Gasoline overflows when filling tank

There's a common misconception that if part of a car's cooling system fails, the failing part can be replaced, the system closed up, fluid topped off, and the car will be ready to go. Many people have overheating problems, replace the offending component (thermostat, radiator, etc), top off the fluid, and then wonder why they still overheat.

This is because when the cooling system (which operates as a sealed system) is opened up and new components are installed, air bubbles become trapped in the system when it's reassembled. Coolant is added, but the bubbles displace some of the system's volume and become trapped in the cooling system.

The way to alleviate the problem is to burp the cooling system. It's easy to do, and only takes half an hour to an hour. It can be done at home very easily.

The first step is to reassemble the system after you replace whatever components are failing. Tighten all clamps, connect all hoses, and then fill the radiator or coolant holding tank, and fill the overflow reservoir to the indicated level (there's a small hose that typically runs from the radiator flange where the cap is positioned, over to the overflow container). Find the thermostat (trace the lower radiator hose back to the engine from the radiator - where it attaches to the engine is either exactly where, or very near, the location of the thermostat). Jack up the car so that the thermostat is pointed upward (the hose would be attaching at a downward angle). Now start the car.

You jack it up in this way so that the thermostat points upward. The thermostat will open downward in this position. Watch your temperature gauge as it rises to, and then beyond, the normal operating temperature. If it is rising very slowly, you can rev the engine, or hold it at 2000 RPM or so, to help build the heat. Eventually the engine will begin to heat up beyond normal and the gauge will climb. This is what you want. Allow it to climb to somewhere between 2/3 and 3/4 of the way to a full overheat, and then shut the engine off. Allow it to cool, and then CAREFULLY open the radiator cap. You'll hear a purge of pressure, and will probably see bubbling in the overflow container. Check the level of the coolant in the overflow and the radiator, top them off as needed, and repeat this procedure. Keep doing so until the car no longer overheats. Now, take it for a drive around the block a few times, and see if it overheats then (sometimes putting the engine under load will cause it to overheat even when it won't while sitting in the driveway). If it does not overheat, you are done. If it does, pull over, turn off the engine, and turn on the heat full blast (this will extract heat from the engine). Get the car home and burp it again.

Why are you doing this? Here's why. Those air bubbles in the system that I mentioned are the root of your evil. When you start the engine, the water pump spins and circulates the coolant (and air bubbles) throughout the engine. At some point, those bubbles come to the thermostat, which stays closed until the car gets to a certain temperature, at which point it opens and allows the coolant to go to the radiator to cool off. When the air bubbles get to the closed thermostat, they get stuck. In turn, having the bubbles pinned against the back side of the thermostat keeps it from opening since the system is pressurized and the thermostat can't open against the pressure of the bubbles. This is why the car begins to overheat. By waiting until you are most of the way to a full overheat, you get as many bubbles stuck there as possible.

Once you allow the car to cool enough that the coolant won't explode out of the radiator when you open the cap, you can open it. This relieves the pressure in the cooling system and allows the thermostat to open. The bubbles travel through the thermostat and hose to the radiator, burble their way to the top, and "burp" out of the cap's opening. With the bubbles out, the coolant level drops some (which is why the coolant as to be topped off), and you repeat the process since the coolant doesn't always follow the same pathway. You want to be sure that all the bubbles are removed from the system, so you do this a few times.

Hopefully this will help you with overheating problems and with diagnosing future issues. I know this is listed under Chevy cars, but that is only because I had to select something, and those are commonly owned cars. This process is important on ANY car, regardless of manufacturer or engine.