87 camaro 305 motor, if i leave the car idle for like 5-10 minutes to warm up then the coolant will come out of the overflow container....but i could be driving it for 45 minutes and even hot rodding it and this wont happen....then turn it off for 10 mintues.....let it idle and it will do it again...turn it off for 10 minutes and drive it around it wont do it. switched the thermostat today..still does it...what elese?

kinda hard to understand what i said..basically coolant comes out of the overflow when it is in idle for awhile and not when im driving

Are you saying that the overflow container is filling up and over flowing on to the ground? My first guess would be a bad radiator cap. Also, did you notice there are two full marks on the overflow container? There should be a cold and a hot mark. When the engine is hot, is the upper hose (or whatever hose you can reach) get hard? Should have about 13 lbs of pressure in the system if the cap is doing it's job. Does it suck coolant back into the radiator from the overflow as it cools down?

well the coolant just goes into the overflow...bubbles around like its boiling...and then comes out the vent in the overflow cap for like 5 minutes once i have turned the car off...and smokes like crazy...top hose feels likes its getting hard yes...u mean the one going to the t-stat right? the level goes down when i turn the car off yes

Yes, Dave is asking about the tightness of the top radiator hose. If a good closed system and no leaks, the top radiator hose will get taut. This is due to the radiator cap, a pressure relief valve. The good thing you mentioned is after cooldown, the reservior has a lower fluid level, this is good.

However, you suggest the engine still boils over into the reservior after shutdown. This suggests heat soak of the engine, no moving coolant, it sets and bakes, so to speak. One cure for this boiling over is to increase the radiator cap relief pressure by replacing the cap with a higher rating, from 13 PSI to 16-18 PSI. The more pressure generated during operation, the less of boiling after shutdown.

There may be some problems you might want to check out:
1. Do the fan(s) come on? Drvr side fan is controlled by the coolant switch, either in the radiator tank or engine block. The pass fan is controlled by selecting A/C or DEFrost, both fans come on.
2. Is the bottom spoiler intact under the radiator? Alot of times, going over speed bumps and the like, these air dams get damaged.
3. After engine shutdown, are there any cool spots on the radiator? Feel with the hand or shoot the rad core with a temp gun. If bottom few rows of the radiator are cool, radiator may need replacing. Have you cleaned the outside front of the radiator/A/C condensor? Soak with 409 or Simple Green and rinse off.
4. Water pump belt tight and not slipping? More than 4 years old?
5. Same with hoses & coolant, more than 4 years old? All might need changing.
6. With overflow boiling, could be a leaky head gasket. Remove the spark plugs and find the cleanest plug, this is the leaky cylinder. If a head gasket is leaking combustion pressure into the coolant system, bubbles will appear in the coolant/reservior tank. The bigger the leak, the more violent the release of bubbles. This step may not be your case, but something to consider.

1.yes the fan comes on, theres only one of them.
2.spoiler in front is fine
3.havent checked that but i will
4.belt is fine, water pump is no more than 2 and a half years old max
5.changed all the hoses when i changed water pump
6.well i have had the cap off and i can see bubbles in there...small bubbles..thers is milky oil on the oil cap but i think its due to condensation because the oil itself looks fine.

if the cap doesn't work looks like its probably the head gasket... in 3 days i'm not going to be driving it for about 6 months until i put my new motor in anyways...just gunna scrap that 305 i was just interested so i knew what was going on. thanks alot guys!

Milky oil is concerning. I'd crack the drain plug until it drips and see if water or oil comes out.

found out the problem..radiator fan not working...could have sworn it was the other day. would explain why its only happening in idle. thanks for all the responses

Stranger things have happened. Let us know how the repair turns out.