I have a 2003 Ford F-150 pickup with the 5.4L and about 134k miles and is primarily driven in the winter for daily commuting and can sit around for months at a time. I will tow with it, but since I have owned it starting at 89k I have never overheated the engine. The problem I noticed was an intermittent coolant leak under the front of the engine (haven't been able to pinpoint exactly) and only see the leak during a cooldown period. I have added maybe 1-2 quarts of coolant over the past 6 weeks and last night I checked for the usual signs of a bad head gasket. No water in oil No oil in water No puff of white smoke on startup No rough idle or any other odd symptoms including check engine light The occasional bubble coming up through the coolant overflow tank and increases when the engine is revved (at least when the engine is decelerating since I haven't had a friend help me). I have read a lot people have problems with the coolant overflowing the coolant reservoir and the engine overheats. I don't have anything close to this. My question is what is the right plan of attack to figure out the problem? 1. The first thing I was going to do today is the combustion leak test to check for hydrocarbons 2. Do a compression test 3. Inspect spark plugs while they are out to look for antifreeze or signs of water contamination 4. UOA to test for contamination Is there any other reasons I would see bubbles in the coolant? Maybe a leaking water pump (source of my leak as well)? Bad intake manifold gasket? Thanks for your help.