I had the same issue last year and they replaced my heater core. Here we are one year later and the same exact thing is happening. I only get heat when I accelerate. Any additional thoughts ?

 

Tale tell sign of a bubble in the coolant system. Heat up the truck and see if the upper radiator hose is hot. If not, there is a bubble and it needs purged.

 

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Low coolant....real question is where did it go? Had this happen on my GM 5.3. Luckily for me it was leaking out onto the exhaust manifold & evaporating rather than going into the engine. Never dripped a drop on the ground.

 

Same here…it was low coolant, but no sign of a leak. This was in late November early December..topped off the coolant and been good since. Did the passenger side blend door last winter.

 

I am also having this issue. Coolant is full, upper radiator hose is warm, recently replaced both the water pump and thermostat. Any ideas on this situation?

 

s...Also...

Consider putting your complete vehicle information (year, model, cab, trim, engine, wheelbase, _x4 drive, mileage, maintenance, modifications, etc., etc.) in your signature to have it automagically included when you post. *Eliminates playing 20 questions when/if a post is made pertaining to your vehicle.
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Check your y pipe n coolant