hey folks, I got 2007 hummer H3 around 135k miles. recently all the sudden I been having bubbles coming from the coolant reservoir and overheating like 220+. and when I drive on it, the coolant in the radiator is low but the reservoir still full. I did a chemical block test for blown head gasket and it came negative, no leak visible anywhere outside, no white smoke from exhaust and no mix coolant with oil. I did a pressure test and it's not holding pressure when I go 15PSI, it immediately start to go down slowly while bubbling coming from the coolant reservoir. I changed the thermostat thinking it's maybe not opening but still the same problem.. anyone knows what's going on? is it a bad head gasket???

thank you

 

Check your transmission level - transmission cooler might be leaking internally? I don't know where the bubbles would be coming from though...

 

i checked that and checked the engine oil but when I use the pressure tester as soon as I put air in there or pressure it start to bubble from the reservoir

 

Do you smell antifreeze? Sounds like a pinhole leak in your radiator to me.

 

no I don’t smell anything . And I don’t see any leak anywhere. The radiator is like less than a year old but you never know maybe it got a hole somewhere???
But if there is a hole wouldn’t it leak coolant?? Because I don’t see any leak! One day I was driving and all the sudden it start to get hot luckily I was close to home so I popped the hood and looked everywhere and I didn’t see anything but noticed air coming from the coolant reservoir .. I don’t know if it’s a head gasket since the chemical test came negative but the pressure test as soon as I pump 15 psi it immediately start to leak air and pressure into the coolant reservoir and bubbles and the pressure start to drop slowly

 

The only route from the radiator into the coolant reservoir is through the thermostat.

 

and I replaced that with brand new one and it still does it . Maybe there is a crack into gasket that the air passes theo to the coolant

 

Radiator Cap!

 

How a Weak Radiator Cap Causes Overheating

• Loss of Pressure:

A radiator cap's job is to seal the cooling system and maintain pressure, which significantly raises the boiling point of the coolant. A weak or damaged cap can't hold this pressure.

• Coolant Boils Prematurely:
• Without the proper pressure, the coolant in the engine can reach its boiling point sooner than it should, even at normal operating temperatures.
• Coolant Loss:
• As the coolant boils, it expands and can be forced out of the system through the overflow tank.
• Reduced Heat Transfer:
• When the coolant boils, it becomes less effective at absorbing and transferring heat from the engine.
• Overheating:
• The loss of coolant and the reduced efficiency of the remaining coolant lead to rising engine temperatures, causing overheating.
  
  Symptoms of a Failing Radiator Cap
  • Overheating Engine: The most obvious sign, with the temperature gauge frequently reading high.
• Coolant Leaks: Puddles under the vehicle or a low coolant level can indicate a leaky cap.
• Overflowing Coolant Reservoir: Coolant spilling into the overflow tank due to a failing cap.
• Collapsed Hoses: A faulty cap might create a vacuum, causing radiator hoses to collapse or flatten.
• Steam from the Engine: Escaping steam or a sweet smell often signals coolant escaping the system due to a bad cap.

What to Do

• Inspect the Cap: Check the cap's seal for wear, damage, or corrosion.

• Replace the Cap: If it's old, worn, or shows signs of failure, replace it with a new cap that matches your vehicle's specifications. Replacing a radiator cap is a relatively inexpensive and often effective fix for overheating issues.

 

That was the first thing I replaced I got a oem radiator cap and still does it