Coolant Flush, Water Boiling Over.

[Bobbafett 49 2 Cars]

I really should start my own stupid questions thread but here we go.

 

I removed my radiator the other day and the coolant was literally a brown mess so as well as flushing everything I bought some 2-part coolant flush (Holts).

 

On the instructions it says to add the flush, refill and run the engine on fast idle for 30 mins with the expansion cap OFF and a wet towel over the cap. Do not allow to boil.

 

Well after about 5 mins of running the engine the water is boiling and pissing out the expansion tank. I fitted a new thermostat and it is definitely opening and pushing around the rad. The fan does not come on before the coolant boils out of the tank. There is no voltage going to the fan at any point before it boils. I fitted a brand new rad temp sensor. The gauge on the dash is here just before boiling:

 

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Now I understand how pressuring the system raises the boiling point, but then how is it possible to run for 30 mins without the cap on? Should the fan be coming on before the coolant boils?

 

[erbs 42 1 Cars]

it should get to a certain temperature then the fan kicks in,

 

http://forum.205gtidrivers.com/index.php?showtopic=153098