I really don't see why it would not produce enough heat. Even at idle the engine produces tons of heat. When I had it out in -20 (-60 including wind-chill), I left it idling until the inside warmed up. It got very warm inside the passenger compartment. Two possibilities:
1) Your engine is, in fact, running cold. That would likely be due to an incorrect thermostat (or a new thermostat that is defective and opens at too low of a temperature). With the engine completel cold, take of the radiator cap and let it run for a while to completely warm up-- 20 minutes should do. Use a meat thermometer (I have a digital one I got for $5 at a grocery store) and see how hot the coolant in the radiator is. It should be around 190-- the factory service manual says it should be at least 170.
2) Your engine is running at the proper temperature, but your heating system is defective. This could be a clogged heater core, which you could back-flush or replace. It could be a problem with the air blend door. Not sure if your's has it, but my 94 has a water valve that can shut off the flow of hot coolant into the heater core-- that could be the problem.