I went into town today to check how much money I need to borrow for new tires and some jackstands (see brake rotor post) and coming back I revved the motor quite hard going up a hill, and immediately smelled warm coolant and heard a hissing noise on the intake side of the car..

I pulled over straight away (I'm learning that one fast) and couldn't see anything leaking. The expansion tank was still full, so, with a careful eye on the temp gauge, I drove it gently home. Half a mile later I'd lost about 3 litres of coolant and aquired a nice pool of green stuff on the floor of the car. There's no coolant coming out in the engine bay, so if I just bypass the heater I can drive it no problem.

What am I most likely to have blown in the heater system? And how difficult is it to check?

Also, does anyone have any tips on bypassing the heater?

Thanks to all of you for helping keep my disaster rolling.
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Drive it like you hate it