This is from an old post. Before performing this test, make sure to park your car on a slope where the front of the car is higher than the rear, and let your car idle for 10-15 minutes. This way all the bubbles rise toward the radiator, and leave the engine block. Then drive the car home and let it sit for a long while and get completely cool.
The other option is that it may be a bad head gasket, or small fissure in the block/head.
1) To check this you can let your car sit until the engine is completely cold.
2) Open the radiator cap and make sure the coolant/water level is toped off in the radiator.
3) Making sure the motor and coolant are completely cold, start the motor.
3) Then watch the radiator's cap.
4) If the water/coolant gushes/bubbles out non-stop (not dribbles over the stem/neck that the radiator cap secures to) at idle or if you rev the engine to around 1k -2k rpms and hold it for ~10-15 seconds(via throttle body is easiest) then there is a good chance the head gasket is bad or the block/head has a crack in it.
I just used this product (bar's block seal) for my overheating engine and it worked. Follow the instructions exactly. I almost did follow the instructions exactly, except at the last step, I did not remove the product. I drove with the product in the engine/radiator for 3-5 days....actually truth be told, I need to check because I may have forgotten to remove it. If so, it has been almost 2 months.
It is the Bar Leaks - head gasket repair block seal (part no. 1100) in the grey bottle. It has a grey outline of a 3 cylinder head gasket in the background, and it costs ~$12. It is far cheaper than the $750 head gasket parts and labor I had to spend on my last 2jzge (only to have it fail again in less than a year).
check the reviews and stories on amazon.com.. there are nothing but good things to say.
Bar's Leaks 1100 Head Gasket Repair - 20 oz.
after I finished with the product, I did a gradual 15 min. triple digit speed test on the motor in the early-early morning. I had to make sure that the engine was reliable for the week to follow. Everything went excellent. the engine runs slightly hotter (need to check if I removed the product from cooling system.), but that is not an issue at all.
I hope this helps.