coolant leak into exhaust?-PAA

[james accordino]

What are the implications of coolant under pressure
flowing through a crack in the head into the exhaust
tract?  Does the burnt coolant contaminate the oxygen
sensor?  Sensor looks pretty clean and is 3 months
old.  4th O2 sensor in a year.  All other (fuel/elec.)
components repeatedly verified
electronically/physically or replaced.  Symptoms are
poor mileage and soot in the tailpipe.  Code reads O2
sensor reads lean.  Coolant "disappears" at a slow
rate (1 qt. per month).  There are no external coolant
leaks-period.  There were small radius cracks around
the exhaust valve seat, which the manual says are
allowable.  If the crack/cracks penetrate the coolant
jacketing (which is very close here) just downstream
of the exhaust valve, would this limit the
pressurisation effect of the combustion chamber on the
cooling system?  Would this also allow coolant to be
wicked/sucked through the crack under certain
conditions, i.e. exhaust scavenging?  Let your tech
heads rattle those questions.  Meanwhilst.

Primary questions are:

1.  Effect of burnt/partially burnt green coolant on 
    3/4 wire O2 sensors?

2.  Effective (cheap/easy) way to check if coolant is 
    leaking through the exhaust valve pocket and being

    burned.

TIA
Jim Accordino
PAA-possible Audi applicability