98 chevy cavalier...water is boiling out of the overflow lid...it is plastic and that is where the radiator cap is. car runs hot, and water is gone when i get home...15 min commute. fill it up and happens again. shows signs of water all around radiator cap. replaced water pump, thermometor twice( thought it might be a defect one) flushed all hoses and connections, because there is no way to flush radiator unless u take it completely off. also run defroster, and in 15 min is blowing cold air, put water in, and it blows hot air again, hot cold hot cold like every 5 min...please any advice of what it might be? thanks

Pam are you sure it's boiling? If the Antifreeze is boiling I'd think you'd see steam coming out of your reservoir as the water expands into the reservoir, but you didn't say you see anything like that.
Of course engines get hot, that's why we have the cooling systems but it doesn't sound like it's "boiling" to me with no signs of visible steam unless your antifreeze level is really low?

If it's really boiling, and you antifreeze level is good, then your reservoir should be getting filled up with expanded hot water that goes into it to catch it when it expands.

If the water isn't going into the reservoir, then maybe your antifreeze level in the radiator is too low and you need to add antifreeze?
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Best to fill the radiator with the engine cooled down. If you ever have to open the radiator when it's hot, NEVER open it with your bare hand as you could get scolded with boiling water. Instead use a big thick towel to remove the radiator cap off a hot engine. Again, if you can, just wait until the engine cools then open the radiator cap.

With the engine cool, you can fill the radiator with a 50/50 antifreeze mix. The 50/50 mix come premixed 50 percent water, and 50 percent antifreeze. So you can throw that stuff straight into the radiator.

Non mixed antifreeze needs to be mixed 50/50. Some people use 50 percent distilled water when they mix it because distilled water doesn't have the minerals in it that may get stuck in your radiator. That's being a bit picky about it though. Tap water works fine in a pinch. The other 50 percent is antifreeze. You can mix it right in the radiator too, put in a measured amount of the antifreeze, followed by the same measured amount of water. The water pump will mix it all together when you run the engine.

Put the cap back on and run the engine and if/when the water "boils" it should go into your reservoir. If it does boil in go in the reservoir then yes...your engine is getting hot and the antifreeze is boiling.

When the antifreeze boils, it's probably because your thermostat is stuck closed. That means the thermostat isn't letting water circulate through the radiator to get cooled. Thermostats are cheap...no more than $30 I'd say for most cars. Changing them is pretty easy too as they are usually right at the top of the engine. A mechanic may charge $100 to change it....but it's an easy job on most vehicles requiring just a couple sockets/socket wrench.

So did you check the gauge and the sending unit ?
Either component could be faulty.

Well, I would say that the radiator would be next on your list, but usually the overflow is attached to the radiator, so that should be impossible to boil before radiator. How does the temperature gauge look? Even if any parts are hot to the touch that doesn't necessarily mean they are overheated. It may be that the system is still working out bubbles, hence the overflow boiling over.

Hello,

I just had a similar problem.
Replace the reservoir cap for the radiator--or if you have a radiator cap replace that.
I hope this helps.

thermostat is stuck closed need to replace it

Its normal for the car to have antifreeze going to the overflow tank if thats what your asking.

Physically look in the radiator under the cap(if one exsist's) when cool, for coolant level, and check the valve in the cap for sticking. If air is present in the radiator it can't get out, and more coolant can't get in,(from the overflow). Either clean the cap or replace it and check that it is the CORRECT cap.

If radiator is low on coolant fill it to overflowing, replace cap, and check that coolant overflow is filled to the COLD line or mark. The overflow is normally where you would add coolant. (never overfill the overflow)After refilling as outlined above, check the level every morning for 2 or 3 days until coolant level stops dropping below the cold mark and add coolant to the overflow ONLY. Do not open the radiator cap. Also, if you have even a tiny coolant leak anywhere in the system, air will get drawn in, instead of coolant from the overflow.

For more OVERHEATING PROBLEMS try these...
Radiator fins dirty, clean with a strong stream of water, not high pressure water.
Radiator clogged, try backflushing it, or replace.
Thermostat stuck open or shut, replace it.
Water pump worn out, can no longer move enough coolant, replace it.
Fan shroud broken or missing....
Electric Fan(s) not working, Check the fan, relay, fuse and engine temperature sensor's.
Belt driven fan, belt slipping, fan clutch is bad, fan blades have flattened out.
Air dam under front bumper is gone, loose, or broken. It actually has a purpose other than scraping on the driveway or curbs. It forces air up into and thru the condenser and radiator. If it's loose, airflow can actually push it out of place making it useless or blocking the airflow. Along with that, there may also be a plastic piece attached to the bottom of, and wraps up behind the bumper. If it

Two ideas.

1. Try a new radiator reservoir cap (trust me, it is very important in holding pressure which keeps the coolant from boiling out).

2. This one you will need an entire weekend to spend on this. You need to remove the thermostat and replace the thermostat housing. pull one of the radiator hoses and attach a garden hose with good pressure. It needs to flush for at least 20 minutes. Then switch directions and flush water where was coming out. Then put it back together and drive until it gets hot. Allow it to cool for 45mins (to prevent engine from cracking due to rapid cooling) and repeat until no crud comes out when flushing.

bad thermostat and coolant maybe low. when a engine gets to hot at certain boiling point it evaporates

Bad thermostat, or cracked head. If it has been run hot, most likely the head is bad.