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 I'll post this here but I believe this could all lead to the thermostat. 

 Anyway, not long ago I replaced the heater core and evap. Removed hvac, cleaned, inspected the foam seals and reinstalled.

 Now that it's getting cooler outside I'm noticing that it's not putting out the expected heat. I read the heater output write up in the articles section and checked for the movement of the blend door motor, checked to see if the heater hoses were hot indicating flow of coolant, blower motor is functioning as it should. All good.

 I have noticed though that when I come to a stop light or if I'm sitting still much more than about 30-45 seconds the engine begins to cool down per that gauge. Like from just above 190° to below 190° (again, on the factory gauge). Temp output at the floor vent is good while fan is on low but cools down considerably when moved to 3 or on high. 

 I fear that when I really does get cold around here I will not have enough heat output to warm the cab.

 When the outside temp was in the 60's the heat output seemed to be in spec with the article. I'm thinking thermostat, any other things I should check out? 

 The first two pics are going down the road, engine at temp, outside temp was about 45° I think.

IMG_20201027_171942507.jpg

IMG_20201027_171929554.jpg

Next pic is sitting at a red light, notice the drop in engine temp.

 

IMG_20201027_172449971.jpg

I've been google searching this issue, I've read that it might be possible to have air trapped in the heater core. Anyone heard of this? Is there a certain way to bleed it out?

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Mine has done this to varying degrees ever since I bought it. I spent some time in Indiana the first winter of ownership in 10* weather. The only way to warm it up was drive to work on the interstate. Across town at every light you could watch the temp drop as doon as you stopped. I had to shove cardboard between the intercooler and radiator to help. Alot of us have winter fronts that block the grill in cold winter months. I still use cardboard at times if its only briefly cold.

You could have some air trapped I suppose but the coolong system in thes truck is very good.

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 I agree the cooling systems are very good in these trucks. There is alot of heat from the Cummins to keep under control. I just figured I would get better heat from the heater with all the heat generated by the engine.

 It hasn't been cold enough ( at least I didn't think so) to have to put cardboard in somewhere yet.

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1 hour ago, Doubletrouble said:

 I agree the cooling systems are very good in these trucks. There is alot of heat from the Cummins to keep under control. I just figured I would get better heat from the heater with all the heat generated by the engine.

 It hasn't been cold enough ( at least I didn't think so) to have to put cardboard in somewhere yet.

I re read your post and I guess i forgot your heater out put. Seeing temp drop in the engine at a stop light sitting for a bit is not unusual but it should come back fairly quick. In that 10* weather mine would get over 140 and loose alot of that at any red light l came to when l went thru town. Still a bit warm for cardboard or fronts.

 My heat output on the heater seemed to diminish some after l did my heater and box several back also. Figured it was something l did since the foam for redoig the hvac vents was a bit different than what was oe. The heat want run me out of the cab now but l stay nice and warm just on higher fan setting.

 Mine is stuck on ac at the moment. Figured the actuator had gone bad. Just pulled it and that was not the case. New thread soon to be posted.

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12 hours ago, Doubletrouble said:

That ain't good. I'll have to try changing the T-stat and flushing things out good first before replace a brand new heater core.

It's probably just your tstat, you can just pop the hoses off the heater core and flush it out, try both directions see if anything comes out.

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Are you anywhere near a Cat dealer. Cat do cooling system flush fluid. Bought my heater back from the dead. I have a tub at home if you need the part number. I just added to coolant, ran it for 1/2 hour then back flushed heater and engine and then fitted a 190 Stat 

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Anytime I do a heater core it always gets flushed out and new coolant. Rather dumb to put the old stuff back and still all the debris in the block to just fill the heater core up with debris.

 

Like doing a engine rebuild and putting the old oil back in. Doesn't make any sense... Coolant system should of been flushed before even installing. So now when your done you just fill with fresh coolant and water and your done.

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 What is the cooling system capacity? I think I saw somewhere 6 gallons?

 I didn't flush the while system and change that stat at the time of heater core install because I was laid off at the time, money was tight. 

 I'll get a new T-stat and the required amount of coolant and do a flush here very soon. 

 

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20 hours ago, Doubletrouble said:

What is the cooling system capacity? I think I saw somewhere 6 gallons?

 

That is correct for a true empty system but since most folks don't get the last bit of water out of the system then its about 5.0 to 5.5 gallons typically on a refill. I just dump the first gallon straight to make up for that water left from a block flush. 

20 hours ago, Doubletrouble said:

 I'll get a new T-stat and the required amount of coolant and do a flush here very soon. 

 

I know the feeling well.

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