Heater Core Performance
Before performing the following tests, refer to Cooling for the procedures to check the engine coolant level and flow, engine coolant reserve/recovery system operation, accessory drive belt condition and tension, radiator air flow and the fan drive operation. Also, be certain that the accessory vacuum supply line is connected at the engine vacuum source.
Maximum Heater Output
Engine coolant is delivered to the heater core through two heater hoses. With the engine idling at normal operating temperature, set the temperature control knob in the full hot position, the mode control switch knob in the floor position, and the blower motor switch knob in the highest speed position. Using a test thermometer, check the temperature of the air being discharged at the HVAC housing floor outlets. Compare the test thermometer reading to the Temperature Reference chart
If the floor outlet air temperature is too low, refer to Cooling to check the engine coolant temperature specifications. Both of the heater hoses should be hot to the touch. The coolant return heater hose should be slightly cooler than the coolant supply heater hose. If the return hose is much cooler than the supply hose, locate and repair the engine coolant flow obstruction in the cooling system. Refer to Cooling for the procedures. An alternate method of checking heater performance is to use a DRBII It scan tool to monitor the engine coolant temperature. The floor outlet air temperature reading should be no more than 4.5° C (40°F) lower than the engine coolant temperature reading.
Obstructed Coolant Flow
Possible locations or causes of obstructed coolant flow:
Faulty water pump.
Faulty thermostat. Coolant temp should still reach 190-195*F for proper operation.
Pinched or kinked heater hoses.
Improper heater hose routing.
Plugged heater hoses or supply and return ports at the cooling system connections.
A plugged heater core.
If proper coolant flow through the cooling system is verified, and heater outlet air temperature is still low, a mechanical problem may exist.
Possible locations or causes of insufficient heat:
An obstructed cowl air intake.
Obstructed heater system outlets.
A faulty, obstructed or improperly installed blend door.
A faulty blower system.
A faulty A/C heater control.
Also, want to note that you need to pull the HVAC case completely to do the job right. I've done several heater cores for locals here and it seem fairly typical the evaporator is partially plugged up with dirt, dust, grass, pine needles, leaves, hay, etc.
Heater Core Diagnostics
Are you fighting to get heat from your heater in the winter time. Barely even luke warm? Well I'm going to guide you on how to resolve this issue.
Heater Core Performance
I hear it all the time, "If both hoses to the heater core are hot then the heater core is working good." This is totally wrong. Heater core is just minature radiator. If you check your main radiator it should be hot on the upper hose which is return from the engine and cold on the lower hose which is the return to the engine. Same this goes for heater cores. The hose from the head is the supply hot lead. Then the other hose hook to the steel tube is the return. So with just your bare hand if you grab the hot side hose it should be pretty darn hot and you shouldn't be able to hang on to that hose at full 190-195*F coolant temperature. So remember this could burn you be careful. As for the cold side hose it will be warm yet but much cooler than the supply hose it should be about 110-120*F roughly if your heater core is working well. Because your transfering the heat into the cab. If not you'll most likely end up replacing the heater core because its plugged up and both hoses are hot.
If the heater core is working properly you should be capable of reaching 150-160*F vent temperatures on any fan setting.
You might have to check the blend door operation. Typically with the ignition key in the ON position and the engine OFF you should be able to turn the temperature knob from cold to hot and see the stem of the blend door move. If your not seeing a good control of the cold to hot you might have to pull the stepper motor and replace the blend door stem. This is supplied from Heater Treater. This stepper motor is on the passenger side of the dash right along the transmission hump you can miss it.
If you got green coolant and its been over 50k miles you should consider a cooling system flush. If your using yellow or orange coolants they are rated for 100k miles typically. But I suggest you consider using coolant test strips to test the pH level of the coolant if your going for long change periods. I normal would do a full system flush with fresh water to rid the system of any corrosive coolant and any debris.
I see it quite often where people post on forum where there engine temperture rarely even gets close to 190*F. This is a sign of a failed thermostat. Stock all 1998.5 to 2002 Dodge Ram with Cummins should have 190*F thermostat. 180*F thermostat is not recommended. If your having problem getting to 190*F or holding 190*F coolant temperatures then the thermostat has failed. A good thermostat will typically float right around 193-197*F in normal operation. So this should be dispalyed on your gauge as being right at or just right of the 190*F mark. You can get a replacement thermostat at your local NAPA store.