Corsa C 2004 z10xep
Corsa c 1.0 2004. If have heating on in car and stud at lights heater goes cold with fans on. Just blows cold or cooler. Start moving along and heat comes back even at 20mph. Windy day and can feel wind blowing though vents heater is hot. Just not when you have fans on. Any ideas. Checked temperature in pipes into heater core are same on in and out sides. Normal holds at around 88c.