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Coolant System Malfunction

2002 GMC Sonoma 4.3L V6. 70,000 miles. When the engine heats up the coolant expands and raises the level in the overflow reservior as it should. Upon cool down the reservior level stays elevated and the radiator is low on coolant. No leakage. I have changed the radiator cap and had the coolant system power flushed by the GMC dealer. Problem is constant, not come and go. Air in the system sounds like a waterfall under the dash until the engine reaches operating temperature. No coolant leaking under the dash, in the cab or anywhere else that I can find. Any ideas?

Classic symptoms of a blown head gasket…Combustion gasses are blowing the coolant out of the radiator…They do not condense when they cool…

To get the air out of the cooling system on my 2000 S10 Blazer (4.3) I drive it up a set of ramps to get the radiator cap as high as possible.

Before starting the engine I remove the radiator cap. As the engine warms up the air should start working its way out of the system and the coolant level in the radiator should drop. I keep the coolant level topped off until no more air comes out.

Ed B.

Thanks for the info. I’m going to get it checked out.

Thanks for the suggestion. I did try that and thought I removed all the air from the system. Next time the engine heated and cooled the air was back.

I would bet on leaking intake gaskets. Very common on this engine.

It could be as simple as a bad hose or clamp. A pressure test on the system would be my first step.

Check the hose connected to the reservoir and the radiator filler neck. Make sure there are no cracks or air leaks. I had to put a small hose clamp on mine due to it being loose at the radiator filler neck. Also, replace the radiator cap. Cheap insurance.

Did you look at the cap of the overflow reservoir to see if the hose is attached to the underside that goes down into the coolant and that it is on the right port and is not cracked around the port?

If your tank is one of those that has the “hose” molded into the side, it could have a hairline crack in it.

For the coolant recovery system to work, the radiator has to suck the coolant back in, so the “straw” can’t have any holes in it and it must be submerged in the liquid in the reservoir.

You don’t mention an almost immediate indication of overheating (or any level of overheating for that matter).When exhaust gas contacts the coolant this coolant gets hot fast.

If I read this right you are pretty much asking “why is the coolant not drawn back into the radiator upon cooldown” What comes to mind is (as mentioned) a leak or crack in the hose between the rediator and the recovery bottle.

I will go over the hoses and caps again tomorrow morning. Your saying a very small crack in a hose can break the suction and draw air back into the rad rather than coolant.
Thanks. gives me something to look for.

I checked and the cap on my reservior is just a cap. no hose connection. nothing laying in the reservior that would indicate anything came loose. I’ll check my hoses and connections again.

I found a the plastic hose clamp at the rad fill neck was loose. I could spin the hose around the spout quite easily. I replaced it with a metal hose clamp and will test drive tomorrow. I changed the rad cap prior.
Thank you very much.

Any leak in the cooling system will let air into the system and the coolant recovery won’t work. The waterfall sound under the dash is a classic symptom of air in the cooling system. It could ne that it was never filled right, or you could have a leak anyplace in the cooling system, or you could have a bad head gasket.
First step is to make sure you have filled the radiator and gotten all the air out. If the recovery system still won’t work have the rad. pressure tested to find a leak and have them check for exhaust contamination in the coolant.

If you have a traditional type of radiator and overflow tank, the overflow hose should go from just under the radiator cap to the overflow cap. If so, then you have to have a hose that goes from the bottom of the cap to the coolant in the tank.

Sometimes the hose goes to a separate port on top of the tank and then to the coolant through another tube or a molded tube that goes down the side of the tank. A third option is that the hose goes to a port at the bottom of the overflow tank.

The type of system in my 02 Saturn does not have a radiator cap. The overflow tank actually serves as a top tank for the radiator. The cap of the overflow tank serves as the radiator cap too. If you have this type system, then there must be some gelling of the Dexcool in your system that the flush didn’t get out.