Still blowing cold air after heater core replacement...what gives?

Replaced the core, buttoned everything back up, added coolant and bled the system, still no heat. Its moving the blend door, i can cycle through the vent positions and the air changed each time.

Any suggestions? Really dont want to pull the dash again if i can avoid it


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How new is your thermostat?

August 2021..less than 5k miles. I rebuilt the entire system and upgraded to a 3 row rad and high flow pump, along with an HD fan clutch


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What year is it? Do you have a control valve?

High flow pump has been known to cause problems, may be moving the coolant too fast. Also when it's super cold I have to cardboard my grill to get any meaningful heat.

99 4.0 aw4

I kinda thought about cardboard over half the rad. I beefed up the system for towing ane maybe went overkill for daily driving in the winter

I dont get any warm air, even if i rev at idle. Part of me suspects blend door related


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Just because air changes what vents it comes through doesn't mean the mixer door is working. 99 has the electric one that is prone to failing...
No air in system, operating temp and no heat isalmost always this issue

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I had the blend door motor off for replacement while it was out...the SKP one rock auto sells is junk and the gear is 180* off, plus soft plastic went snap.

Going to try bleeding again. Heater hosese get hot to touch, so i know theres flow going thru the core


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There is no way that coolant is not going through a new heater core. When the tstat is shut, the heater core circuit is the only means for the coolant system to circulate coolant, and the pressure built by the water pump will easily push out any air in the heater core and associated lines. I've tried it with clear vinyl tubing. Its impressive to watch how fast the coolant moves through that circuit, even when the tstat is removed.

If your motor is getting up to operating temperature, the only possible explanation is that the blend door is not working.

I find it hard to believe your cooling system is built so overkill you can't get heat. I am running the Champion BC1193 radiator. There are not many radiators any more heavy duty than this one. Ron Davis is the only one I can think of. I have no problem getting heat when the temps are below freezing.

Air pocket trapped in the system. Took about 30 miles to work it out after i burped the system.

Went out the morning after thanksgiving and turned the heat on, on a whim, and it was blowing warm. Today shes cranking out the heat really well, although it does take a bit longer to get up to temp than i would expect. It works at least *

Fwiw, im using the blackhorse racing 3 row aluminum rad from Ebay with the GMB hi flow pump from Rockauto, Hayden upgraded heavy duty fan clutch (ZJ i think) and 195* superstat. G Power silicone rad hoses and custom cut silicone heater hoses. ultrapower heater core and GPD evap coil from Rockauto. Running Zerex g-05 coolant after full descaling and flush with CLR and distilled water


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I may have to put a 205* tstat in for winter if the cardboard doesnt work


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