Reader Question: Getting Rid of Windshield Fog?

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Here’s the latest reader question, along with my reply!

Rob asks: Is there anything that can be done to get rid of fog on the inside surface of a windshield? I wipe it away using glass cleaner but it seems to never go away.

My reply: Yes, there is. Replace the heater core. It is probably leaking and the fog you’re seeing is the result of the greasy mist produced by leaking engine coolant inside the ductwork of your car.

The heater core is a kind of mini radiator buried inside the dashboard – a terrible idea in terms of the inevitable need to replace it when it fails. Hot coolant flows through the core and hot air flows into the car’s cabin. But over time, the core develops cracks because of fatigue/wear and then it springs a leak. This leak will get worse and eventually, you’ll have an interior soaked with sticky, impossible to get rid of coolant. You may have to replace the carpet and do a massive cleaning job to get rid of it – after you have the core replaced, of course.

I’d attend to this job sooner rather than later, to limit the mess and the expense.

. . .

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  1. IF it’s the heater core, the fogging likely stops after the car gets warmed up, and may fog again immediately after shutting it off. (Just in case…but it likely is the heater core).

    A good tip to prevent random fogging: Use shaving cream to clean the inside of the windows… better than that No Fog stuff.

    • Hi Bill,

      The heater core thing is miserable. My buddy – the mechanic – had to replace one recently in a late model F-150 and that entailed literally removing most of the dashboard to get at the leaky old one. Satan himself approved the location.

      • eric, my first bad heater core just busted and filled the floorboard with coolant, no warning. The second was on my 82 K30 diesel. I saw the windshield getting misty and could smell the coolant. 30 miles from the house I saw in my mind’s eye that sumbitch just gushing into the floor in the middle of nowhere. I bought a big bottle of Bar’s Leak and installed it. In a few minutes I didn’t smell it. I got home and cleaned the windshield. I bought a couple gallons of anti-freeze and a couple gallons of of distilled water and kept driving it. I’d just block off the delivery hose and add what was needed till I could get somewhere and buy a new one. Years later after I had installed Bar’s Leak in everything I had I had to buy new anti-freeze having used it in some piece of equipment, a big tractor. I didn’t need new coolant in that pickup till the water pump went out.

        I was headed to work and the wife wanted to know if she could use that pickup. I said “Yep, it’s got a new water pump in the seat that has to be installed since it’s out of coolant”. I came home and found her hooked up to a trailer load of rip rap she’d loaded up at an old lake that had been drained. I just shook my head and kept on going. My best friend was helping her and I later asked if they’d both managed to get the water pump on. Shit, she said, he’s useless. I did it myself. I figured as much. Give him a shop of tools and the simplest chore and when you got back you could fix it. Some people are useless as mechanics and he definitely was.

          • Thanks much joeallen. My health could be much better but it was much worse so I’m proud to feel like I’m on the mend again. But it’s one of those “nobody knows” things, I guess typical for counting up in the seventh decade. I hope you’re doing well.

            • Counting the 2 days till retirement, a busy one at that. Looking forward to leaving the workplaces due to this conjob crap posing as covid19.


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