I am at my wits end. I have a fluid leak. Not sure what fluid. Not oil because it evaporates during the day and leaves no spot on driveway. But here's what has me confused, it only leaks at night. I drove it yesterday morning, parked it, and checked it about every hour until dark. NO LEAK. I get up this morning and have this huge spot under the engine. The fluid is not really oily feeling and I'm not sure what color it is. Best I can tell, it's clear(ish). What is it? Why only at night? How can I find out what is going on?
2001 F 150 5.4 L 4X4

 

Condensation?

 

No it's not condensation.

 

Try putting an old bowl or something to help catch it. This will be the best first step, figure out WHAT it is. Seemly water based tho (since it's disappearing with no marks) so that would be coolant or water (like you said). Where under the truck is it hitting the ground? Driver side or passenger? Front/mid/back of motor? Under the cab? Under the bed? I get condensation dripping mid engine passenger side I suspect from AC condensation. Same motor on an 02. Coolant hoses would be heater hoses on top PS from firewall to from of intake manifold and one that goes under the manifold. Top and bottom rad hoses, (bottom on driver side). Coolant reservoir on front DS. Where it's dripping is the first step. And are you losing any coolant?

 

I know you said not condensation, but bear with me a sec here. (I had this on a different vehicle and it drove me nuts)

Lets say that at night you get dew on the truck, as more accumulates it starts to make big drops and starts to run off. There are body drains that catch runoff and such, 2 happen to dump out right under the A pillars, which could make the spot you are describing.
Another this is the AC drain dumps centerish back of engine

 

If it evaporates, then it's not oil, tranny fluid, brake fluid, power steering fluid (tranny fluid), or differential oil. Which means it can only be A/C condensate, washer fluid, or coolant. Check your coolant reservoir. If it's fine you don't have much of a problem.

 

Ahhh I forgot washer fluid. Good call. Again for the OP, location location location.