No heat idling, only while driving????

[lowreese26]

99 GMC Yukon 4x4
Flushed my cooling system two months ago and changed the heater control valve and flushed the heater core. Then my radiator busted the tank, so it was retanked. Now the heat is cold while idling. Driving is when it gets warm. Get to a stop light and it goes cold again. Thanks

 

[SAATR]

Sounds like s lack of coolant circulation at low engine speeds. Water pump impeller may be shot.

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[jkeaton]

What's the level like? May have air in the system?

 

[lowreese26]

The water pump, radiator, hoses, thermostat and heater control valve are all less than a year old. I have been fighting a blown oil cooler line. So I have had the radiator in and out a few times recently. Is there a way to bleed the air out of the system?

 

[sewlow]

Point the front of the truck up so that the rad cap is higher than the rest of the engine. Ramps or a steep driveway works.
Put the cap on so that it is on the first catch. Let the truck run until it gets up to temp. You can bring the revs up slowly so that the pump circulates the coolant. This'll help to push any air out of the system. Just make sure to bring the revs down slowly as well, otherwise, if the coolant is close to full, it'll gush out if you just drop the rpm's down to idle.
The system won't be pressurized with the cap on that first catch so the engine will not quite get up to full operating temp.
Slowly remove the cap with a rag over top, & check the radiator fluid level. The rag is because the cap will be anywhere from warm to hot, but as the cap hasn't been cranked on all the way, the system isn't pressurized so it's not gonna spew when you open it.
Add anti-freeze to top up.
You may have to do this several times in order to completely burp any air out of the system.

 

[lowreese26]

Thanks a lot guys. I will try tomorrow after work.