no mixing. i just did the fan clutch job 3 weeks ago and i drained all the coolant - no oil in coolant. I check my oil level every week - no coolant in oil.

i am thinking maybe there is a crack in the cylinder head. also, why wouldn't this pressure get relieved via the expansion tank cap - i thought that the cap valve would open once the pressure went over a given value.

all the same, are there any tests (other than checking for oil in coolant) that i can conduct - maybe i just happened to replace a defective reservoir cap with another defective reservoir cap.

I read in another thread that once one component of a system fails, it maybe an indicator that the other components of that same system are waiting in line to fail also - so i just happened to buy a car that had reached end of life for the cooling system component. At 159 000 miles - is this more or less expected behavior?