Heater blowing cold

[Jbiggs1102]

Ok I have a 2006 6.0 lariat got the auto hvac system. Heat is blowing cold after it is warmed up and the. When i am going down the road it heats up but as soon as I sit still it goes right back cold any ideas where to start.

[amdriven2liv]

Check coolant level.

Turn it to "air conditioning", does it only blow out the defrost?

Might check for vacuum leaks.

[Term3]

When your checking the coolant level check for any evidence of coolant around the recovery bottle cap, wetness, white deposits. If there is you have a bad head gasket and its filling up the block heater with gases since that's a high point and once you are up in RPMs the water pump has enough force to push those gases out of the heater core and then give you good heat inside. You can also verify this with the coolant test strips for exhaust gasses from your local parts store. Sorry in advance for the bad news.

[Fordtrucktexan]

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Mine's doing something similar, I had the heater on while flushing coolant last week and it felt great, got blistering hot, but the outside ambient temp was in the 80's, so not exactly a major feat. This morning (about 40 degrees F outside ambient temp) I started the truck and let it get up to 134 degrees according to my CTS. I could barely feel any warmth in the air coming out of the vents. I noticed when I revved up on the engine, it instantly got hotter, but as soon as it returned to idle, the air got cooler again.

I was thinking either I haven't got all the air out of my system yet from the flush, or I hadn't let my truck run long enough to heat up all of the coolant. No evidence of deposits from the degas cap.

[Term3]

Take the hose off that is coming out of the middle of the block and stick a small screw driver in that fitting to make sure it's clear as its not that big of an opening. Then make sure there's no restriction in that hose going up to the degas bottle. All the air should be out of the system after a week of driving unless its barely getting up to temperature each time.

[Adamv7010]

Originally Posted by Jbiggs1102

Ok I have a 2006 6.0 lariat got the auto hvac system. Heat is blowing cold after it is warmed up and the. When i am going down the road it heats up but as soon as I sit still it goes right back cold any ideas where to start.

Recently learned this can be caused by head gaskets

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[Jbiggs1102]

Turns out it was a little low on coolant. Added about a gallon and she is blowing good now thanks for the help guys.

[amdriven2liv]

Now the next step, why was it a gallon low?

[Adamv7010]

Originally Posted by Jbiggs1102

Turns out it was a little low on coolant. Added about a gallon and she is blowing good now thanks for the help guys.

You havent fixed anything...you just patched it until next time

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[turbocharged]

Once my truck is warmed up to op temp, the heat blows HOT, but when I come to a stop light it just blows less hot. hotter than warm, but cooler than when revved. I would think that's normal in my case. Just keep her full of coolant, and try to find out where the coolant is going.