That's good advice. When coolant mixes into the engine oil, it emulsifies & looks like a chocolate milkshake. If you see that, put the cap back on, thank the seller for his/her time, walk away and don't go back. Check the dipstick, too.Evildenny said:I heard form a friend of mine that after the test drive that I should take off the oil cap, Too see if any steam or smoke comes out and too look on the oil cap for a white substance. Is that true?