Auto Repair Help
Carl’s Corner – Auto Repair Help


By Carl O’Reilly

A fine mist or film of liquid that collects on the inside of the windshield (windows are fogged up). May be more noticeable after the operation of the front defrosters.

Mist on the windshield is usually caused by a leak in the heater core. Water is circulated into the passenger compartment through the heater core. The heater core is heated by the engines coolant. Air is then blown over the heater core, by the blower motor when the heater is on, to heat the passenger compartment. If this core develops a leak, it will usually leak into the passenger compartment floor board. A heater core that has only a pin-hole sized leak, may not be leaking on the floor but may produce a fine mist or oily film that collects on the inside of the windshield.

Engine coolant or antifreeze is usually green and will feel slightly greasy if rubbed between two fingers. Collecting some of the fluid on a white piece of paper or napkin can help identify the fluid.

The only corrective action for a leak in the heater core is replacement. Do not use radiator stop leak or other sealant products in an attempt to fix a leak in a heater core.

A heater core that is leaking cannot be repaired and must be replaced. On most vehicles, replacing the heater core is an extensive and time consuming project that may require special tools. The evaporator core that contains the refrigerant (freon) for the air conditioner may be contained in the same housing as the heater core. On these vehicles, the housing is removed as a single unit and requires the refrigerant to be removed from the air conditioning system using special equipment. You should consult a repair manual specific to your vehicle or consult the advice of a professional technician prior to undertaking this job.

3 responses to “Diagnose Foggy Windshield”

  1. Richard jackson says:

    Replaced heater core, radiator,cooling fan, thermostat. Still see a smokey film on windshield. I don’t know what else to look for

  2. Bill Thomas says:

    Thank you for this. Now I know as I already experienced a foggy windshield. Sometimes it becomes ok when I turn on the aircon.

  3. What can i use on my foggy windshield until i get heater core fixed?

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