2018 f150 5L, no flow to heater core at idle.

[lmorris]

Earlier this year this had the heater core replaced and cooling system flushed for the obvious reason. A few months later the truck came back with a lack of heat complaint. Technician couldn't verify the lack of heat but did find that the water pump was leaking. The water pump was replaced, cooling system bled and truck returned. Comes back now with lack of heat control. HVAC tech found and replaced a faulty temp. door motor. On the road test he noticed the heat drops off at idle. That's when I get it. Initial testing led to plugged heater core. Hotline confirmed and instructed us to replace the heater core again and flush the system, which I did. Still has issue. Digging further I found that I have not enough flow to the heater core at idle, off idle is fine. I ran separate heater hoses to verify it was not a restriction in the existing hoses. Pulled the water pump for inspection thinking the impeller was loose, water pump is good. Put it all back together and bled the system again per WSM. Not fixed. Using a infrared temp. gun on the block revealed heat inconsistencies on right head at idle. Hotline instructed to remove heads and inspect for restrictions in the coolant passages of the block and heads. Nothing was found.

Anyone have any insight as to what could be going on?

[Steve Mutter]

I have not heard of anyone in our shop coming across this problem, did you end up finding anything ?

[lmorris]

Only thing notable found was the bleed holes in the head gasket looked burnt. Right bank only. Left side looked fine. Replaced the right cylinder head and both head gaskets. All good after that. What ever the cause was remains a mystery.