So at 277k on the old girl, im startin to lose roughly a gallon of coolant every 500 miles or so...I checked for leaks everywhere and dont see anything, also pull the dipstick and check oil almost every day to see if its making its way into the pan...and nothing....Any ideas or anyone else with this prob...BTW..my work van with 5.4 and 171k is startin to do the same thing.........

 

WHAT ENGINE? Have you checked from underneath? Follow the trail of coolant forward & up to its source. If you have a UV flashlight & yellow glasses, it may be easier to see. Look in the valley, and near the front of the intake around the spark plug wells.

 

both 5.4l