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Diagnose lack of heater core heat output

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  • Staff

My 01 Cummins has the worst heater output compared to the other two Dodge Cummins 4x4 pickups I had. They would roast me out of the cab and cause me to turn the temperature down.


This pickup I leave on high and it's still not hot like it should be.

I've always had a lack of heat and lack of cold AC with this rig.


I already put a brand new heater core and evaporator core in it, new NAPA 190° t-stat, new water pump, new radiator, new radiator fan clutch, new blower motor, new hvac reastat, and heater treater blend door adapter.


My EJWA says it's running at 190° at full temp while driving it.


I timed the factory blend door actuator per their instructions when I installed the heater treater adapter.


I went ahead and bought a brand new blend door actuator and will be installing it this weekend.


I'm hoping it's just a faulty blend door actuator not allowing the temp blend door to not fully go to full heat or cold positions.


Besides doing that, is there anything else I should look at for troubleshooting?


I'm going to check the temps at the heater hoses and radiator hoses again to see what they're at when it's at full temp.





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  • Owner

The heater hose going to the head is the supply and should be the same temp as coolant. The other hose going to the steel tube is return and should be 50 to 70 degrees cooler than coolant temp while the fan on high and temp set to to full heat. If they are both equally hot then the heater would be plugged up possibly from debris in the block.


Typically on my Quadzilla I range from 186 to 202 degrees. Running still a Napa thermostat. I flush coolants on a regulat schedule. Vent temperature I typically measure about 160 degrees out of the vents on full heat.

Edited by Mopar1973Man
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  • 4 weeks later...
  • Owner
On 11/14/2023 at 3:35 PM, 01_Cummins_4x4 said:

Mike, if my supply heater hose is not as hot (same temp) as the coolant, what would I look at for diagnosis of that issue?


Supply is the hose coming from the head. Pull the hose off and check to see if the fitting is plugged up. Also, it could be the heater core is plugged up and not flowing. This will all the coolant flow to slow and cool in hoses. It's only a loop from the head fitting through the heater core to the return to the steel pipe heading to the water pump. Any restriction will reduce heater performance. 99% of the time the heater core is to blame because of a lack of coolant flushes.

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