2006 F-150 XLT SC with 5.4 V-8 and 42,000 miles

I'd appreciate some thoughts you guys might have on this. I don't drive my pickup often, maybe once every couple of weeks, sometimes every 3 weeks. Beginning about this past November, I noticed when it just sits outside and the temperatures drop, there is a noticeable amount of moisture condensing on the inside of the windshield mostly on the passenger side, also on inside of the passenger side headlight lens. Generally this means there is water getting into the cab of the truck from, my first thought, a leaking heater core. I have checked numerous times and find no evidence of a leaking heater core, no moisture on carpet at all. Since November, I am now noticing the interior moisture on the glass extends across to the driver side as well, getting slightly worse. Still, no moisture on carpet. Checked rear of cab and also find no sign of a leak from glass. When I drive it and get it thoroughly warmed up, this doesn't happen for a day or two, but then it starts again. Recently topped off coolant reservoir bottle with about a pint of 50-50 water antifreeze mix, but it's been slightly low like that for at least two years so I have written that off. We have had unusually mild and warm winter so far and I have noticed that just before the sun goes down and temps are dropping, I can see the windshield starting to steam up around 55 degrees. This obviously means high humidity inside the cab, but where is it coming from? Any thoughts?

 

Finally found it. I now have found a slight dampness on the passenger side carpet so my original thoughts of a leaking heater core must be right. That being said, the carpet is just slightly damp. Anyone have any experience using Motorcraft VC-6 cooling system stop leak pellets?

 

While it certainly could be what you are guessing, your truck is not that old and has low mileage and your coolant has not been a real issue either. Have you confirmed that there is no water leaking from either the windscreen seal, or even through some body sealer that may have cracked in the roof gutters or elsewhere also on the cab where panels join.

 

Taste it (spit it out don't swallow).....I think all coolants have a slightly sweet taste...though not sure about all the new ones.

 

No, I haven't and I also thought about that as well. I have the carpet dried out now and thought I'd watch it. It's supposed to rain here early in the week. Actually, I am not sure how to check for those things other than starting over with dry carpet and just see what happens when it rains.

 

There wasn't enough to taste and not sure I'd try that anyway. It was just damp carpet.

 

We had a warm dry month of March, but now that April is here it has cooled off and rained. The carpet was perfectly dry and no moisture appeared on the inside of the windshield till this week. Now that we have had rain, the moisture is again appearing when outside temps drop to about 60 degrees in the evenings after a warm sunny day. Now I am convinced there is a very slight leak somewhere besides the cooling system. Where do I start looking?

 

It does sound like outside water penetration at this point as a more likely possibility , and this was something I thought about prior also with my earlier posting and idea around the windscreen seal, or body sealant leaking. Perhaps it is time to hold a water hose on the windscreen for a while and check then for water leaks. That is what we did when I was involved with car assembly work. Well we did not use a garden hose but a shower test just the same. All cars would get a drizzle test and some got a major water soak as an audit process. That said, we did find some that leaked once in a while. Not saying that this is your problem either, but based on the age of the truck and what you just described around recent rain conditions this is something that needs to be checked at least.

 

That's also what I was thinking.

And where would you suggest I actually be looking for a possible leak? Without major disassembly I cannot even get near the egde of the glass from under the dash. Remember, the carpet doesn't exactly get wet, just very slightly damp. It's actually difficult to detect the moisture, but when it was colder I could compare the damp area to dry carpet and there was a difference. Maybe I should just live with it and see if it gets worse. OR...drive it more often to keep it dried out.

 

It took a long while, but I finally noticed the rear seat head rest on driver's side was wet after a huge rain. I found the problem, it's the high mounted 3rd brake light. After Googling it, I found it's actually a pretty common leak on this body style F-150. It's sealed now and I haven't had any moisture appear on the inside of the windshield for several weeks. That also explains a dark stain on the rear seat that cleaned up easily last summer, but returned during the winter; it was just because of the water.