The Lisle Coolant Fill Funnel to Remove Trapped Air

First check the coolant level, if the coolant is low enough this alone could be the problem. In many cases with Nissans, only a slightly below full coolant level could be enough to cause poor heater performance. The coolant could be low enough to affect the heater however, not low enough to cause the engine temperature to rise above the normal range. Pressure test the system to make sure there is no leak, even a drip can cause enough coolant loss to affect the heater performance, due to air pockets being introduced.

If the heater does eventually work after the vehicle has been ran for an extended period of time, the thermostat could be stuck in the open position. This allows the coolant to flow continuously, preventing the engine from reaching operating temperature quickly. As you know a stuck closed thermostat presents a much more urgent problem than heater performance.

Particularly if the cooling system on the car you are working on looks like it has been neglected, there is a chance there could be a restriction in the radiator or the heater core. On rare occasion, the fins of the impeller on the water pump could be rusted to a point that it is no longer pumping the coolant. If the impeller is plastic and not subject to corrosion, it is possible for it to stress crack an come loose from the shaft.

If equipped, verify heater control valve operation and control cable adjustment to make sure the valve is being opened completely (this is a common problem with 1996 to 1999 Honda's). Use an infrared thermometer or feel the heater hose temperatures at the heater core. If the hoses are both warm, the coolant is flowing through the heater core. The temperature blend cable or actuator should be checked for proper operation, the blend door may not be moving fully or at all.

If there is a major difference in the temperature of the two hoses, there is a possibility there is air trapped in the heater core or at some portion of the cooling system, preventing the flow through the heater core. Some models have one or more bleeder valves to purge the air out of the cooling system, refer to the service manual for the specific vehicle. The use of a coolant fill funnel like the Lisle LIS24610 works wonders in purging the air from cooling systems. The Lisle coolant fill funnel allows coolant to fill the void as the air slowly works its way to the surface. The air is removed when the heater is blowing hot air and there are no more air bubbles surfacing in the coolant fill funnel.

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