[Mgs] Flushing the cooling system?

Denise Thorpe xyzabcde at earthlink.net
Tue Mar 10 18:12:22 MST 2009


Hi Paul,

Since my last pontificating went over so well, I'll do it again.  A radiator 
boils over when the coolant increases in volume to the point that the 
pressure overcomes the pressure limit of the radiator cap...something about 
PV=nRT.  If a car has an overflow bottle, like a late model B, the air in 
the bottle is displaced and absorbs the extra volume so the coolant never 
boils over.  That's why there's an upper limit on filling the bottle.  Early 
Bs have an overflow tube so that extra volume is dumped on the ground before 
it has a chance to bypass the radiator cap.  The problem with this is that 
the coolant is then gone for good and doesn't get sucked back in like it 
would if it went into a bottle.  So to answer your question, Bs with the 
correct parts should never boil over.

But of course that's not the whole story, at least not from me.  Boiling 
over doesn't automatically cause engine damage.  If your car does boil over 
or run too hot, the best thing you can do is pull over to the side of the 
road, leave it running, and pour water slowly over the top of the radiator. 
This will cool down the engine enough so that you can either turn off the 
engine or add water to the radiator with the car running.  If you turn off 
an overheated engine, the heat in the coolant that's no longer circulating 
will rise to the head and possibly crack it.  Of course, if your car is 
overheating because coolant can't circulate, like if the thermostat is stuck 
shut or the water pump belt is gone, then no amount of water over the 
radiator is going to do you any good and you should turn off the engine as 
soon as possible.

The difference between running temperatures for early and late Bs is partly 
the tuning and partly the thermostat.  When Bs had to pass more stringent 
smog laws, they were set to run with the timing retarded which causes a car 
to run hotter.  Also, the thermostats were changed from 180 degrees to 190 
degrees.  At the same time, the temperature gauges were changed to have no 
numbers on them.  MG was worried that people would be upset if they knew 
that their cars were always running above the boiling point of water.  With 
a 180 degree thermostat, my '67 runs at 185 and with a 160 degree 
thermostat, my car runs at 185.  It just takes longer to get there.

Denise Thorpe


PaulH. asked:

The question still stands, just how hot does it have to be to have an MGB in 
good condition boil over?




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