After I parked my car I kept hearing this bubbling noise. Popped my hood, and looked in my coolant reservoir and it was bubbling. Its not boiling or anything like that, it’s more like air is going into the reservoir. Before I drove it today, I added some coolant into the reservoir but I don't think that has anything to do with it. I searched and didn't find a whole lot of info except that air could be getting in from a blown headgasket? Any ideas on what I should do, or possible causes? EDIT: I remember the shop told me that I had one loose head bolt on the passenger side head, and that a smidge of coolant would sep. No puddles under the car, but the coolant reservoir is usually empty. I was going to get to it when I installed some headers(wont be for a while), but I never thought of it till now. So this could probably be my problem, but any other ideas?

apart from the obvious, replace headgasket, I would check to make sure your radiator cap isn't rusted or otherwise busted looking. Mine does this too sometimes. Though, i'm fairly sure my gaskets are crap.

Radiator cap is fairly new so it’s probably not that. Starting to lean more and more on the loose head bolt.

I was trying to remove any air pockets just now, but ended up having coolant gush out of my radiator, and all over my passenger side fender. Can someone tell me what I did wrong; I removed the radiator cap, coolant level was about 5" down. Started the car, letting it run for about 7mins. Thermostat opened, and you could see the coolant flowing now. (I think this is where I messed up) Before putting the radiator cap back on I shut the car off, and the next thing I see is coolant gushing out of the radiator cap hole. Now I think I was suppose to put the cap on first right?

The inside of the engine is hot. The water pump is not running. That hot water wants to expand somewhere. Somewhere being out of your open radiator instead of under pressure and seeping into your reservoir.

its the thermostat. i had the same problem with my 87 iroc. i think when it makes that bubbling noise, the thermostat is stuck closed, so the radiator isnt circulating... or something like that. its a fairly simple job. just remove the t-stat housing and yank out the old one and get a new one. its like 5 to 8 bucks. i recomend tryin that first.

thermostat stuck closed == no water flow. This makes it hard for the hot engine to get the gas to the radiator where it has to get to in order to be released up by the radiator cap into the overflow resevoir.

I'd put my money on the head gasket if the radiator cap is good. That's almost certainly my problem as well, since my head gaskets seem to consist of hopes and dreams.

From what I read in the past, if you head gasket was the problem you would have notice bubbles in the radiator with the cap off and the engine running (followed by a slew of other issues, water in oil/oil in water, rough idle etc). Your issue with seeing bubbles in the overflow could be coming from turning the engine off, then hot water from the engine boiling back into the radiator, and any air that is trapped at the top of the radiator is forced out into the overflow tank.

Exhaust to coolant leaks maybe? that way you dont realy get a leak of oil to coolant. Not sure if that's technically even possible though.

The only reason why i put my money on the gasket (assuming a non-oil leak is even possible), was because the bubbles dont occur with any reasonable pattern. Though, now that i'm thinking about it. i dont think the bubbles occur until the car is turned off, so my second idea and yours (da.slyboy) seem more realistic. Sitting water will boil and i guess it's just pot luck if the hot spots are still hot enough when the engine is turned off.