Hello All,


I'm new here on the forum. I have a 2004 Silverado 1500 4x4 with a 4.8 V8. I live in Maine. It seems that when I start my truck in the morning, with blower on high and temp selector on hot, even after The engine is up to 200 degrees, I get nothing but cold air out the vents. It's hard to defrost your windshield this way. As soon as I rev it up to 2000 rpm, heat starts coming out like crazy. like in 5 seconds. When I let it idle again, it slowly cools to cold again. This is repeatable. I did a cooling system flush back in August. The coolant level in the reservoir is where it should be. Any ideas?

 

Sounds to me that you are not getting the coolant to flow through the heater core. When you rev it up, you get some flow. I think the heater core is mostly plugged

With the engine warmed up, feel the heater hoses going into and from the heater core. If one is hot and one cold, you are not getting flow. If both are hot, you are

 

Could be a partly plugged heater core.
Or a worn and tiered water pump
Or (and this is what I think it is) air lock in your cooling system.
Your heater core is probably the highest point in the cooling system.

 

I agree with all of the above. Probably a plugged up heater core, or the most likely cause is air stuck in your cooling system.

If it were me, I'd let the truck get warm, and reach under the hood and feel the heater hoses. If one feels hot, and the other not as hot, or just warm, you most likely have poor flow through the heater core when the truck is idling and the water pump isn't pushing quite so hard. If one is hot or less than hot, and the other is cold, then I would try and bleed the air out of the heater core.

I'd open up the cooling system while cold, install a spill free funnel and fill it half way up with new coolant. Then start the truck and let it run till the thermostat opens up and the cooling system is fully warmed up. At that point, rev the engine to 3K RPM and hold for maybe 10 seconds and repeat maybe 30 times. Usually you can see the coolant in the funnel being sucked into the engine as the air that used to be int here is replaced with coolant. You'll also notice that your heat starts to work. It's best to have your heat on full blast while you're doing this.

Good luck!