On my 1999 Blazer ZR2 the heather works as it is supposed to but when the car is standing still with engine running on idle the air starts to blow cold after a short period
Any here knows why this happens?

Since i had to repace the water pump i also replaced the original Dex Cool Chevy anti freeze with red Valvoline Max Life coolant anti freeze and the mix is 50/50 percent
I also bleed the cooling system for air
This if the coolant could have anything to do with it which i do not think because the if i only used water the car should blow warm air with a warm engine.

 

Did you flush all the Dex cool out well including the heater core before switching? If not you now have brown sludge in the heater core plugging it instead of coolant as most anti freeze's are not compatible with Dex cool

 

Does the heater work when driving at highway speeds but then it goes cold when idling? Does the temperature control allow you to change the temperature and allow you to redirect the heat to where you want when the engine is driving at normal speeds?

If so, then the only thing I can think of is that there might be air in the heater core that when the motor is revving is able to displace that it cannot when idling.

 

I have to agree with you that this looks like air in the heather core when the car is standing still idling.

But after i had chaned that terrible Dex Cool coolant that Chevy use i had the heater on when i bled the system for air.
So i suppose the only way to fix this is to bleed the sytem for air once more

Thanks for your help

 

Be sure to report back what ended up working for you. You are not the first person to have a problem like this.

 

christine_208. I have just now bleed the coolant system again after i replaced the water pump last summer.
The thing is that i had to fill 1,5 liters of coolant (on the radiator) which i find strange since there are no leaks on the car and it was coolant in the reservoir

I also parked with the front uphill and the heat turned on max with the blower motor on the first timei filled the coolant

Anyway the car is blowing hot now when standing still just idling

But the original Dex Cool stuff that GM used on this car is really the worst coolant i have ever seen.

When i replaced the water pump the radiator lid was almost impossible to get off since that Dex Cool stuff had built up a hard lair of almost a stone like mass under the radiator cap so it was impossible to push the cap down to get it off..

I replaced the Dex Cool stuff with Valvoline Long Life red coolant wich is an okay coolant

Thanks for your help


Valvoline Max Life coolant is much better than that Dex Cool acid stuff GM used

 

Well, since Dex-Cool is the same as it has always been and I have been running it in cars since 2006 without any issues, I will say that it's not the problem. The issue was with the poor gasket design GM used back then and that the coolant system in the cars was not properly sealed, allowing air to seep in and contaminate the coolant. These issues were fixed some years back. Also, don't believe a manufacturer's nonsense about leaving coolant in the system for 5 years or more. Change it around every three years. We have been over this repeatedly, so let's not turn this thread into another 'Dex-Cool thread'.

 

I took some of the new green "compatible with everything" coolant and mixed it with the Prestone "Dexcool" "mixes with everything" coolant and could not get any precipitation even with steel and aluminum added to the mix. I'm pretty sure the dexcool now is much better than the original. A BIG also is the what rriddle3 said about the poor sealing and the bad LIM gaskets.