My 1989 190e, with the A/C on is blowing hot air out of the side vents only on heavy or uphill accelerations. Once the car reaches coasting speed the air goes cold again. Center vents continue to blow cold all the time. I replaced the vacuum regulated heater control valve but still have the same problem. I've looked at the vacuum lines and they look good. Is there a vacuum check valve that goes bad in this car? If so, where is it? Thanks!

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jayhawk