If exhaust is getting into the coolant system it should be building up alot of
pressure in the tank and blowing water out of the tank or blowing some piece
of your coolant system apart (radiator, heater core, etc.)  If you are not
getting this pressure you could take it to a mechanic and he can do a pressure
test on the coolant system to make sure that you don't have a leak.  Don't
think it could be your water pump or that your radiator fan is not coming on,
or something like that (bad thermostat, slipping waterpump belt, etc.)  Hope I
helped.
Ken Wilford
John 3:16