1998 GMC Jimmy SLT warm air only from heating ducts

If the temp gauge is now more stable, the problem is in your heater system, not the engine cooling system.

This sounds like a thermostat that is fully open all the time. It allows the engine coolant to warm when sitting still (no air being forced thru the radiator), but even that goes away when moving, since the air through the radiator will remove that heat from the coolant. Does you van have a temp gauge? Is so, at full operating temp. most vehicles have coolant temps of 180-195 degrees F, depending on the thermostat installed. Anything less indicates a thermostat that is starting to malfunction.

Air can be purged. While your engine is cool remove radiator and reservoir caps. Place funnel in radiator. Turn heat to the highest position and start vehicle. Add coolant to radiator as needed while vehicle warms up. You should see bubbles coming out of radiator. Fill reservoir to between cold and hot fill lines. Replace caps when bubbling stops but before coolant temperature reaches normal operating temperature. Never remove the radiator cap until engine has fully cooled down.

Hi John,
Your right with your diagnostics, you have a blend door problem. If your switching to cool air and it is still hot then you also have a heat exchange unit fault, this is the component that changes the water flow from hot to cool by shutting off. If it is not operating correctly this will give you constant hot air no matter where you temperature switch is positioned.
As for the warm air while the engine is cold, sounds like the blend door is still open to atmosphere tempreture and if it is hot outside will pull that hot air into the inside of the vehicle.
Have the heat exchange valve checked, and the only way the check blend door operation is to remove the wiring from the blend doors and have the circuit tested. If the circuit is ok then the blend doors will need to be replaced.
Hope this helps

Suggestion: see if the blend door opens or closes completely to allow all air through the heater core. To see the blend door moving, remove the panel that says "Cut here for service" under the hood along the firewall. I have a 99 Bravada where the blend door keeps moving back and forth constantly. Changed the blend door actuator but still keeps moving. The air inside is only warm since the door keeps allowing bypassed air through.

try skewing the engine coolant temperature sensor with a multi-meter and a temp vs resistance or voltage chart. the ect could be out of range and not set a code. also check actual engine temp with an infrared thermometer. hope this helps

The heater core might need back flushed, I just did mine this winter. Go to dakota-durango.com and introduce your self and your rare as ever truck, a v6 durango, and have fun.

You should see heat well prior to this heat, is your coolant level exceeding low? I have seen situations where the coolant level is so low it will not get to the heater until it warms up (by warming up the coolant has expanded thus filling up the heating system more.
I hope this may have helped,
Tom

BLEND DOOR STUCK OR BROKEN. NEEDS TO REMOVE DASH AND HEATER/EVAP CORE BOS TO REPAIR REPLACE.

The Engine Coolant Temperature (ECT) sensor is mounted in the intake manifold and sends engine temperature information to the ECM. The ECM supplies 5 volts to the coolant temperature sensor circuit. The sensor is a thermistor which changes internal resistance as temperature changes. When the sensor is cold (internal resistance high), the ECM monitors a high signal voltage which it interprets as a cold engine. As the sensor warms (internal resistance low), the ECM monitors a low signal voltage which it interprets as warm engine.
Fig. 1: View of the Engine Coolant Temperature (ECT) sensor Fig. 2: Engine Coolant Temperature (ECT) sensor locationTESTING
See Figures 3 and 4

1. Remove the ECT sensor from the vehicle.
2. Immerse the tip of the sensor in container of water.
3. Connect a digital ohmmeter to the two terminals of the sensor.
4. Using a calibrated thermometer, compare the resistance of the sensor to the temperature of the water. Refer to the engine coolant sensor temperature vs. resistance illustration.
5. Repeat the test at two other temperature points, heating or cooling the water as necessary.
6. If the sensor does not meet specification, it must be replaced.

Fig. 3: Intake Air Temperature (IAT) and Engine Coolant Temperature (ECT) sensor wiring diagram Fig. 4: ECT sensor temperature vs. resistance values