Overheating after shutdown

[FM3V6M]

My engine runs at  normal operating temp while driving but once shut down the guage goes way up and it boils over, is this a thermostat issue?  Thanks!

[Bullitt-]

I would be more suspect of the radiator cap & maybe the coolant.

[moparmaniac59]

I think a rise in the gage would be normal as the ambient temperature of the engine at shutdown wouldn't be much different than while running until it has time to cool and since the water pump isn't moving coolant through the block, a rise in the temp would be expected, but boil over is another story. I am not sure what would be the cause of that.

                                                Matt B.

[Chryco Psycho]

do you have alum heads ?
 it is typical for the coolant to jump 20* on shutdown as the heat soaks into the non circulating coolant

[Bullitt-]

I know a guy with a purple Barracuda 440 6-Pack that runs cool but after shutdown piddles a bit, tried different radiator caps...just needs an overflow tank i think.

[6packCuda]

I little dribble is normal after shut down but boiling over is not. Sounds like a radiator cap problem to me. If the cap isn't keeping the cooling system under pressure, it will boil pretty easy.

[Aloid]

I little dribble is normal after shut down but boiling over is not. Sounds like a radiator cap problem to me. If the cap isn't keeping the cooling system under pressure, it will boil pretty easy.

I found this old thread because I had this issue today (the first time the car has been out for an extended time).   I had a few pints of coolant flush into my overflow tank just as I shutdown...  it was really boiling like crazy in the overflow...

If it's the radiator cap, how do you test it?  Or do you just get another one?

Thanks,

Alan

[Bullitt-]

If there's any question that the cap may be bad just get a new one.
Inspect the radiators sealing surface as well.
   Brother hood bought a $5 radiator cap from O'Reilly's .... Murray Caps & Stats - Standard Radiator Cap  Part Number: 7616 that solved his problem.
   Any shop would have a tester, to much to invest just to test 1-cap IMO.

[Aussie Challenger]

It does sound like the cap but if motor has been sitting for an extended period it may pay to flush out the radiator and possibly replace the thermostat as a precaution.   
I know that you have just bought this car so you don't know its earlier history, the radiator could even be too small.