Matt Y says:
You may need to do some more research. You can definitely start a car with the radiator cap off. The coolant will not spew all over the place on start up. I know because I have done it many many times. The only time the coolant spews out everywhere is if you take the cap off while the engine is very hot. Of course if you start the car with the cap off and leave it off for more than lets say 4-5 minutes the coolant will begin to spew out because the engine has heated up, as well as the coolant. next time try following the directions to a tee before assuming you know better than the scientist that invented the stuff.
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J Mullins says:
If you can run your engine with the cap off without coolant spewing out, you dont have a blown head gasket.
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Expert note:
Above customer ended up using another bottle with success. Thanks Matt!