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Air in cooling system

When I first start my '97 Z71 and begin to pull out of the driveway, I hear a girgle coming from the heater core. It goes away as I drive down the road, but every time I start it cold, the sound comes back. My coolant overflow tank is filled to the cold mark when the engine is cold and fills to the hot mark when the engine it hot (like it should). How do I “burp” the air out of the system?

Some vehicles actually have a bleeder screw to do this-- consult a service manual to see if your truck has this. On most vehicles, though, the radiator cap is the high point in the cooling system so what you can do is, with the engine cold, pop the radiator cap off and let the motor idle for a little bit. Keep an eye on it-- it will probably overflow eventually, and that’s when you need to put the cap back on. Parking the truck pointing a little uphill will help too. Also, now might be a good time to replace the radiator cap-- a bad rad cap can cause weird little problems like this.

for about 10 bucks, the new radiator cap is the first place i’d start.

I had a pin-hole in my heater core and had the same symptom for a month or two. It didn’t leak enough to smell, or see coolant, but it was enough to let air back into the core every night. After a couple of months, the leak got much bigger and became obvious. Up till then, it did not go down enough to have to add any coolant.