It might be complicated but I'll try to keep it as short as possible. My car is a 2009 Pontiac Montana with 210k km (131 k Miles).

I use my car as a working van for delivery and it serves well until 2 weeks ago. It was a really hot afternoon and I saw some white steam coming out from the hood. I opened the hood and found coolant was spraying. I drove the car to a repair shop 2 minutes away and they diagnosed my radiator was leaking. I repaired my car with a new radiator, and it served for 3-4 days without any issues.

On another hot afternoon, while I was driving, I heard some weird boiling noise from the hood. I pulled over, opened the hood, and found the coolant in the coolant tank was "boiling". The pressure was so high that even pushed the coolant out of the tank. I parked on the side of the road for a couple of hours then it returned to normal. On the next day, at about the same time, hot afternoon, the coolant in the tank was bubbling again.

The next day, I took it to a retired mechanic's home and ask him to diagnose it, he said it could be caused by a blown gasket. He checked the fan and some other parts which I can't tell and he said they are all good. He did the gasket check with the blue liquid test kit, but the color did not change which shows there was no leaking in the head gasket. He said it may not be accurate and he feel it was due to a blown head gasket anyway. He put the head gasket sealer into the coolant and ask me to wait and see how it works. While the car was idling, we saw bubbles coming up from the bottom of the coolant tank constantly, about one small bubble per second, he said that was the sign of a bad gasket.

2 days later, while I was driving on another hot afternoon, it came up again. I went to another mechanic and asked for help, and he asked me a couple of questions, during the whole process, the car never leaked any liquid under, never had white smoke from the exhaust pipe, and never go overheated. The ac does not blow cold air due to no freon but blows hot air without any issue. He suggested changing to a new radiator cap and I did. Today, I drove the car for a whole day and the car survived even the hot afternoon. I almost believed it was just a simple problem of a leaking radiator cap. I heard the bubbling sound at 6 pm again. I opened a hood, the bubbles were back in the tank again. After about 5-10 minutes, the bubbles do not pop anymore and I drive it back. I checked the coolant tank a couple of times today and the constant bubbles never showed up in the tank, until the boiling bubbles at 6 pm.

I really have no idea what's going on. Really need the car as a working tool and can't stand the car dropping me on the side of the road again and again and again. What could be the problem and what should I do? Should I change to another radiator cap and give it another try?

Thanks a lot.