So my heat doesn't work when I start driving. I have plenty of heat when I am driving slow or standing still, but once my car starts going over 40 MPH, only cold air comes in. I have a 1988 420 SEL. Is there a door open in the heating system somewhere or a guard missing? Can someone help?

Thank

Do a search on monovalve, specifically replacing the monovalve cartridge. Or don't do a search and just replace the monovalve cartridge

Sixto
93 300SD

Thanks Sixto, I read your conversatiion on Monovalve where another person was trying to figure out if they shorted their monovalve. Would it be my monovalve if I have heat when I am standing still? Seems like plenty of coolant is getting into the core because I have heat when standing still. Thanks for your help.

C. Landry

Sixto, sorry to jump the gun, I read further about the Monovalve and have noticed that one of the symptoms is heat at low RPM's going away to no heat when RPM's are raised. I found a new monovalve online, but could you direct me in a direction, where I can buy a rebuild kit? Trying to save a little bread.

Thanks again,

C. Landry