How long after starting without radiator cap before coolant comes out?

Okay, odd, and arguably stupid question, but has anyone started a cold Jeep (with a fully filled open style cooling system) with an open radiator cap? How long before the coolant rises and starts dribbling out the filler neck?

Not very long unless it is very cold outside, assuming the radiator is full.
Why?

lazyxj said:

Not very long unless it is very cold outside, assuming the radiator is full.
Why?

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Thanks, that helps .. Yesterday WOT ~60mph on interstate entrance suddenly got a hissing noise.
5mi drive home, no smoking, sprays, smells, overheating .. still hissing so popped hood to try to find it.
Nothing to see, couldn't locate by ear other than seemed to be top front area of engine. Shut off Jeep and shortly thereafter, coolant flows *into* overflow bottle .. almost all the way to the top.
Head gasket seemed possible explanation, so I went out today to check and see if coolant would spray out radiator fill tube. It didn't move for half a minute or a minute and then rose slowly and started drooling over the side.
Radiator cap is obviously broken (into pieces), so that's the obvious next step, but I'm not sure bad cap would account for noise/pressurization so trying to get more data to figure out if after cap I should do a combustion gas test, compression test, change the water pump, or just replace cap and keep an eye on coolant/oil/temp levels

A faulty radiator cap most certainly will cause radiator boil over as evidenced by hissing of steam, and leaking coolant filling the coolant overflow bottle.

Tim_MN said:

A faulty radiator cap most certainly will cause radiator boil over as evidenced by hissing of steam, and leaking coolant filling the coolant overflow bottle.

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AGREE ....Test your cap, or just get a new one, as yours is very likely the problem. infact buy two, keep one as a handy spare for them remote long offroad trips.

Pay attention to the correct cap pressure rating, dont get the wrong pressure. I dont know what yours should be rated as I deal with the Renix sealed type system in my 1990 XJ, with its own funky plastic pressure tank and special cap.