows COLD air like the a/c is on for all the gauges. The thermostat was replaced again and still no HOT air. What is causing the problem with the heater and is it related to the coolant leak?? The car has 140,00 miles on it, so does it pay to have this all fixed? I don't mind adding coolant if that leak is expensive to repair, but I need a heater with the cold Wisconsin winters arriving. Thanks for your assist.

THe coolant leak is causing your no heat issue. When air gets in the cooling system on these cars, it will often settle in the heater core causing a vapor-lock situation that will prevent coolant from circulating. You must get the coolant leak fixed if you want heat (not to mention your motor to last). If the air pocket finds it's way to the engine, it will creat a steam pocket and blow the head gasket(s). If the coolant is leaking internally due to the intake gasket issue the 3100 is known for, you could also destroy the lower end bearings. If it's an external leak, from the quick disconnect hose fitting on the driver's side end of the intake for example, then it's a relatively simple fix.

thank you so much for responding quickly. I have copied your response and am off to the mechanic today. This site is awesome.