I have to add about one quart of coolant every 300 to 400 miles. I have let the car idle for extended periods, no visable leaks. The is no vapor from the exhaust or anti-freeze smell. No wet carpets or any stray collant any where I can see. Should I pull spark plugs? What evidence of coolant burning would I see? Other ideas? The car has 183K miles. Thanks, Dave Johnson